Chapter 8: Loops | Think-Embedded

Sometimes a program needs to execute the same block of code multiple times. Writing the same statements repeatedly is highly inefficient and difficult to manage. In C, loops are used to repeat a block of code until a condition becomes false.

C provides three main types of loops: the for loop, the while loop, and the do-while loop.

1. for Loop

The for loop is commonly used when the exact number of iterations is known in advance.

Syntax

for(initialization; condition; update)
{
    // code to repeat
}

How It Works

First, the initialization statement executes exactly once. Next, the condition is evaluated. If true, the loop body executes. After the body runs, the update statement executes, and the condition is checked again. The loop stops immediately when the condition becomes false.

Example Program

main.c

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

Console Output

Execution Flowchart

2. while Loop

The while loop is used when the number of iterations is not fixed beforehand but depends on a dynamically changing condition.

Syntax

while(condition)
{
    // code executes as long as condition is true
}

How It Works

The condition is checked before each iteration. As long as it remains true, the loop body executes. If the condition is false initially, the loop body will not execute even once.

Example Program

while_demo.c

#include <stdio.h>

int main()
{
    int count = 1;

    while(count <= 3)
    {
        printf("Running\n");
        count++;
    }

    return 0;
}

Console Output

Running
Running
Running

Execution Flowchart

3. do-while Loop

The do-while loop is a post-test loop that executes the loop body first and checks the condition afterward.

Syntax

do
{
    // code to execute
}
while(condition);

How It Works

Unlike for and while, the do-while loop always executes at least once because the condition is evaluated after the loop body completes.

Example Program

do_while_demo.c

#include <stdio.h>

int main()
{
    int value = 1;

    do
    {
        printf("%d\n", value);
        value++;
    }
    while(value <= 3);

    return 0;
}

Console Output

1
2
3

Execution Flowchart

Key Differences

Understanding when to use each loop is essential for writing clean, optimized C code:

Loop Type	Condition Check	Best Used For
for loop	Before execution (Pre-test)	When the exact number of iterations is known beforehand.
while loop	Before execution (Pre-test)	When looping until a specific event or variable change occurs.
do-while loop	After execution (Post-test)	When the loop body must execute at least once (e.g., dynamic menus).

Embedded Focus

Loops are heavily used in embedded systems because firmware applications must run continuously while monitoring registers, pins, and sensors.

main_loop.c

while(1)
{
    read_sensor(); // Microcontroller runs this code forever
}

In embedded software design, infinite loops keep the central microcontroller alive, running background operations, and responding to physical events continuously.

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