[sequence structure, branch structure, loop structure, continue statement, break statement, return statement] (learning Note 6 -- C language process control)

C In language, different programs are executed according to the program , It's divided into sequential structures 、 Branching structure 、 And circular structure . Here is a simple case to show C Linguistic 3 It's a process structure .

One 、 Sequential structure

Sequential structure is the simplest process structure , It executes each statement one by one in a top-down manner .

Divide two integers （ The result is required to retain two valid decimal places ）

#include<stdio.h>
int main()
{
    
	int a,b;
	float quotient;
	printf("Please enter two integer:\n");
	scanf("%d%d",&a,&b);
	quotient = (float)a / b;
	printf("a / b = %.2f\n",quotient);
	return 0;
}

When the program calculates the result , The type conversion operator is used , It is the int Type variable a The type of is converted to float type , The purpose of this is to get a small number with a decimal point . Because of the variable a Converted to float type , So the variable b It will also be automatically converted to float type , Naturally you will get one float Operation result of type . And assign the result to the variable quotient. Finally through printf Function to print information on the control window by retaining two significant decimal places .

The execution flow of this program is ：
1. The standard library header file “stdio.h” Include .
2. Execute the main function .
3. Define two int Type variable a and b.
4. Definition float Type variable quotient.
5. Use printf Function to print a prompt message .
6. Use scanf Function to get user input , And save it to the variable a and b in .
7. Yes a and b Use the division operator to perform an operation , And assign the result to quotient.
8. Use printf Function to print the final result
9. Exit main function , The program is finished
It can be seen that , The whole program is executed sentence by sentence from top to bottom .

Two 、 Branching structure

C The branch structure of the language can control whether part of the process of the program is executed , Or select one of multiple execution paths to execute .

1.if sentence

Format ：

if( expression )
	 sentence 

The expression in parentheses is the judgment condition , When the value of the expression is true , Execute statement ; When the value of the expression is false , Do not execute statements .
Code ：

#include<stdio.h>
int main()
{
    
	int a,b;
	float quotient;
	printf("Please enter two integer:\n");
	scanf("%d%d",&a,&b);
	if(b)
	{
    
		quotient = (float)a / b;
		printf("a / b = %.2f\n",quotient);
	}
	return 0;
}

Can pass if Statement to divide by 0 Check the condition of , Only the divisor is not 0 when , To divide .
if(b) And if(b!=0) It is also a judgment variable b Whether the value of is not equal to 0, While using if(b) It will make the code more concise . Their The effect is actually the same .

2.if…else sentence

Format ：

if( expression )
	 sentence 1
else
	 sentence 2

if…else Statement and if Statement than , More else part , A process of choosing one from the other Implementation .
Code ：

#include<stdio.h>
int main()
{
    
	int a,b;
	float quotient;
	printf("Please enter two integer:\n");
	scanf("%d%d",&a,&b);
	if(b)
	{
    
		quotient = (float)a / b;
		printf("a / b = %.2f\n",quotient);
	}
	else
	{
    
		printf("Data is error!\n");
	}
	return 0;
}

When the divisor is not 0 when , The program prints out the normal results , Divisor is 0 when , Print a message that clearly informs the user .

3.if…else Statement nesting

Through to if…else Nesting of statements , You can let the program implement the process execution except one of many .

Ask the user to enter a 0 To 100 The integer of , To show students' test scores , The program can evaluate the corresponding grade according to this score , share A,B,C,D Four levels , among 90 Points and above are excellent （ use A Express ）,80 To 89 Divided into good （ use B Express ）,60 To 79 A pass （ use C Express ）,60 Below is a failing grade （ use D Express ）.

#include<stdio.h>
int main()
{
    
	int score;
	printf("Please enter a score between 0 and 100:\n");
	scanf("%d",&score);
	if(score >= 0 && score <= 100)
	{
    
		if(score>=90)
		{
    
			printf("A\n");
		}
		else
		{
    
			if(score>=80)
		    {
    
				printf("B\n");
		    }
			else
			{
    
				if(score>=60)
				{
    
					printf("C\n");
				}
				else
				{
    
					printf("D\n");
				}
			}
		}
	}
	else
	{
    
		printf("Data is error!\n");
	}
	return 0;
}

4.if…else if…else sentence

Its execution process and utilization if…else The execution process of nested statements is the same , It can be considered that if…else if…else The sentence is right if…else A variant of statement nesting .

#include<stdio.h>
int main()
{
    
	int score;
	printf("Please enter a score between 0 and 100:\n");
	scanf("%d",&score);
	if(score >= 0 && score <= 100)
	{
    
		if(score>=90)
		{
    
			printf("A\n");
		}
		else if(score>=80)
		{
    
			printf("B\n");
		}
		else if(score>=60)
		{
    
			printf("C\n");
		}
		else
		{
    
			printf("D\n");
		}
	}
	else
	{
    
		printf("Data is error!\n");
	}
	return 0;
}

5.switch…case sentence

It can also achieve the effect of selecting one from many , And make the logic of the code clearer .

#include<stdio.h>
int main()
{
    
	int score;
	printf("Please enter a score between 0 and 100:\n");
	scanf("%d",&score);
	if(score >= 0 && score <= 100)
	{
    
		switch(score / 10)
		{
    
			case 10:
			case 9:
				printf("A\n");
				break;
			case 8:
				printf("B\n");
				break;
			case 7:
			case 6:
				printf("C\n");
				break;
			default:
				printf("D\n");
				break;
		}
	}
	else
	{
    
		printf("Data is error!\n");
	}
	return 0;
}

break Statement can terminate switch…case Statement statement execution , If something is missing break, be switch…case Statement will not be terminated , The program will continue to execute the following statements , Until I met break Statement or represent the whole switch…case Closing brace at the end of the statement .
The integer expression is score/10, It's the original 0 To 100 The scores of , Conversion to 0 To 10 The scope of the .
default Labels can be placed anywhere , It can even appear in all case Before the label , therefore default Label under break Don't omit sentences at will .

3、 ... and 、 Loop structure

C The loop structure in language is to make the statement repeatable 、 A process structure that is executed many times .

1.while sentence

Format ：

while( expression )
	 sentence 

#include<stdio.h>
int main()
{
    
	int i,n,score,sum = 0;
	float aver; 
	printf("Please enter the number of students:\n");
	scanf("%d",&n);
	printf("Please enter the scores of %d students:\n",n);
	i = n;
	while(i--)
	{
    
		scanf("%d",&score);
		sum += score;
	}
	aver = (float)sum / n;
	printf("Total:%d , Average:%.2f\n",sum,aver);
	return 0;
}

stay while The statement before , adopt i=n, Put variables n The value of is assigned to the variable i, Because we need to keep the number of students , It is also needed to calculate the average score later . namely while In the statement, we only operate and modify variables i, Not for variables n Have an impact on .
stay sum When defining a variable, you must initialize it , Let the variable sum The initial value of 0, If you don't do that , At the beginning sum The value of the variable is unknown , Even if students' grades are added up , The final calculated total score will not be correct .

2.do…while sentence

do…while Statement and while The sentences are similar ,do…while Is to execute the loop body first , Then check the value of the expression .
Format ：

do
	 sentence 
while( expression );

#include<stdio.h>
int main()
{
    
	int i,n,score,sum = 0;
	float aver; 
	printf("Please enter the number of students:\n");
	scanf("%d",&n);
	printf("Please enter the scores of %d students:\n",n);
	i = n;
	do
	{
    
		scanf("%d",&score);
		sum += score;
	}while(--i);
	aver = (float)sum / n;
	printf("Total:%d , Average:%.2f\n",sum,aver);
	return 0;
}

3.for sentence

Format ：

for( expression 1; expression 2; expression 3);
	 sentence 

expression 1 Loop variables are usually initialized or assigned here （ Execute once during initialization ）
expression 2 The loop body is executed when the value of the expression is true , End when false for sentence （ Before the loop body is executed ）
expression 3 Loop variables are usually modified here 、 update operation （ After the loop body is executed ）

If the expression 2 Omitted , The condition is always true , So it becomes an infinite loop , Lead to for Statement cannot end , We usually call this infinite loop an infinite loop .

#include<stdio.h>
int main()
{
    
	int i,n,score,sum = 0;
	float aver; 
	printf("Please enter the number of students:\n");
	scanf("%d",&n);
	printf("Please enter the scores of %d students:\n",n);
	for(i = 0;i < n; ++i)
	{
    
		scanf("%d",&score);
		sum += score;
	}
	aver = (float)sum / n;
	printf("Total:%d , Average:%.2f\n",sum,aver);
	return 0;
}

4. Nested use of loops

#include<stdio.h>
int main()
{
    
	int n;
	printf("Please enter a data!\n");
	scanf("%d",&n);
	for(int i=1;i <= n; ++i)
	{
    
		if(i % 2)
			continue;
		printf("%d ",i);
	}
	return 0;
}

Outer layer for Statement , Except for an inner layer for Out of statement , There is a printf Function call statements , It will be inside for Statement execution finished （ The inner loop body is executed 9 Time ） after , Perform a line feed operation .

Four 、 Flow control statement

1.continue sentence

characteristic ：

1. Can only be used in statements with a loop structure
2. Usually and if Statement collocation
3. Once executed , The following statements will be skipped , Go straight to the next iteration
4. It only works on the loop that contains it

Format ：

continue;

for(int i = 1; i <= 10; ++i)
{
    
	if(i & 2)
		continue;
	printf("%d",i);
}

Print the results ：

2 4 6 8 10

When variables i When the value of is odd , The value of the expression is true , This can lead to continue Statement executed . once continue Statement executed , Then the following printf Function call statements are no longer executed .

2.break sentence

A sharp weapon against the dead circle .
What to pay attention to ：

1. Can only be used in switch…case Statement or loop structure
2. Usually with if Statements use
3. Once executed , Will force the process to be interrupted , End the execution of the statement

for(int i = 1; i <= 10; ++i)
{
    
	if(i > 5)
		break;
	printf("%d",i);
}

result ：

1 2 3 4 5

3.return sentence

return Statement can perform the function of forcibly stopping a function , That is, if there is in the function return Statement executed , Even if there are other statements , The execution process of the entire function will also end .

#include<stdio.h>
int main()
{
    
	printf("AA\n");
	return 0;
	printf("BB\n");
}

result ：

AA

return Statement must be used inside a function , Whether in sequential structure 、 Branching structure 、 Or a circular structure , Will cause the function execution process to be forced to end , If the main function is closed , It will lead to the end of the whole program .