A Sample Program
As before, you'll find some unfamiliar wrinkles that we'll soon iron for you.
#include "stdio.h"
int main(void) {
float weight; //user weight
float value; //platinum equivalent
printf("Are you worth your weight in platinum?\n");
printf("Let's check it out.\n");
printf("Please enter your weight in pounds: ");
scanf("%f", &weight); //get input from the user
value = weight * 1700 * 14.5833;
//assume platinum is $1700 per ounce
//14.5833 converts pounds avd. to ounces troy
printf("Your weight in platinum is worth %0.2f.\n", value);
printf("You are easily worth that! If platinum prices drop.\n");
printf("eat more to maintain your value.\n");
return 0;
}Tip Errors and Warnings
If you type this program incorrectly and, say, omit a semicolon, the compiler gives you a syntax error message. Even if you type it correctly, however, the compiler may give you a warning similar to “Warning—conversion from ‘double’ to ‘float,’ possible loss of data.” An error message means you did something wrong and prevents the program from being compiled. A warning , however, means you’ve done something that is valid code but possibly is not what you meant to do. A warning does not stop compilation. This particular warning pertains to how C handles values such as 1700.0. It’s not a problem for this example, and the chapter explains the warning later.
When you type this program, you might want to change the 1700.0 to the current price of the precious metal platinum. Don’t, however, fiddle with the 14.5833 , which represents the number of ounces in a pound. (That’s ounces troy, used for precious metals, and pounds avoirdupois, used for people—precious and otherwise.)
Note that “entering” your weight means to type your weight and then press the Enter or Return key. (Don’t just type your weight and wait.) Pressing Enter informs the computer that you have finished typing your response. The program expects you to enter a number, such as 156 , not words, such as too much . Entering letters rather than digits causes problems that require an if statement (C hapter 7 , “C Control Statements: Branching and Jumps”) to defeat, so please be polite and enter a number. Here is some sample output:
Are you worth your weight in platinum?
Let's check it out.
Please enter your weight in pounds: 156
Your weight in platinum is worth $3867491.25.
You are easily worth that! If platinum prices drop,
eat more to maintain your value.
Program Adjustments
Did the output for this program briefly flash onscreen and then disappear even though you added the following line to the program, as described in Chapter 2 , “Introducing C”?
getchar();For this example, you need to use that function call twice:
getchar(); getchar();The getchar() function reads the next input character, so the program has to wait for input. In this case, we provided input by typing 156 and then pressing the Enter (or Return) key, which transmits a newline character. So scanf() reads the number, the first getchar() reads the newline character, and the second getchar() causes the program to pause, awaiting further input.
What’s New in This Program?
- Perhaps the most outstanding new feature is that this program is interactive. The computer asks you for information and then uses the number you enter. An interactive program is more interesting to use than the noninteractive types. More important, the interactive approach makes programs more flexible. For example, the sample program can be used for any reasonable weight, not just for 156 pounds. You don’t have to rewrite the program every time you want to try it on a new person. The scanf() and
printf() functions make this interactivity possible. The scanf() function reads data from the keyboard and delivers that data to the program, and printf() reads data from a program and delivers that data to your screen. Together, these two functions enable you to establish a two-way communication with your computer (see F igure 3.1 ), and that makes using a computer much more fun.
Data Variables and Constants
A computer, under the guidance of a program, can do many things. It can add numbers, sort names, command the obedience of a speaker or video screen, calculate cometary orbits, prepare a mailing list, dial phone numbers, draw stick figures, draw conclusions, or anything else your imagination can create. To do these tasks, the program needs to work with data , the numbers and characters that bear the information you use. Some types of data are preset before a program is used and keep their values unchanged throughout the life of the program. These are constants. Other types of data may change or be assigned values as the program runs; these are variables . In the sample program, weight is a variable and 14.5833 is a constant. What about 1700.0 ? True, the price of platinum isn’t a constant in real life, but this program treats it as a constant. The difference between a variable and a constant is that a variable can have its value assigned or changed while the program is running, and a constant can’t
Data: Data-Type Keywords
The int keyword provides the basic class of integers used in C. The next three keywords ( long , short , and unsigned ) and the C90 addition signed are used to provide variations of the basic type, for example, unsigned short int and long long int . Next, the char keyword designates the type used for letters of the alphabet and for other characters, such as # , $ , % , and
* . The char type also can be used to represent small integers. Next, float , double , and the combination long double are used to represent numbers with decimal points. The _Bool type is for Boolean values ( true and false ), and _Complex and _Imaginary represent complex and imaginary numbers, respectively.
The types created with these keywords can be divided into two families on the basis of how they are stored in the computer: integer types and floating-point types.
Bits, Bytes, and Words
The terms bit , byte , and word can be used to describe units of computer data or to describe units of computer memory. We’ll concentrate on the second usage here.
The smallest unit of memory is called a bit . It can hold one of two values: 0 or 1 . (Or you can say that the bit is set to “off” or “on.”) You can’t store much information in one bit, but a computer has a tremendous stock of them. The bit is the basic building block of computer memory.
The byte is the usual unit of computer memory. For nearly all machines, a byte is 8 bits, and that is the standard definition, at least when used to measure storage. (The C language, however, has a different definition, as discussed in the “Using Characters: Type char" section later in this chapter.) Because each bit can be either 0 or 1, there are 256 (that’s 2 times itself 8 times) possible bit patterns of 0s and 1s that can fit in an 8-bit byte. These patterns can be used, for example, to represent the integers from 0 to 255 or to represent a set of characters. Representation can be accomplished with binary code, which uses (conveniently enough) just 0s and 1s to represent numbers. ( Chapter 15 , “Bit Fiddling,” discusses binary code, but you can read through the introductory material of that chapter now if you like.)
A word is the natural unit of memory for a given computer design. For 8-bit microcomputers, such as the original Apples, a word is just 8 bits. Since then, personal computers moved up to 16-bit words, 32-bit words, and, at the present, 64-bit words. Larger word sizes enable faster transfer of data and allow more memory to be accessed.
Integer Versus Floating-Point Types
For a human, the difference between integers and floating-point numbers is reflected in the way they can be written. For a computer, the difference is reflected in the way they are stored.
The Integer
An integer is a number with no fractional part. In C, an integer is never written with a decimal point. Examples are 2, –23, and 2456. Numbers such as 3.14, 0.22, and 2.000 are not integers. Integers are stored as binary numbers. The integer 7, for example, is written 111 in binary. Therefore, to store this number in an 8-bit byte, just set the first 5 bits to 0 and the last 3 bits to 1.
The Floating-Point Number
A floating-point number more or less corresponds to what mathematicians call a real number . Real numbers include the numbers between the integers. Some floating-point numbers are 2.75, 3.16E7, 7.00, and 2e–8. Notice that adding a decimal point makes a value a floating-point value. So 7 is an integer type but 7.00 is a floating-point type. Obviously, there is more than one way to write a floating-point number. We will discuss the e-notation more fully later, but, in brief, the notation 3.16E7 means to multiply 3.16 by 10 to the 7th power; that is, by 1 followed by 7 zeros. The 7 would be termed the exponent of 10.
The key point here is that the scheme used to store a floating-point number is different from the one used to store an integer. Floating-point representation involves breaking up a number into a fractional part and an exponent part and storing the parts separately. Therefore, the 7.00 in this list would not be stored in the same manner as the integer 7, even though both have the same value. The decimal analogy would be to write 7.0 as 0.7E1. Here, 0.7 is the fractional part, and the 1 is the exponent part. F igure 3.3 shows another example of floating-point storage. A computer, of course, would use binary numbers and powers of two instead of powers of 10 for internal storage. You’ll find more on this topic in C hapter 15 . Now, let’s concentrate on the practical differences:
■ An integer has no fractional part; a floating-point number can have a fractional part.
■ Floating-point numbers can represent a much larger range of values than integers can. See Table 3.3 near the end of this chapter.
■ For some arithmetic operations, such as subtracting one large number from another, floating-point numbers are subject to greater loss of precision.
■ Because there is an infinite number of real numbers in any range—for example, in the range between 1.0 and 2.0—computer floating-point numbers can’t represent all the values in the range. Instead, floating-point values are often approximations of a true value. For example, 7.0 might be stored as a 6.99999 float value—more about precision later. ■ Floating-point operations were once much slower than integer operations. However, today many CPUs incorporate floating-point processors that close the gap.
Basic C Data Types
The int Type
The int type is a signed integer. That means it must be an integer and it can be positive, negative, or zero. The range in possible values depends on the computer system. Typically, an int uses one machine word for storage. Therefore, older IBM PC compatibles, which have a 16-bit word, use 16 bits to store an int . This allows a range in values from –32768 to 32767 . Current personal computers typically have 32-bit integers and fit an int to that size. Now the personal computer industry is moving toward 64-bit processors that naturally will use even larger integers. ISO C specifies that the minimum range for type int should be from –32767 to 32767 . Typically, systems represent signed integers by using the value of a particular bit to indicate the sign.
Declaring an int Variable
The keyword int is used to declare the basic integer variable. First comes int , and then the chosen name of the variable, and then a semicolon. To declare more than one variable, you can declare each variable separately, or you can follow the int with a list of names in which each name is separated from the next by a comma. The following are valid declarations:
int erns;
int hogs, cows, goats;You could have used a separate declaration for each variable, or you could have declared all four variables in the same statement. The effect is the same: Associate names and arrange storage space for four int-sized variables.
These declarations create variables but don’t supply values for them. How do variables get values? You’ve seen two ways that they can pick up values in the program. First, there is assignment:
cows = 112; Second, a variable can pick up a value from a function—from scanf() , for example. Now let’s look at a third way.
Initializing a Variable
To initialize a variable means to assign it a starting, or initial , value. In C, this can be done as part of the declaration. Just follow the variable name with the assignment operator ( = ) and the value you want the variable to have. Here are some examples:
int hogs = 21;
int cows = 32, goats = 14;
int dogs, cats = 94; /* valid, but poor, form */In the last line, only cats is initialized. A quick reading might lead you to think that dogs is also initialized to 94 , so it is best to avoid putting initialized and noninitialized variables in the same declaration statement.
In short, these declarations create and label the storage for the variables and assign starting values to each.
Printing int Values
You can use the printf() function to print int types. The %d notation is used to indicate just where in a line the integer is to be printed. The %d is called a format specifier because it indicates the form that printf() uses to display a value. Each %d in the format string must be matched by a corresponding int value in the list of items to be printed. That value can be an int variable, an int constant, or any other expression having an int value. It’s your job to make sure the number of format specifiers matches the number of values; the compiler won’t catch mistakes of that kind.
#include "stdio.h"
int main(void) {
int ten = 10;
int two = 2;
printf("Doing it right:");
printf("%d minus %d is %d.\n", ten, 2, ten - two);
printf("Doing it wrong: ");
printf("%d minus %d is %d.\n", ten); //forgot 2 arguments
return 0;
}Compiling and running the program produced this output on one system:
Doing it right: 10 minus 2 is 8
Doing it wrong: 10 minus 16 is 1650287143
For the first line of output, the first %d represents the int variable ten , the second %d represents the int constant 2 , and the third %d represents the value of the int expression ten - two . The second time, however, the program used ten to provide a value for the first %d and used whatever values happened to be lying around in memory for the next two! (The numbers you get could very well be different from those shown here. Not only might the memory contents be different, but different compilers will manage memory locations differently.)
You might be annoyed that the compiler doesn’t catch such an obvious error. Blame the unusual design of printf(). Most functions take a specific number of arguments, and the compiler can check to see whether you’ve used the correct number. However, printf() can have one, two, three, or more arguments, and that keeps the compiler from using its usual methods for error checking. Some compilers, however, will use unusual methods of checking and warn you that you might be doing something wrong. Still, it’s best to remember to always check to see that the number of format specifiers you give to printf() matches the number of values to be displayed.
Octal and Hexadecimal
Normally, C assumes that integer constants are decimal, or base 10, numbers. However, octal (base 8) and hexadecimal (base 16) numbers are popular with many programmers. Because 8 and 16 are powers of 2, and 10 is not, these number systems occasionally offer a more convenient way for expressing computer-related values. For example, the number 65536, which often pops up in 16-bit machines, is just 10000 in hexadecimal. Also, each digit in a hexadecimal number corresponds to exactly 4 bits. For example, the hexadecimal digit 3 is 0011 and the hexadecimal digit 5 is 0101. So the hexadecimal value 35 is the bit pattern 0011 0101, and the hexadecimal value 53 is 0101 0011. This correspondence makes it easy to go back and forth between hexadecimal and binary (base 2) notation. But how can the computer tell whether 10000 is meant to be a decimal, hexadecimal, or octal value? In C, special prefixes indicate which number base you are using. A prefix of 0x or 0X (zero-ex) means that you are specifying a hexadecimal value, so 16 is written as 0x10 , or 0X10 , in hexadecimal. Similarly, a 0 (zero) prefix means that you are writing in octal. For example, the decimal value 16 is written as 020 in octal.
Displaying Octal and Hexadecimal
Just as C enables you write a number in any one of three number systems, it also enables you to display a number in any of these three systems. To display an integer in octal notation instead of decimal, use %o instead of %d . To display an integer in hexadecimal, use %x . If you want to display the C prefixes, you can use specifiers %#o , %#x , and %#X to generate the 0 , 0x , and 0X prefixes respectively. Listing 3.3 shows a short example. (Recall that you may have to insert a getchar(); statement in the code for some IDEs to keep the program execution window from closing immediately.)
#include <stdio.h>
int main(void) {
int x = 100;
printf("dec = %d; octal = %o; hex = %x\n", x, x, x);
printf("dec = %d; octal = %#o; hex = %#x\n", x, x, x);
return 0;
}Compiling and running the program produces this output:
dec = 100; octal = 144; hex = 64
dec = 100; octal = 0144; hex = 0x64
You see the same value displayed in three different number systems. The printf() function makes the conversions. Note that the 0 and the 0x prefixes are not displayed in the output unless you include the # as part of the specifier.
Other Integer Types
C offers three adjective keywords to modify the basic integer type: short , long , and unsigned . Here are some points to keep in mind:
■ The type short int or, more briefly, short may use less storage than int, thus saving space when only small numbers are needed. Like int , short is a signed type.
■ The type long int, or long, may use more storage than int, thus enabling you to express larger integer values. Like int , long is a signed type.
■ The type long long int , or long long (introduced in the C99 standard), may use more storage than long . At the minimum, it must use at least 64 bits. Like int, long long is a signed type.
■ The type unsigned int , or unsigned , is used for variables that have only nonnegative values. This type shifts the range of numbers that can be stored. For example, a 16-bit unsigned int allows a range from 0 to 65535 in value instead of from –32768 to 32767 . The bit used to indicate the sign of signed numbers now becomes another binary digit, allowing the larger number.
■ The types unsigned long int , or unsigned long , and unsigned short int , or unsigned short , are recognized as valid by the C90 standard. To this list, C99 adds unsigned long long int , or unsigned long long .
■ The keyword signed can be used with any of the signed types to make your intent explicit. For example, short , short int , signed short , and signed short int are all names for the same type.
Declaring Other Integer Types
Other integer types are declared in the same manner as the int type. The following list shows several examples. Not all older C compilers recognize the last three, and the final example is new with the C99 standard.
long int estine;
long johns;
short int erns;
short ribs;
unsigned int s_count;
unsigned players;
unsigned long headcount;
unsigned short yesvotes;
long long ago;
Why Multiple Integer Types?
Why do we say that long and short types “may” use more or less storage than int ? Because C guarantees only that short is no longer than int and that long is no shorter than int. The idea is to fit the types to the machine. For example, in the days of Windows 3, an int and a short were both 16 bits, and a long was 32 bits. Later, Windows and Apple systems moved to using 16 bits for short and 32 bits for int and long . Using 32 bits allows integers in excess of 2 billion. Now that 64-bit processors are common, there’s a need for 64-bit integers, and that’s the motivation for the long long type.
The most common practice today on personal computers is to set up long long as 64 bits, long as 32 bits, short as 16 bits, and int as either 16 bits or 32 bits, depending on the machine’s natural word size. In principle, these four types could represent four distinct sizes, but in practice at least some of the types normally overlap.
The C standard provides guidelines specifying the minimum allowable size for each basic data type. The minimum range for both short and int is –32,767 to 32,767, corresponding to a 16-bit unit, and the minimum range for long is –2,147,483,647 to 2,147,483,647, corresponding to a 32-bit unit. (Note: For legibility, we’ve used commas, but C code doesn’t allow that option.) For unsigned short and unsigned int , the minimum range is 0 to 65,535, and for unsigned long , the minimum range is 0 to 4,294,967,295. The long long type is intended to support 64-bit needs. Its minimum range is a substantial –9,223,372,036,854,775,807 to 9,223,372,036,854,775,807, and the minimum range for unsigned long long is 0 to 18,446,744,073,709,551,615. For those of you writing checks, that’s eighteen quintillion, four hundred and forty-six quadrillion, seven hundred forty-four trillion, seventy-three billion, seven hundred nine million, five hundred fifty-one thousand, six hundred fifteen using U.S. nomenclature (the short scale or échelle courte system), but who’s counting?
When do you use the various int types? First, consider unsigned types. It is natural to use them for counting because you don’t need negative numbers, and the unsigned types enable you to reach higher positive numbers than the signed types.
Use the long type if you need to use numbers that long can handle and that int cannot. However, on systems for which long is bigger than int , using long can slow down calculations, so don’t use long if it is not essential. One further point: If you are writing code on a machine for which int and long are the same size, and you do need 32-bit integers, you should use long instead of int so that the program will function correctly if transferred to a 16-bit machine. Similarly, use long long if you need 64-bit integer values.
Use short to save storage space if, say, you need a 16-bit value on a system where int is 32-bit. Usually, saving storage space is important only if your program uses arrays of integers that are large in relation to a system’s available memory. Another reason to use short is that it may correspond in size to hardware registers used by particular components in a computer.
Integer Overflow
What happens if an integer tries to get too big for its type? Let’s set an integer to its largest possible value, add to it, and see what happens. Try both signed and unsigned types. (The printf() function uses the %u specifier to display unsigned int values .)
#include <stdio.h> int main(void) { int i = 2147483647; unsigned int j = 4294967295; printf("%d %d %d\n", i, i + 1, i + 2); printf("%u %u %u\n", j, j + 1, j + 2); return 0; }Here is the result for our system:
The unsigned integer j is acting like a car’s odometer. When it reaches its maximum value, it starts over at the beginning. The integer i acts similarly. The main difference is that the unsigned int variable j , like an odometer, begins at 0, but the int variable i begins at –2147483648. Notice that you are not informed that i has exceeded (overflowed) its maximum value. You would have to include your own programming to keep tabs on that.2147483647 -2147483648 -2147483647 4294967295 0 1
The behavior described here is mandated by the rules of C for unsigned types. The standard doesn’t define how signed types should behave. The behavior shown here is typical, but you could encounter something different
long constants and long long constants
Normally, when you use a number such as 2345 in your program code, it is stored as an int type. What if you use a number such as 1000000 on a system in which int will not hold such a large number? Then the compiler treats it as a long int , assuming that type is large enough. If the number is larger than the long maximum, C treats it as unsigned long . If that is still insufficient, C treats the value as long long or unsigned long long , if those types are available.
Octal and hexadecimal constants are treated as type int unless the value is too large. Then the compiler tries unsigned int . If that doesn’t work, it tries, in order, long , unsigned long , long long , and unsigned long long .
Sometimes you might want the compiler to store a small number as a long integer. Programming that involves explicit use of memory addresses on an IBM PC, for instance, can create such a need. Also, some standard C functions require type long values. To cause a small constant to be treated as type long , you can append an l (lowercase L ) or L as a suffix. The second form is better because it looks less like the digit 1. Therefore, a system with a 16-bit int and a 32-bit long treats the integer 7 as 16 bits and the integer 7L as 32 bits. The l and L suffixes can also be used with octal and hex integers, as in 020L and 0x10L .
Similarly, on those systems supporting the long long type, you can use an ll or LL suffix to indicate a long long value, as in 3LL . Add a u or U to the suffix for unsigned long long , as in 5ull or 10LLU or 6LLU or 9Ull .
Printing short , long , long long , and unsigned Types
To print an unsigned int number, use the %u notation. To print a long value, use the %ld format specifier. If int and long are the same size on your system, just %d will suffice, but your program will not work properly when transferred to a system on which the two types are different, so use the %ld specifier for long . You can use the l prefix for x and o , too. So you would use %lx to print a long integer in hexadecimal format and %lo to print in octal format. Note that although C allows both uppercase and lowercase letters for constant suffixes, these format specifiers use just lowercase.
C has several additional printf() formats. First, you can use an h prefix for short types. Therefore, %hd displays a short integer in decimal form, and %ho displays a short integer in octal form. Both the h and l prefixes can be used with u for unsigned types. For instance, you would use the %lu notation for printing unsigned long types. Listing 3.4 provides an example. Systems supporting the long long types use %lld and %llu for the signed and unsigned versions.
#include <stdio.h>
int main(void) {
unsigned int un = 3000000000; //system with 32-bit int and 16-bit short
short end = 200;
long big = 65537;
long long verybig = 12345678908642;
printf("un = %u and not %d\n", un, un);
printf("end = %hd and %d\n", end, end);
printf("verybig = %lld and not %ld\n", verybig, verybig);
return 0;
}Here is the output on one system(results can vary):
un = 3000000000 and not -1294967296
end = 200 and 200
verybig = 12345678908642 and not 12345678908642This example points out that using the wrong specification can produce unexpected results. First, note that using the %d specifier for the unsigned variable un produces a negative number! The reason for this is that the unsigned value 3000000000 and the signed value –129496296 have exactly the same internal representation in memory on our system. So if you tell printf() that the number is unsigned, it prints one value, and if you tell it that the same number is signed, it prints the other value. This behavior shows up with values larger than the maximum signed value. Smaller positive values, such as 96, are stored and displayed the same for both signed and unsigned types.
Next, note that the short variable end is displayed the same whether you tell printf() that end is a short (the %hd specifier) or an int (the %d specifier). That’s because C automatically expands a type short value to a type int value when it’s passed as an argument to a function. This may raise two questions in your mind: Why does this conversion take place, and what’s the use of the h modifier? The answer to the first question is that the int type is intended to be the integer size that the computer handles most efficiently. So, on a computer for which short and int are different sizes, it may be faster to pass the value as an int . The answer to the second question is that you can use the h modifier to show how a longer integer would look if truncated to the size of short . The third line of output illustrates this point. The value 65537 expressed in binary format as a 32-bit number is 00000000000000010000000000000001. Using the %hd specifier persuaded printf() to look at just the last 16 bits; therefore, it displayed the value as 1. Similarly, the final output line shows the full value of verybig and then the value stored in the last 32 bits, as viewed through the %ld specifier.
Earlier you saw that it is your responsibility to make sure the number of specifiers matches the number of values to be displayed. Here you see that it is also your responsibility to use the correct specifier for the type of value to be displayed.
Match the Type printf() Specifies
Remember to check to see that you have one format specifier for each value being displayed in a printf() statement. And also check that the type of each format specifier matches the type of the corresponding display value.
Using Characters: Type
The char type is used for storing characters such as letters and punctuation marks, but technically it is an integer type. Why? Because the char type actually stores integers, not characters. To handle characters, the computer uses a numerical code in which certain integers represent certain characters. The most commonly used code in the U.S. is the ASCII code given in the table on the inside front cover. It is the code this book assumes. In it, for example, the integer value 65 represents an uppercase A . So to store the letter A , you actually need to store the integer 65 . (Many IBM mainframes use a different code, called EBCDIC, but the principle is the same. Computer systems outside the U.S. may use entirely different codes.)
The standard ASCII code runs numerically from 0 to 127. This range is small enough that 7 bits can hold it. The char type is typically defined as an 8-bit unit of memory, so it is more than large enough to encompass the standard ASCII code. Many systems, such as the IBM PC and the Apple Macs, offer extended ASCII codes (different for the two systems) that still stay within an 8-bit limit. More generally, C guarantees that the char type is large enough to store the basic character set for the system on which C is implemented.
Many character sets have many more than 127 or even 255 values. For example, there is the Japanese kanji character set. The commercial Unicode initiative has created a system to represent a variety of characters sets worldwide and currently has over 110,000 characters. The International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) have developed a standard called ISO/IEC 10646 for character sets. Fortunately, the Unicode standard has been kept compatible with the more extensive ISO/IEC 10646 standard.
The C language defines a byte to be the number of bits used by type char , so one can have a system with a 16-bit or 32-bit byte and char type.
Declaring Type char Variables
As you might expect, char variables are declared in the same manner as other variables. Here are some examples:
char response;
char itable, latan; This code would create three char variables: response , itable , and latan .
Character Constants and Initialization
Suppose you want to initialize a character constant to the letter A . Computer languages are supposed to make things easy, so you shouldn’t have to memorize the ASCII code, and you don’t. You can assign the character A to grade with the following initialization:
char grade = 'A'; A single character contained between single quotes is a C character constant . When the compiler sees 'A' , it converts the 'A' to the proper code value. The single quotes are essential. Here’s another example:
char broiled; //declare a variable
broiled = 'T'; //OK
broiled = T //NO! Thinks T is a variable
broiled = "T"; //NO! Thinks T is a stringIf you omit the quotes, the compiler thinks that T is the name of a variable. If you use double quotes, it thinks you are using a string.
Because characters are really stored as numeric values, you can also use the numerical code to assign values:
char grade = 65; /* ok for ASCII, but poor style */ In this example, 65 is type int , but, because the value is smaller than the maximum char size, it can be assigned to grade without any problems. Because 65 is the ASCII code for the letter A , this example assigns the value A to grade. Note, however, that this example assumes that the system is using ASCII code. Using 'A' instead of 65 produces code that works on any system. Therefore, it’s much better to use character constants than numeric code values.
Somewhat oddly, C treats character constants as type char rather than type char . For example, on an ASCII system with a 32-bit char and an 8-bit char , the code
char grade = 'B'; represents 'B' as the numerical value 66 stored in a 32-bit unit, but grade winds up with 66 stored in an 8-bit unit. This characteristic of character constants makes it possible to define a character constant such as 'FATE' , with four separate 8-bit ASCII codes stored in a 32-bit unit. However, attempting to assign such a character constant to a char variable results in only the last 8 bits being used, so the variable gets the value 'E' .
Nonprinting Characters
The single-quote technique is fine for characters, digits, and punctuation marks, but if you look through the table on the inside front cover of this book, you’ll see that some of the ASCII characters are nonprinting. For example, some represent actions such as backspacing or going to the next line or making the terminal bell ring (or speaker beep). How can these be represented? C offers three ways.
The first way we have already mentioned—just use the ASCII code. For example, the ASCII value for the beep character is 7, so you can do this:
char beep = 7; The second way to represent certain awkward characters in C is to use special symbol sequences. These are called escape sequences .
Escape sequences must be enclosed in single quotes when assigned to a character variable. For example, you could make the statement
char nerf = '\n';and then print the variable nerf to advance the printer or screen one line.
Now take a closer look at what each escape sequence does. The alert character ( \a ), added by C90, produces an audible or visible alert. The nature of the alert depends on the hardware, with the beep being the most common. (With some systems, the alert character has no effect.) The C standard states that the alert character shall not change the active position. By active position , the standard means the location on the display device (screen, teletype, printer, and so on) at which the next character would otherwise appear. In short, the active position is a generalization of the screen cursor with which you are probably accustomed. Using the alert character in a program displayed on a screen should produce a beep without moving the screen cursor.
Next, the \b , \f , \n , \r , \t , and \v escape sequences are common output device control characters. They are best described in terms of how they affect the active position. A backspace ( \b ) moves the active position back one space on the current line. A form feed character ( \f ) advances the active position to the start of the next page. A newline character ( \n ) sets the active position to the beginning of the next line. A carriage return ( \r ) moves the active position to the beginning of the current line. A horizontal tab character ( \t ) moves the active position to the next horizontal tab stop (typically, these are found at character positions 1, 9, 17, 25, and so on). A vertical tab ( \v ) moves the active position to the next vertical tab position.
These escape sequence characters do not necessarily work with all display devices. For example, the form feed and vertical tab characters produce odd symbols on a PC screen instead of any cursor movement, but they work as described if sent to a printer instead of to the screen.
The next three escape sequences ( \\ , \' , and \" ) enable you to use \ , ' , and " as character constants. (Because these symbols are used to define character constants as part of a printf() command, the situation could get confusing if you use them literally.) Suppose you want to print the following line:
Gramps sez, "a \ is a backslash."Then use this code:
printf("Gramps sez, \"a \\ is a backslash.\"\n");The final two forms ( \0oo and \xhh ) are special representations of the ASCII code. To represent a character by its octal ASCII code, precede it with a backslash ( \ ) and enclose the whole thing in single quotes. For example, if your compiler doesn’t recognize the alert character ( \a ), you could use the ASCII code instead:
beep = '\007'; You can omit the leading zeros, so '\07' or even '\7' will do. This notation causes numbers to be interpreted as octal, even if there is no initial 0 .
Beginning with C90, C provides a third option—using a hexadecimal form for character constants. In this case, the backslash is followed by an x or X and one to three hexadecimal digits. For example, the Ctrl+P character has an ASCII hex code of 10 (16, in decimal), so it can be expressed as '\x10' or '\X010' . F igure 3.5 shows some representative integer types.
When you use ASCII code, note the difference between numbers and number characters. For example, the character 4 is represented by ASCII code value 52. The notation '4' represents the symbol 4, not the numerical value 4.
At this point, you may have three questions:
- Why aren’t the escape sequences enclosed in single quotes in the last example ( printf("Gramps sez, \"a \\ is a backslash\"\"n"); )? When a character, be it an escape sequence or not, is part of a string of characters enclosed in double quotes, don’t enclose it in single quotes. Notice that none of the other characters in this example ( G , r , a , m , p , s , and so on) are marked off by single quotes. A string of characters enclosed in double quotes is called a character string . (C hapter 4 explores strings.) Similarly, printf("Hello!\007\n"); will print Hello! and beep, but printf("Hello!7\n"); will print Hello!7 . Digits that are not part of an escape sequence are treated as ordinary characters to be printed.
- When should I use the ASCII code, and when should I use the escape sequences? If you have a choice between using one of the special escape sequences, say ' \f' , or an equivalent ASCII code, say '\014' , use the '\f' . First, the representation is more mnemonic. Second, it is more portable. If you have a system that doesn’t use ASCII code, the '\f' will still work.
- If I need to use numeric code, why use, say, '\032' instead of 032 ? — First, using '\032' instead of 032 makes it clear to someone reading the code that you intend to represent a character code. Second, an escape sequence such as \032 can be embedded in part of a C string, the way \007 was in the first point.
Printing Characters
The printf() function uses %c to indicate that a character should be printed. Recall that a character variable is stored as a 1-byte integer value. Therefore, if you print the value of a char variable with the usual %d specifier, you get an integer. The %c format specifier tells printf() to display the character that has that integer as its code value.
#include <stdio.h>
int main(void) {
char ch;
printf("Please enter a character.\n");
scanf("%c", &ch);
printf("The code for %c is %d.\n", ch, ch);
return 0;
}Here is a sample run:
Please enter a character.
C
The code for C is 67.When you use the program, remember to press the Enter or Return key after typing the character. The scanf() function then fetches the character you typed, and the ampersand ( & ) causes the character to be assigned to the variable ch . The printf() function then prints the value of ch twice, first as a character (prompted by the %c code) and then as a decimal integer (prompted by the %d code). Note that the printf() specifiers determine how data is displayed, not how it is stored.
Signed or Unsigned?
Some C implementations make char a signed type. This means a char can hold values typically in the range –128 through 127. Other implementations make char an unsigned type, which provides a range of 0 through 255. Your compiler manual should tell you which type your char is, or you can check the limits.h header file, discussed in the next chapter.
As of C90, C enabled you to use the keywords signed and unsigned with char . Then, regardless of what your default char is, signed char would be signed, and unsigned char would be unsigned. These versions of char are useful if you’re using the type to handle small integers. For character use, just use the standard char type without modifiers.
The _Bool Type
The _Bool type is a C99 addition that’s used to represent Boolean values—that is, the logical values true and false . Because C uses the value 1 for true and 0 for false , the _Bool type really is just an integer type, but one that, in principle, only requires 1 bit of memory, because that is enough to cover the full range from 0 to 1.
Programs use Boolean values to choose which code to execute next.
Portable Types: stdint.h and inttypes.h
By now you’ve probably noticed that C offers a wide variety of integer types, which is a good thing. And you probably also have noticed that the same type name doesn’t necessarily mean the same thing on different systems, which is not such a good thing. It would be nice if C had types that had the same meaning regardless of the system. And, as of C99, it does—sort of.
What C has done is create more names for the existing types. The trick is to define these new names in a header file called stdint.h . For example, int32_t represents the type for a 32-bit signed integer. The header file on a system that uses a 32-bit int could define int32_t as an alias for int . A different system, one with a 16-bit int and a 32-bit long , could define the same name, int32_t , as an alias for int . Then, when you write a program using int32_t as a type and include the stdint.h header file, the compiler will substitute int or long for the type in a manner appropriate for your particular system.
The alternative names we just discussed are examples of exact-width integer types ; int32_t is exactly 32 bits, no less or no more. It’s possible the underlying system might not support these choices, so the exact-width integer types are optional.
What if a system can’t support exact-width types? C99 and C11 provide a second category of alternative names that are required. This set of names promises the type is at least big enough to meet the specification and that no other type that can do the job is smaller. These types are called minimum width types . For example, int_least8_t will be an alias for the smallest available type that can hold an 8-bit signed integer value. If the smallest type on a particular system were 16 bits, the int8_t type would not be defined. However, the int_least8_t type would be available, perhaps implemented as a 16-bit integer.
Of course, some programmers are more concerned with speed than with space. For them, C99 and C11 define a set of types that will allow the fastest computations. These are called the fastest minimum width types. For example, the int_fast8_t will be defined as an alternative name for the integer type on your system that allows the fastest calculations for 8-bit signed values.
Finally, for some programmers, only the biggest possible integer type on a system will do; intmax_t stands for that type, a type that can hold any valid signed integer value. Similarly, uintmax_t stands for the largest available unsigned type. Incidentally, these types could be bigger than long long and unsigned long because C implementations are permitted to define types beyond the required ones. Some compilers, for example, introduced the long long type before it became part of the standard.
C99 and C11 not only provide these new, portable type names, they also provide assistance with input and output. For example, printf() requires specific specifiers for particular types. So what do you do to display an int32_t value when it might require a %d specifier for one definition and an %ld for another? The current standard provides some string macros (a mechanism introduced in Chapter 4 ) to be used to display the portable types. For example, the inttypes.h header file will define PRId32 as a string representing the appropriate specifier ( d or l , for instance) for a 32-bit signed value. Listing 3.6 shows a brief example illustrating how to use a portable type and its associated specifier. The inttypes.h header file includes stdint.h , so the program only needs to include inttypes.h.
#include <stdio.h>
#include <inttypes.h>
int main(void) {
int32_t me32; //me32 a 32-bit signed variable
me32 = 45933945;
printf("First, assume int32_t is int: ");
printf("me32 = %d\n", me32);
printf("Next, let's not make any assumptions.\n");
printf("Instead, use a \"macro\" from inttypes.h: ");
printf("me32 = %" PRId32 "\n", me32);
return 0;
}In the final printf() argument, the PRId32 is replaced by its inttypes.h definition of "d" , making the line this:
printf("me16 = %" "d" "\n", me16);But C combines consecutive quoted strings into a single quoted string, making the line this:
printf("me16 = %d\n", me16);Here’s the output; note that the example also uses the \" escape sequence to display double quotation marks:
First, assume int32_t is int: me32 = 45933945
Next, let's not make any assumptions.
Instead, use a "macro" from inttypes.h: me32 = 45933945It’s not the purpose of this section to teach you all about expanded integer types. Rather, its main intent is to reassure you that this level of control over types is available if you need it. Reference Section VI, “Extended Integer Types,” in Appendix B provides a complete rundown of the inttypes.h and stdint.h header files.
Types float , double , and long double
The various integer types serve well for most software development projects. However, financial and mathematically oriented programs often make use of floating-point numbers. In C, such numbers are called type float , double , or long double . They correspond to the real types of FORTRAN and Pascal. The floating-point approach, as already mentioned, enables you to represent a much greater range of numbers, including decimal fractions. Floating-point number representation is similar to scientific notation, a system used by scientists to express very large and very small numbers. Let’s take a look.
In scientific notation, numbers are represented as decimal numbers times powers of 10. Here are some examples.
The first column shows the usual notation, the second column scientific notation, and the third column exponential notation, or e-notation, which is the way scientific notation is usually written for and by computers, with the e followed by the power of 10. F igure 3.7 shows more floating-point representations.
The C standard provides that a float has to be able to represent at least six significant figures and allow a range of at least to
. The first requirement means, for example, that a float has to represent accurately at least the first six digits in a number such as 33.333333. The second requirement is handy if you like to use numbers such as the mass of the sun (2.0e30 kilograms), the charge of a proton (1.6e–19 coulombs), or the national debt. Often, systems use 32 bits to store a floating-point number. Eight bits are used to give the exponent its value and sign, and 24 bits are used to represent the nonexponent part, called the mantissa or significand , and its sign.
C also has a double (for double precision) floating-point type. The double type has the same minimum range requirements as float , but it extends the minimum number of significant figures that can be represented to 10. Typical double representations use 64 bits instead of 32. Some systems use all 32 additional bits for the nonexponent part. This increases the number of significant figures and reduces round-off errors. Other systems use some of the bits to accommodate a larger exponent; this increases the range of numbers that can be accommodated. Either approach leads to at least 13 significant figures, more than meeting the minimum standard.
C allows for a third floating-point type: long double . The intent is to provide for even more precision than double . However, C guarantees only that long double is at least as precise as double.
Declaring Floating-Point Variables
Floating-point variables are declared and initialized in the same manner as their integer cousins. Here are some examples:
float noah, jonah;
double trouble;
float planck = 6.63e-34;
long double gnp;
Floating-Point Constants (Literals)
There are many choices open to you when you write a literal floating-point constant. The basic form of a floating-point literal is a signed series of digits, including a decimal point, followed by an e or E , followed by a signed exponent indicating the power of 10 used. Here are two valid floating-point constants:
-1.56E+12
2.87e-3
You can leave out positive signs. You can do without a decimal point (2E5) or an exponential part (19.28), but not both simultaneously. You can omit a fractional part (3.E16) or an integer part (.45E–6), but not both (that wouldn’t leave much!). Here are some more valid floatingpoint constants:
3.14159.2
4e16
.8E-5
100.
Don’t use spaces in a floating-point constant.
Wrong: 1.56 E+12
By default, the compiler assumes floating-point constants are double precision. Suppose, for example, that some is a float variable and that you have the following statement:
some = 4.0 * 2.0;Then 4.0 and 2.0 are stored as double , using (typically) 64 bits for each. The product is calculated using double precision arithmetic, and only then is the answer trimmed to regular float size. This ensures greater precision for your calculations, but it can slow down a program.
C enables you to override this default by using an f or F suffix to make the compiler treat a floating-point constant as type float ; examples are 2.3f and 9.11E9F . An l or L suffix makes a number type long double ; examples are 54.3l and 4.32e4L . Note that L is less likely to be mistaken for 1 (one) than is l . If the floating-point number has no suffix, it is type double .
Since C99, C has a new format for expressing floating-point constants. It uses a hexadecimal prefix ( 0x or 0X ) with hexadecimal digits, a p or P instead of e or E , and an exponent that is a power of 2 instead of a power of 10. Here’s what such a number might look like:
0xa.1fp10The a is 10 in hex, the .1f is 1/16th plus 15/256 th ( f is 15 in hex), and the p10 is 2 10 , or 1024, making the complete value (10 + 1/16 + 15/256) x 1024, or 10364.0 in base 10 notation.
Not all C compilers have added support for this feature.
Printing Floating-Point Values
The printf() function uses the %f format specifier to print type float and double numbers using decimal notation, and it uses %e to print them in exponential notation. If your system supports the hexadecimal format for floating-point numbers, you can use a or A instead of e or E . The long double type requires the %Lf , %Le , and %La specifiers to print that type. Note that both float and double use the %f , %e , or %a specifier for output. That’s because C automatically expands type float values to type double when they are passed as arguments to any function, such as printf() , that doesn’t explicitly prototype the argument type.
#include <stdio.h>
int main(void) {
float aboat = 32000.0;
double abet = 2.14e9;
long double dip = 5.32e-5;
printf("%f can be written %e\n", aboat, aboat);
//next line requires C99 or later compliance
printf("And it's %a in hexadecimal, powers of 2 notation\n", aboat);
printf("%f can be written %e\n", abet, abet);
printf("%Lf can be written %Le\n", dip, dip);
return 0;
}This is the output, provided your compiler is C99/C11 compliant:
32000.000000 can be written 3.200000e+04 And it's 0x1.f4p+14 in hexadecimal, powers of 2 notation 2140000000.000000 can be written 2.140000e+09 0.000053 can be written 5.320000e-05
Floating-Point Overflow and Underflow
Suppose the biggest possible float value on your system is about 3.4E38 and you do this:
float toobig = 3.4E38 * 100.0f;
printf("%e\n", toobig);What happens? This is an example of overflow —when a calculation leads to a number too large to be expressed. The behavior for this case used to be undefined, but now C specifies that toobig gets assigned a special value that stands for infinity and that printf() displays either inf or infinity (or some variation on that theme) for the value.
What about dividing very small numbers? Here the situation is more involved. Recall that a float number is stored as an exponent and as a value part, or mantissa . There will be a number that has the smallest possible exponent and also the smallest value that still uses all the bits available to represent the mantissa. This will be the smallest number that still is represented to the full precision available to a float value. Now divide it by 2. Normally, this reduces the exponent, but the exponent already is as small as it can get. So, instead, the computer moves the bits in the mantissa over, vacating the first position and losing the last binary digit. An analogy would be taking a base 10 value with four significant digits, such as 0.1234E-10, dividing by 10, and getting 0.0123E-10. You get an answer, but you’ve lost a digit in the process. This situation is called underflow , and C refers to floating-point values that have lost the full precision of the type as subnormal . So dividing the smallest positive normal floating-point value by 2 results in a subnormal value. If you divide by a large enough value, you lose all the digits and are left with 0. The C library now provides functions that let you check whether your computations are producing subnormal values.
There’s another special floating-point value that can show up: NaN , or not-a-number. For example, you give the asin() function a value, and it returns the angle that has that value as its sine. But the value of a sine can’t be greater than 1, so the function is undefined for values in excess of 1. In such cases, the function returns the NaN value, which printf() displays as nan , NaN , or something similar.
Floating-Point Round-off Errors
Take a number, add 1 to it, and subtract the original number. What do you get? You get 1. A floating-point calculation, such as the following, may give another answer:
#include <stdio.h> int main(void) { float a, b; b = 2.0e20 + 1.0; a = b - 2.0e20; printf("%f\n", a); return 0; }The output is this:
- 0.000000 (older gcc on Linux)
- -13584010575872.000000 (Turbo C 1.5)
- 4008175468544.000000 (XCode 4.5, Visual Studio 2012, current gcc)
The reason for these odd results is that the computer doesn’t keep track of enough decimal places to do the operation correctly. The number 2.0e20 is 2 followed by 20 zeros and, by adding 1, you are trying to change the 21st digit. To do this correctly, the program would need to be able to store a 21-digit number. A float number is typically just six or seven digits scaled to bigger or smaller numbers with an exponent. The attempt is doomed. On the other hand, if you used 2.0e4 instead of 2.0e20, you would get the correct answer because you are trying to change the fifth digit, and float numbers are precise enough for that.
Floating-Point Representation
The preceding sidebar listed different possible outputs for the same program, depending on the computer system used. The reason is that there are many possible ways to implement floating-point representation within the broad outlines discussed earlier. To provide greater uniformity, the Institute of Electrical and Electronics Engineers (IEEE) developed a standard for floating-point representation and computation, a standard now used by many hardware floatingpoint units. In 2011 this standard was adopted as the international ISO/IEC/IEEE 60559:2011 standard. This standard is incorporated as an option in the C99 and C11 standards, with the intention that it be supported on platforms with conforming hardware. The final example of output for the floaterr.c program comes from systems supporting this floating-point standard. C support includes tools for catching the problem.
Complex and Imaginary Types
Many computations in science and engineering use complex and imaginary numbers. C99 supports these numbers, with some reservations. A free-standing implementation, such as that used for embedded processors, doesn’t need to have these types. (A VCR chip probably doesn’t need complex numbers to do its job.) Also, more generally, the imaginary types are optional. With C11, the entire complex number package is optional.
In brief, there are three complex types, called float _Complex, double _Complex , and long double _Complex . A float _Complex variable, for example, would contain two float values, one representing the real part of a complex number and one representing the imaginary part. Similarly, there are three imaginary types, called float _Imaginary , double _Imaginary , and long double _Imaginary .
Including the complex.h header file lets you substitute the word complex for _Complex and the word imaginary for _Imaginary , and it allows you to use the symbol I to represent the square root of –1.
You may wonder why the C standard doesn’t simply use complex as the keyword instead of using _Complex and then adding a header file to define complex as _Complex . The standards committee is hesitant to introduce a new keyword because that can invalidate existing code that uses the same word as an identifier. For example, prior to C99, many programmers had already used, say, struct complex to define a structure to represent complex numbers or, perhaps, psychological conditions. Making complex a keyword would make these previous uses syntax errors. But it’s much less likely that someone would have used struct _Complex , especially since using identifiers having an initial underscore is supposed to be reserved. So the committee settled on _Complex as the keyword and made complex available as an option for those who don’t have to worry about conflicts with past usage.
Beyond the Basic Types
Summary: The Basic Data Types
Keywords:
The basic data types are set up using 11 keywords: int , long , short , unsigned , char , float , double , signed , _Bool , _Complex , and _Imaginary .
Signed Integers:
These can have positive or negative values:
int — The basic integer type for a given system. C guarantees at least 16 bits for int .
short or short int — The largest short integer is no larger than the largest int and may be smaller. C guarantees at least 16 bits for short .
long or long int — Can hold an integer at least as large as the largest int and possibly larger. C guarantees at least 32 bits for long .
long long or long long int — This type can hold an integer at least as large as the largest long and possibly larger. The long long type is least 64 bits.
Typically, long will be bigger than short , and int will be the same as one of the two. For example, old DOS-based systems for the PC provided 16-bit short and int and 32-bit long , and Windows 95–based systems and later provide 16-bit short and 32-bit int and long .
You can, if you want, use the keyword signed with any of the signed types, making the fact that they are signed explicit.
Unsigned Integers:
These have zero or positive values only. This extends the range of the largest possible positive number. Use the keyword unsigned before the desired type: unsigned int , unsigned long , unsigned short . A lone unsigned is the same as unsigned int .
Characters:
These are typographic symbols such as A , & , and + . By definition, the char type uses 1 byte of memory to represent a character. Historically, this character byte has most often been 8 bits, but it can be 16 bits or larger, if needed to represent the base character set.
- char — The keyword for this type. Some implementations use a signed char , but others use an unsigned char . C enables you to use the keywords signed and unsigned to specify which form you want.
Boolean:
Boolean values represent true and false ; C uses 1 for true and 0 for false .
- _Bool — The keyword for this type. It is an unsigned int and need only be large enough to accommodate the range 0 through 1.
Real Floating Point:
These can have positive or negative values:
- float — The basic floating-point type for the system; it can represent at least six significant figures accurately.
- double — A (possibly) larger unit for holding floating-point numbers. It may allow more significant figures (at least 10, typically more) and perhaps larger exponents than float .
- long double — A (possibly) even larger unit for holding floating-point numbers. It may allow more significant figures and perhaps larger exponents than double.
Complex and Imaginary Floating Point:
The imaginary types are optional. The real and imaginary components are based on the corresponding real types:
- float _Complex
- double _Complex
- long double _Complex
- float _Imaginary
- double _Imaginary
- long double _Imaginary
Summary: How to Declare a Simple Variable
1. Choose the type you need.
2. Choose a name for the variable using the allowed characters.
3. Use the following format for a declaration statement:
type-specifier variable-name ;
The type-specifier is formed from one or more of the type keywords; here are examples of declarations:int erest; unsigned short cash;4. You can declare more than one variable of the same type by separating the variable names with commas. Here’s an example:
char ch, init, ans;5. You can initialize a variable in a declaration statement:
float mass = 6.0E24;
Type Sizes
What type sizes does your system use?
#include <stdio.h>
int main(void) {
//C99 provides a %zd specifier for sizes
printf("Type int has a size of %zd bytes.\n", sizeof(int));
printf("Type char has a size of %zd bytes.\n", sizeof(char));
printf("Type long has a size of %zd bytes.\n", sizeof(long));
printf("Type long long has a size of %zd bytes.\n", sizeof(long long));
printf("Type double has a size of %zd bytes.\n", sizeof(double));
printf("Type long double has a size of %zd bytes.\n", sizeof(long double));
return 0;
}C has a built-in operator called sizeof that gives sizes in bytes. C99 and C11 provide a %zd specifier for this type used by sizeof . Noncompliant compilers may require %u or %lu instead. Here is a sample output:
Type int has a size of 4 bytes. Type char has a size of 1 bytes. Type long has a size of 8 bytes. Type long long has a size of 8 bytes. Type double has a size of 8 bytes. Type long double has a size of 16 bytes.
This program found the size of only six types, but you can easily modify it to find the size of any other type that interests you. Note that the size of char is necessarily 1 byte because C defines the size of 1 byte in terms of char . So, on a system with a 16-bit char and a 64-bit double , sizeof will report double as having a size of 4 bytes. You can check the limits.h and float.h header files for more detailed information on type limits.
Incidentally, notice in the last few lines how a printf() statement can be spread over two lines. You can do this as long as the break does not occur in the quoted section or in the middle of a word.
Using Data Types
When you develop a program, note the variables you need and which type they should be. Most likely, you can use int or possibly float for the numbers and char for the characters. Declare them at the beginning of the function that uses them. Choose a name for the variable that suggests its meaning. When you initialize a variable, match the constant type to the variable type. Here’s an example:
int apples = 12; //RIGHT
int oranges = 3.0; //POOR FORMC is more forgiving about type mismatches than, say, Pascal. C compilers allow the second initialization, but they might complain, particularly if you have activated a higher warning level. It is best not to develop sloppy habits.
When you initialize a variable of one numeric type to a value of a different type, C converts the value to match the variable. This means you may lose some data. For example, consider the following initializations:
int cost = 12.99; /* initializing an int to a double */
float pi = 3.1415926536; /* initializing a float to a double */ The first declaration assigns 12 to cost ; when converting floating-point values to integers, C simply throws away the decimal part ( truncation ) instead of rounding. The second declaration loses some precision, because a float is guaranteed to represent only the first six digits accurately. Compilers may issue a warning (but don’t have to) if you make such initializations.
Many programmers and organizations have systematic conventions for assigning variable names in which the name indicates the type of variable. For example, you could use an i_ prefix to indicate type int and us_ to indicate unsigned short , so i_smart would be instantly recognizable as a type int variable and us_verysmart would be an unsigned short variable.
Arguments and Pitfalls
It’s worth repeating and amplifying a caution made earlier in this chapter about using printf() . The items of information passed to a function, as you may recall, are termed arguments . For instance, the function call printf("Hello, pal.") has one argument: "Hello, pal." . A series of characters in quotes, such as "Hello, pal." , is called a string.
Similarly, the function call scanf("%d", &weight) has two arguments: "%d" and &weight . C uses commas to separate arguments to a function. The printf() and scanf() functions are unusual in that they aren’t limited to a particular number of arguments. For example, we’ve used calls to printf() with one, two, and even three arguments. For a program to work properly, it needs to know how many arguments there are. The printf() and scanf() functions use the first argument to indicate how many additional arguments are coming. The trick is that each format specification in the initial string indicates an additional argument. For instance, the following statement has two format specifiers, %d and %d :
printf("%d cats ate %d cans of tuna\n", cats, cans); This tells the program to expect two more arguments, and indeed, there are two more— cats and cans.
Your responsibility as a programmer is to make sure that the number of format specifications matches the number of additional arguments and that the specifier type matches the value type. C now has a function-prototyping mechanism that checks whether a function call has the correct number and correct kind of arguments, but it doesn’t work with printf() and scanf() because they take a variable number of arguments. What happens if you don’t live up to the programmer’s burden? Suppose, for example, you write a program like that:
#include <stdio.h>
int main(void) {
int n = 4;
int m = 5;
float f = 7.0f;
float g = 8.0f;
printf("%d\n", n, m); //too many arguments
printf("%d %d %d\n", n); //too few arguments
printf("%d %d\n", f, g); //wrong kind of values
return 0;
}
Here’s a sample output from XCode 4.6 (OS 10.8):
4 4 1 -706337836 1606414344 1Next, here’s a sample output from Microsoft Visual Studio Express 2012 (Windows 7):
4 4 0 0 0 1075576832
Note that using %d to display a float value doesn’t convert the float value to the nearest int . Also, the results you get for too few arguments or the wrong kind of argument differ from platform to platform and can from trial to trial.
None of the compilers we tried refused to compile this code; although most did issue warnings that something might be wrong. Nor were there any complaints when we ran the program. It is true that some compilers might catch this sort of error, but the C standard doesn’t require them to. Therefore, the computer may not catch this kind of error, and because the program may otherwise run correctly, you might not notice the errors either. If a program doesn’t print the expected number of values or if it prints unexpected values, check to see whether you’ve used the correct number of printf() arguments.
One More Example: Escape Sequences
The following program shows how the backspace ( \b ), tab ( \t ), and carriage return ( \r ) work. These concepts date from when computers used teletype machines for output, and they don’t always translate successfully to contemporary graphical interfaces.
#include <stdio.h>
int main(void) {
float salary;
printf("\aEnter your desired monthly salary:"); /* 1 */
printf(" $_______\b\b\b\b\b\b\b"); /* 2 */
scanf("%f", &salary);
printf("\n\t$%.2f a month is $%.2f a year.", salary, salary * 12.0); /* 3 */
printf("\rGee!\n"); /* 4 */
return 0;
}
What Happens When the Program Runs
Let’s walk through this program step by step as it would work under a system in which the escape characters behave as described. (The actual behavior could be different. For instance, XCode 4.6 displays the \a , \b , and \r characters as upside down question marks!)
The first printf() statement (the one numbered 1 ) sounds the alert signal (prompted by the
\a ) and then prints the following:
Enter your desired monthly salary: Because there is no \n at the end of the string, the cursor is left positioned after the colon.
The second printf() statement picks up where the first one stops, so after it is finished, the screen looks as follows:
Enter your desired monthly salary: $_______ The space between the colon and the dollar sign is there because the string in the second printf() statement starts with a space. The effect of the seven backspace characters is to move the cursor seven positions to the left. This backs the cursor over the seven underscore characters, placing the cursor directly after the dollar sign. Usually, backspacing does not erase the characters that are backed over, but some implementations may use destructive backspacing, negating the point of this little exercise.
At this point, you type your response, say 4000.00 . Now the line looks like this:
Enter your desired monthly salary: $4000.00 The characters you type replace the underscore characters, and when you press Enter (or Return) to enter your response, the cursor moves to the beginning of the next line.
The third printf() statement output begins with \n\t . The newline character moves the cursor to the beginning of the next line. The tab character moves the cursor to the next tab stop on that line, typically, but not necessarily, to column 9. Then the rest of the string is printed. After this statement, the screen looks like this:
Enter your desired monthly salary: $4000.00
$4000.00 a month is $48000.00 a year. Because the printf() statement doesn’t use the newline character, the cursor remains just after the final period.
The fourth printf() statement begins with \r . This positions the cursor at the beginning of the current line. Then Gee! is displayed there, and the \n moves the cursor to the next line. Here is the final appearance of the screen:
Enter your desired monthly salary: $4000.00
Gee! $4000.00 a month is $48000.00 a year. Flushing the Output
When does printf() actually send output to the screen? Initially, printf() statements send output to an intermediate storage area called a buffer . Every now and then, the material in the buffer is sent to the screen. The standard C rules for when output is sent from the buffer to the screen are clear: It is sent when the buffer gets full, when a newline character is encountered, or when there is impending input. (Sending the output from the buffer to the screen or file is called flushing the buffer .) For instance, the first two printf() statements don’t fill the buffer and don’t contain a newline, but they are immediately followed by a scanf() statement asking for input. That forces the printf() output to be sent to the screen.
You may encounter an older implementation for which scanf() doesn’t force a flush, which would result in the program looking for your input without having yet displayed the prompt onscreen. In that case, you can use a newline character to flush the buffer. The code can be changed to look like this:
printf("Enter your desired monthly salary:\n");
scanf("%f", &salary);This code works whether or not impending input flushes the buffer. However, it also puts the cursor on the next line, preventing you from entering data on the same line as the prompting string. Another solution is to use the fflush() function described subsequently.
Key Concepts
C has an amazing number of numeric types. This reflects the intent of C to avoid putting obstacles in the path of the programmer. Instead of mandating, say, that one kind of integer is enough, C tries to give the programmer the options of choosing a particular variety (signed or unsigned) and size that best meet the needs of a particular program.
Floating-point numbers are fundamentally different from integers on a computer. They are stored and processed differently. Two 32-bit memory units could hold identical bit patterns, but if one were interpreted as a float and the other as a long , they would represent totally different and unrelated values. For example, on a PC, if you take the bit pattern that represents the float number 256.0 and interpret it as a long value, you get 113246208. C does allow you to write an expression with mixed data types, but it will make automatic conversions so that the actual calculation uses just one data type.
In computer memory, characters are represented by a numeric code. The ASCII code is the most common in the U.S., but C supports the use of other codes. A character constant is the symbolic representation for the numeric code used on a computer system—it consists of a character enclosed in single quotes, such as 'A' .
Summary
C has a variety of data types. The basic types fall into two categories: integer types and floatingpoint types. The two distinguishing features for integer types are the amount of storage allotted to a type and whether it is signed or unsigned. The smallest integer type is char , which can be either signed or unsigned, depending on the implementation. You can use signed char and unsigned char to explicitly specify which you want, but that’s usually done when you are using the type to hold small integers rather than character codes. The other integer types include the short , int , long , and long long type. C guarantees that each of these types is at least as large as the preceding type. Each of them is a signed type, but you can use the unsigned keyword to create the corresponding unsigned types: unsigned short , unsigned int , unsigned long , and unsigned long long . Or you can add the signed modifier to explicitly state that the type is signed. Finally, there is the _Bool type, an unsigned type able to hold the values 0 and 1 , representing false and true.
The three floating-point types are float , double , and, since C90, long double . Each is at least as large as the preceding type. Optionally, an implementation can support complex and imaginary types by using the keywords _Complex and _Imaginary in conjunction with the floating-type keywords. For example, there would be a double _Complex type and a float _Imaginary type.
Integers can be expressed in decimal, octal, or hexadecimal form. A leading 0 indicates an octal number, and a leading 0x or 0X indicates a hexadecimal number. For example, 32 , 040 , and 0x20 are decimal, octal, and hexadecimal representations of the same value. An l or L suffix indicates a long value, and an ll or LL indicates a long long value.
Character constants are represented by placing the character in single quotes: 'Q' , '8' , and '$' , for example. C escape sequences, such as '\n' , represent certain nonprinting characters. You can use the form '\007' to represent a character by its ASCII code.
Floating-point numbers can be written with a fixed decimal point, as in 9393.912 , or in exponential notation, as in 7.38E10 . C99 and C11 provide a third exponential notation using hexadecimal digits and powers of 2, as in 0xa.1fp10 .
The printf() function enables you to print various types of values by using conversion specifiers, which, in their simplest form, consist of a percent sign and a letter indicating the type, as in %d or %f .