[explain C language in detail] takes you to play with loop structure (for_while_do while)

executive summary

Life is not just about choices , Also often faced with doing something repeatedly , For example , We study day after day 、 Knock on the code 、

C The language provides —while loop —for loop —do-while Loop to express loop

Next , Introduce the three cycles respectively

while loop

while The flow chart of the cycle is roughly like this

Grammatical structure

while( expression )   // Determine if the expression holds , Execute the loop body when it is established , If it is not established, it will not enter 
 The loop body ;

Next let's use while loop , Print 1-10 The number of

#include <stdio.h>
int main(){
    
    int i = 1;
    while(i < 11)
        printf("%d ",i++);
    return 0;
}

This is while The simplest use , Let's take a look while Medium break and continue

while In circulation break and continue

break

notice break You must be familiar with , Select in structure switch-case Just match break Use

#include <stdio.h>
int main(){
    
    int i = 1;
    while(i < 11)
    {
    
        if( 5 == i)
            break;
        printf("%d ",i++);
    }
    return 0;
}

It can be seen that , When i=5 When , Directly jump out of the whole cycle and no longer execute

break It's about jumping out of the loop , Also jump out directly switch-case

continue

In English ,continue It means to continue , In circulation , It means to skip this cycle and proceed to the next cycle

#include <stdio.h>
int main() {
    
    int i = 1;
    while (i < 11)
    {
    
        if (5 == i)
            continue; //  hold break Switch to continue Have a try 
        printf("%d ", i++);
    }
    return 0;
}

You can guess , As one can imagine , You may think the result is 1 2 3 4 6 7 8 9 10 skip 5, Let's analyze

The cursor keeps flashing , Dead cycle , How can we skip 5 The effect of ,
The core problem is ** such i The value of cannot be changed , Then we put the i Put it in continue Let's try ahead **

#include <stdio.h>
int main() {
    
    int i = 0;
    while (i < 11)
    {
    
        i++;
        if (5 == i)
            continue;
        printf("%d ", i);
    }
    return 0;
}

such continue You won't skip the cycle factor -i The change of , So there's no dead cycle , Look at the results

Successfully skipped 5, Everyone in while Use in loop continue We must pay attention to the problem of circulation factor

continue stay while The role of the loop is :

• In this cycle continue The following code will no longer be executed, but directly jump to while The judgment part of a sentence .
• Make the entry judgment of the next cycle .

for loop

Grammatical structure

for( expression 1 ; expression 2 ; expression 3)
The loop body ;

• expression 1 by ** Initialization part , be used for Initializes the of the loop variable **.
• expression 2 by _ Condition judgment part _, be used for ** Judge when the cycle ends **.
• expression 3 by _ Adjustment part _, be used for **_ Adjustment of cyclic conditions _**.

for In circulation break and continue

break

for In circulation break And while In circulation break Exactly the same , To jump out of a loop

#include <stdio.h>
int main() {
    
	int i = 0;
	for (i = 1; i < 10; i++) {
    
		if (5 == i)
			break;
		printf("%d ", i);
	}
	return 0;
}

break Basically no difference , Just jump out of the loop

continue

for In circulation continue What jumps out is still this cycle , But it doesn't skip the expression 3

#include <stdio.h>
int main() {
    
	int i = 0;
	for (i = 1; i < 10; i++) {
    
		if (5 == i)
			continue;
		printf("%d ", i);
	}
	return 0;
}

Still skip 5,continue Expressions will not be skipped 3, signify i++ It can be executed , It doesn't create a dead cycle

Of course , A man wants to write bug No one can stop him
such as :

for The problem of omitting expressions in loops

for There are in the loop 3 Expression , All can be omitted , But a semicolon cannot be less

int main(){
    
    
    int i = 0;
    // It can be written like this 
    for (i=0;i<10;)
        ;
    // It can be like this 
    for(i = 0;;)
        ;
    // Even so 
    for(;;);  // There can't be less than one semicolon 
    
}

for(;; This code , If there is no... In the loop break, Basically, it's a proper dead cycle , It is not recommended that you use

for A nested loop

When there are two for In the cycle , The problem of cycle control needs to be considered , The ranks of , What does internal and external circulation mean ? You need to think clearly

Write a multiplication table

#include <stdio.h>

int main() {
    

	int i = 0;
	int j = 0;
	for (i = 1; i < 10; i++) {
      // The control line 
		for (j = 1; j <= i; j++) {
    
			printf("%d * %d = %d ", j, i, i * j); // Control the column 
		}
		printf("\n");
	}
	return 0;
}

In general , The control conditions of the inner circulation factor are related to the outer circulation factor

Here are a few for Circular considerations

• Don't be in for Modify the loop variable in the loop body , prevent for The cycle is out of control .
• Suggest for The loop control variable value of the statement adopts “ Front closing back opening section ” How to write it

#include <stdio.h>

int main(){
    
    int arr[10] = {
    1,2,3,4,5,6,7,8,9,10};// Array index from 0 To 9
    int i = 0;
    for(;i <=9;i++)  // Front closed and rear closed 
        printf("%d ",arr[i]);    
    for(;i < 10;i++)// Before closed after opening 
        printf("%d ",arr[i]);
    
    return 0;
}

The writing method of front closed and back open interval , It may make the number have some meaning , For example, in the example 10, Just the number of print elements , Is also the length of the array

Give you a little knowledge
alert for Questions directly followed by semicolons after the cycle , Schools love this stuff , Understand everything

do-while loop

do-while The cycle is very different from the other two cycles , It is also the cycle with the lowest frequency

do
     The loop body 
while( Judge the condition );

do-while The biggest feature of the cycle is anyway , The loop body must be executed once
Only in the corresponding scenario , Should use do-while loop

do-while In circulation ,break as well as continue The usage and while The same in the loop , This article is not going to cover .

goto sentence

about goto sentence , It's a jump , In fact, there is no need to say , Because what can be solved with it can be solved with a cycle

A brief introduction , But it is not recommended that you use

#include <stdio.h>

int main()
{
    
    printf("hehe\n");
    goto flag;
    printf("haha\n");
flag:
    printf("heihei\n");
    return 0;
}

goto Will jump directly to flag At the mark , Skip printing haha

This is just goto Sentence syntax

goto Statements are really used in multi-level loop nesting , Out of the loop , Use break To reach , You can only jump out once , We'll talk about it later

summary

• for Loop for initialization 、 Conditions 、 The control of circulation factors are all together
• do-while The body of the loop executes at least once
• goto It's not recommended

• Each cycle has its merits , According to the corresponding scenario , Select the corresponding cycle , Meet demand
• This article only introduces the circular grammar , For cycles , Everyone should practice hard , Think more

notes :

• All the insights in this article , My humble opinion , Misleading , I would appreciate your comments.
• If it helps you , Please support me