C Language Loop Structures: Differences Between for, while, and do-while

In C language, loop structures are an important part of controlling the flow of program execution. They allow us to repeatedly execute a block of code until a specific condition is met. This article will detail the three main loop structures: <span>for</span>, <span>while</span>, and <span>do-while</span>, and illustrate the differences between them with example code.

1. for Loop

Syntax

for (initialization; condition; update) {    // loop body}

Description

• Initialization: Executed once before the loop starts, used to define and initialize the control variable.
• Condition: Checked before each iteration; if true, the loop body is executed; if false, the loop exits.
• Update: Executed at the end of each iteration, used to update the control variable.

Example Code

#include <stdio.h>
int main() {    for (int i = 0; i < 5; i++) {        printf("This is iteration %d\n", i + 1);    }    return 0;}

Output:

This is iteration 1
This is iteration 2
This is iteration 3
This is iteration 4
This is iteration 5

2. while Loop

Syntax

while (condition) {    // loop body}

Description

• <span>while</span> loop first checks the condition; if true, it enters the loop body and executes; otherwise, it skips directly.
• When using a <span>while</span> loop, ensure that the control variable is modified somewhere to avoid infinite loops.

Example Code

#include <stdio.h>
int main() {    int i = 0;
    while (i < 5) {        printf("This is iteration %d\n", i + 1);        i++; // Update control variable to avoid infinite loop     }
    return 0;}

Output:

This is iteration 1
This is iteration 2
This is iteration 3
This is iteration 4
This is iteration 5

## 3. do-while Loop

### Syntax

do {     // loop body } while (condition); 

### Description

• <span>do-while</span> differs from <span>while</span> in that the latter checks the condition first before deciding whether to enter, while the former will execute at least once because it checks the condition at the end.

Example Code

#include <stdio.h>
int main() {         int i = -1; // Initialize a value less than 5
    do {                 printf("This is also the first output: iteration %d\n", i + 2);                 i++;         } while (i < 5); 
return 0;   }

Output:

This is also the first output: iteration 2
This is also the first output: iteration 3
This is also the first output: iteration 4
This is also the first output: iteration 5
This is also the first output: iteration 6

Summary

Feature	for	while	do-while
Initialization	Yes	No	No
Condition Check	Before each iteration	Before each iteration	After each iteration
At least run once	No	No	Yes