Table of contents
#3. Life cycle and scope of variables
3.1. Scope of local variables: the local scope where the variable is located
3.2. Life cycle (describing a time period)
4.2.const modified constant variables:
4.3.#define defined identifier constants:
5. String + escape character + comment
one,
1. When a local variable is not initialized, its value is random.
Initialization: char ch=W
2. () is a code block
Local variables: Variables defined within () are local variables
Global variables: variables defined outside ()
When the names of local variables and global variables conflict, the local variables take precedence.
int a=100; global variable
int main
{
int a=10;local variable
printf("%d\n",a);
return 0;
}
1.//stdio.h standard input and output header file
#define _CRT_SECURE_NO_WARNINGS
#include
int main()
{
int num1 = 0;
int num2 = 0;
int sum = 0;
scanf("%d %d", &num1, &num2);
sum = num1 + num2;
printf("%d\n", sum);
return 0;
}
Or in the new compiler it can be written as:
#define _CRT_SECURE_NO_WARNINGS
#include
int main()
{
int num1 = 0;
int num2 = 0;
scanf("%d %d", &num1, &num2);
int sum = num1 + num2; The new version of the variable is defined wherever it is used.
printf("%d\n", sum);
return 0;
}
2. Local search tools: Everything
Search: newc++file.cpp
#3. Life cycle and scope of variables
3.1. Scope of local variables: the local scope where the variable is located
Scope of global variables: entire project
(Describe a scope)
int g = 100;
int main()
{
int a = 10;
{
int b = 20;
printf("%d\n", b);
printf("%d\n", a);
printf("%d\n", g);
}
return 0;
}
3.2. Life cycle (describing a time period)
Local variables: Enter the scope of the local variable, the life cycle begins, exit the scope, and the life cycle ends
Global variables: the life cycle of the entire program
int g = 100;
int main()
{
{
int a = 10;
printf("a=%d\n", a);
}
return 0;
}
4. Constants
4.1. Literal constants:
100 3.14 abc w
4.2.const modified constant variables:
Although the num below is a quantity that cannot be modified, it is still a variable in essence.
const int num=10;
printf("%d\n", num);
num=20; cannot be modified here
printf("%d\n", num);————Compilation error
Example: Explain that const is still essentially a variable
const int n=100;
int arr[n]={0};——————Error, n is still a variable, but it cannot be modified. n cannot be placed in arr. Only constants can be placed in arr——only arr[ can be written 100] (Only the c99 standard can be written like this, supports variable-length arrays, and allows variables to specify the array size)
4.3.#define defined identifier constants:
The essence is constant
#define MAX 100
int main()
{
int a=MAX;
int arr[MAX]; OK
printf("%d",a);
return 0;
}————————100
4.4. Enum constants
Enumeration: enumerate one by one
Gender: male, female, confidential
Define enumeration type:
enum Sex
{ MALE,
FEMALE,
SECRET three enumeration constants
};
int main()
{ printf("%d\n", MALE);
printf("%d\n", FEMALE);
printf("%d\n", SECRET);
return 0;
}————————Three enumeration constants 0, 1, 2
、
5. String + escape character + comment
5.1 String:
A string of characters enclosed in double quotes
"abc" "a" " " 空
strlen only seeks the length before \0, for example, abc is 3
#include
int main()
{
//You can put the string in the character array
char arr1[]="abc"; V (hide a \0 after abc)
char arr2[] = { 'a','b','c' }; (should be changed to char arr2[] = { 'a','b','c','\0' };
printf("%d\n", strlen(arr1)); strlen only requires the length before \0, for example, abc is 3
printf("%d\n", strlen(arr2));
return 0;
}——————————3 15 (random value 15)
5.2 Escape characters:
change the original meaning
| escape character |
Definition |
| \? |
When writing multiple question marks in a row, prevent them from being parsed into three letters. |
| \’ |
used to represent character constants' |
| \" |
Used to represent character constants" |
| \\ |
Used to represent character constants\ |
| \a |
Warning character, buzzer sounds |
| \b |
backspace character |
| \f |
paper feed character |
| \n |
newline |
| \r |
Enter |
| \t |
horizontal tab |
| \v |
vertical tab |
\ddd ddd represents 1~3 octal digits\130——>x
\xdd dd represents 2 hexadecimal digits\x30——>0
example:
int main()
{
printf("%d\n",strlen("c:\test\628\test.c"));
return 0;
}————————>14
5.3 Comments
There are two comment methods in C language: c++ comment style // default usage style√
The c comment style /* */ is defective and does not support nested comments.