Knowledge point one: Expressions
1) What is an expression: In C language, one or more operands and zero or more operators form an expression;
example:
100
5 + 10
a / b
a * 10 / b + c
2) The result of the expression:
printf("%d\n", 5 + 10);The operation between an operator and its operand must produce a result, so every expression has a result . For example, the expression 5 + 10 above evaluates to 15.
3) Expression statement:
a) Add a semicolon after the expression to form an expression statement;
b) Expressions in C language cannot exist alone and must exist in the form of expression statements;
example:
100;
5 + 10;
a / b;
a * 10 / b + c;"But there is no semicolon after the following expression 5+10"
printf("%d\n", 5 + 10);#Because 5 + 10 is a subexpression, the function name plus the bracket operator forms a function call expression. Therefore, 5 + 10 is a subexpression of the function call expression, and a semicolon is added after the function call expression.
#The function call expression also has an expression result, which is the return value of the function.
Knowledge point 2: Addition operator +
1) + is the addition operator, which requires an operand on its left and right sides to form an addition operator expression;
2) Expression result : The result of the addition expression is the addition of the left and right expression results ;
int a, b;
a = 200;
b = 100;
a + b; // 表达式结果为300Knowledge point 3: Subtraction operator—
1) - is a subtraction operator, which requires an operand on its left and right sides to form a subtraction operator expression;
2) Expression result : The result of the subtraction expression is the subtraction of the left and right expression results ;
int a, b;
a = 200;
b = 100;
a - b; // 表达式结果为100Knowledge point four: symbolic operators +, -
1) + is used to indicate the sign of an expression, - is used to change the sign of an expression;
2) Unlike the addition and subtraction operators, it only requires one operand on the right side of the operator;
3) Expression result:
a) The result of the + expression is the value of the operand on the right;
b) The result of the - expression is the opposite of the operand value on the right;
+100; // 表达式结果为100
-100; // 表达式结果为-104) Used to change the sign of an expression:
int a = 100;
-a; // 表达式结果为-1005) Please note: + cannot change the sign of an expression, please use negative to get positive;
int a = -100;
+a; // 表达式结果为-100
-a; // 表达式结果为100Knowledge point 5: Multiplication operator *
1) * is a multiplication operator, and its left and right sides each require an operand to form a multiplication operator expression;
2) Expression result: The result of the multiplication expression is the multiplication of the left and right expression results;
3) * is the multiplication operator, be careful not to misuse it with the letter x;
int a, b;
a = 200;
b = 100;
a * b; // 表达式结果为20000Knowledge point 6: Division operator /
1) / is the division operator, which requires an operand on its left and right sides to form a division operator expression;
2) Expression result : The result of the division expression is the left operand divided by the right operand;
int a, b;
a = 200;
b = 100;
a / b; // 表达式结果为23) Problem that integers cannot be divided:
a) For division, please pay special attention to the problem of floating-point data when the integer cannot be divided;
int a, b, c;
a = 5;
b = 2;
c = a / b;# The value of a is 5 and the value of b is 2. So 5 divided by 2 should equal 2.5. However, it seems wrong to use int to receive 2.5, so let's change c to float or double.
int a, b;
a = 5;
b = 2;
float c;
c = a / b;
printf("%f\n", c);
#Obviously the result is still wrong
#In C language, the result of integer and integer operations is still an integer . The fractional part of the result is discarded, a process called truncation . After a / b, the result is still an integer and the value has been truncated. At this time, we assign it to a floating point type c. It can only be converted from integer 2 to floating point 2.0, but it has no effect.
& Then we change a, b, c to float, so that there will be no truncation. a, b will perform floating point operations, and the result will also be a floating point number type float;
float a, b, c;
a = 5;
b = 2;
c = a / b;
printf("%f\n", c)；
Knowledge point 7: Remainder operator %
1) % is the remainder operator, which requires an operand on its left and right sides to form a remainder operator expression;
2) Expression result : The result of the remainder expression is the remainder of the left operand divided by the right operand;
For example: 10%3, 10 divided by 3, equals 3 with a remainder of 1
int a, b;
a = 10;
b = 3;
a % b; // 表达式结果为1Knowledge point 8: Assignment operator =
1) = is the assignment operator, which requires an operand on the left and right sides to form an assignment operator expression;
2) The result of the assignment expression is the value of the operand on the right side of the equal sign;
3) The assignment expression will also transfer the value of the operand on the right side of the equal sign to the operand on the left;
a = 100; // 赋值表达式语句4) The difference between assignment and initialization:
int a = 100; // =表示初始化，不是赋值运算符
a = a + 150; // 赋值运算符，将右边的表达式结果赋值给左边的变量a。In the above code, the first line of code declares a variable a. Please note that the = sign used when declaring the variable and initializing it to 100 is not an assignment operator, but a variable initialization. Although initialization is very similar to assignment, the left side of the = sign is not a simple variable , but a declaration of the variable . You can use this to distinguish them.
5) Find the result of an assignment expression:
The expression a = a + 50 is a compound expression. First, the result of the subexpression a + 50 is obtained, which is 150. Then find the result of the expression a = 150, which is 150. In addition, in addition to calculating the result of the expression, the assignment expression also assigns 150 to the variable a
6) You cannot assign a value to a constant: It is wrong to assign a value to a constant because a constant cannot be changed;
"Hello" = "HelloWorld";
'a' = 'b';
100 = 200;"The above assignment is the wrong way"
Knowledge point 9: Auto-increment and auto-decrement operators
1) ++ refers to the increment operator, and -- refers to the decrement operator. They only require an operand to the left or right of the operator;
2) Placing the operator on the left is called prefix mode. For example: ++i --i;
3) Placing the operator on the right is called suffix mode. For example: i++ i--;
4) Prefix mode:
#include <stdio.h>
int main()
{
int a, b;
a = 10;
b = 10;
printf("%d %d\n", ++a, --b);
printf("%d %d\n", a, b);
return 0;
}
a) Prefix pattern expression result:
++ and the right operand form a prefix auto-increment expression, and the result of the expression is the value of the operand plus 1;
- - forms a prefix decrement expression with the right operand, and the result of the expression is the value of the operand minus 1;
b) Prefix mode operations on operands:
Autoincrement expression: add 1 to the operand;
Decrement expression: decrement the operand by 1;
5) Suffix mode:
#include <stdio.h>
int main()
{
int a, b;
a = 10;
b = 10;
printf("%d %d\n", a++, b--);
printf("%d %d\n", a, b);
return 0;
a) Postfix pattern expression result:
++ and the left operand form a suffix auto-increment expression, and the result of the expression is the value of the operand;
-- forms a suffix decrement expression with the left operand, and the result of the expression is the operand value;
b) Postfix mode operations on operands:
Autoincrement expression: add 1 to the operand;
Decrement expression: decrement the operand by 1;
Knowledge point ten: unary operators, binary operators
1) Unary operator: An operator with only one operand. For example: increment operator ++, decrement operator --;
2) Binary operator: An operator with two operands. For example: assignment operator =, addition operator +;
Knowledge Point 11: Operator Priority
* The precedence of the four arithmetic operators in C language is consistent with that in mathematics. Calculate what is inside the brackets first, multiply and divide first, then add and subtract;
#include <stdio.h>
int main()
{
int a, b;
a = 10 * 2 + 4 * 3;
b = 10 * (2 + 4) * 3;
printf("%d %d", a, b);
return 0;
}
| mark | Operator | type | priority |
| ++ -- | self-increasing, self-decreasing | prefix | 16 |
| ++ -- | self-increasing, self-decreasing | suffix | 15 |
| + - | Positive sign, negative sign | Monocular | 15 |
| * / % | Multiply, divide, remainder | twin eyes | 13 |
| + - | Add, subtract | twin eyes | 12 |
| = | Assignment | twin eyes | 2 |
The higher the priority, the higher the priority in the operation. For example: the priority of multiplication and division is 13, which is greater than the priority of addition and subtraction of 12.