What Is a Loop in C?

A loop is a control structure that repeats a block of code as long as a given condition is true.

Instead of writing the same statements many times:

printf("Hello\\n");
printf("Hello\\n");
printf("Hello\\n");
printf("Hello\\n");
printf("Hello\\n");

We can write a loop that prints “Hello” 5 times:

for (int i = 1; i <= 5; i++) {
    printf("Hello\\n");
}

Types of Loops in C (Beginner Level)

C gives us three main loop types:

• for loop – most commonly used when you know the number of repetitions.
• while loop – repeats while a condition is true; often used when count is not known in advance.
• do-while loop – like while, but guarantees the body runs at least once.

The for Loop in C

The for loop is ideal when we want to repeat something a fixed number of times. Its syntax packs initialization, condition and update in one line.

Syntax

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

• initialization – executed once before the loop starts (e.g., int i = 0;).
• condition – checked before each iteration; if false, the loop stops.
• update – executed after each iteration (e.g., i++;).

Example – Print 1 to 10

#include <stdio.h>

int main() {
    int i;
    for (i = 1; i <= 10; i++) {
        printf("%d\\n", i);
    }
    return 0;
}

Flow:

1. Set i = 1.
2. Check i <= 10. If true, execute body (print value).
3. Execute i++ → increase i by 1.
4. Repeat steps 2–3 until condition becomes false.

Example – Sum of First N Natural Numbers

int n, i;
int sum = 0;

printf("Enter n: ");
scanf("%d", &n);

for (i = 1; i <= n; i++) {
    sum = sum + i;   // or sum += i;
}

printf("Sum = %d\\n", sum);

The while Loop in C

The while loop is useful when we do not know in advance how many times we want to repeat. We just know a condition that must stay true.

Syntax

while (condition) {
    // loop body
}

Example – Count down from 5 to 1:

int count = 5;

while (count > 0) {
    printf("%d\\n", count);
    count--;          // update step
}

In this loop:

• Condition is checked before each iteration.
• If condition is already false at the start, the body will not execute even once.

Technician Example – Reading Values Until Stop Code

Imagine reading device status codes until the user enters 0 (stop).

int code;

printf("Enter status code (0 to stop): ");
scanf("%d", &code);

while (code != 0) {
    printf("Processing code %d...\\n", code);

    // read next code
    printf("Enter status code (0 to stop): ");
    scanf("%d", &code);
}

printf("Stopped.\\n");

The do-while Loop in C

A do-while loop is similar to while, but the condition is checked after running the body. This means the body always runs at least once, even if the condition is false initially.

Syntax

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

Example – Menu That Must Show at Least Once

int choice;

do {
    printf("\\n=== MENU ===\\n");
    printf("1. Start\\n");
    printf("2. Stop\\n");
    printf("3. Status\\n");
    printf("0. Exit\\n");
    printf("Enter choice: ");
    scanf("%d", &choice);

    switch (choice) {
        case 1: printf("Starting...\\n"); break;
        case 2: printf("Stopping...\\n"); break;
        case 3: printf("Status OK\\n"); break;
        case 0: printf("Exiting...\\n"); break;
        default: printf("Invalid choice\\n");
    }
} while (choice != 0);

This is a very common pattern: show menu → take input → process → repeat until the user chooses Exit.

for vs while vs do-while – When to Use Which?

break and continue in Loops

break – Exit the Loop Immediately

The break statement is used to exit from a loop (or switch) immediately, regardless of the loop condition.

int i;
for (i = 1; i <= 10; i++) {
    if (i == 5) {
        break;            // stop loop when i becomes 5
    }
    printf("%d\\n", i);
}

Output: 1 2 3 4

continue – Skip Current Iteration

The continue statement skips the remaining statements in the loop body for the current iteration and jumps to the next iteration.

int i;
for (i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;         // skip printing when i = 3
    }
    printf("%d\\n", i);
}

Output: 1 2 4 5

Nested Loops – Loops Inside Loops

A nested loop is a loop inside another loop. This is very important for pattern printing and handling 2D structures like tables and matrices.

Example – Print a 3x3 Grid of Numbers

int row, col;

for (row = 1; row <= 3; row++) {
    for (col = 1; col <= 3; col++) {
        printf("%d ", col);
    }
    printf("\\n");
}

Output:

1 2 3 
1 2 3 
1 2 3

Here:

• The outer loop controls the rows.
• The inner loop controls the columns.

Pattern Printing with Loops (Star Patterns)

Pattern programs are a classic way to practice loops. Let’s look at some common examples.

1. Simple Triangle Pattern

For n = 5, output:

*
* *
* * *
* * * *
* * * * *

#include <stdio.h>

int main() {
    int n, row, col;

    printf("Enter number of rows: ");
    scanf("%d", &n);

    for (row = 1; row <= n; row++) {
        for (col = 1; col <= row; col++) {
            printf("* ");
        }
        printf("\\n");
    }

    return 0;
}

2. Rectangle of Stars

For 3 rows and 5 columns:

* * * * *
* * * * *
* * * * *

int rows = 3, cols = 5;
int r, c;

for (r = 1; r <= rows; r++) {
    for (c = 1; c <= cols; c++) {
        printf("* ");
    }
    printf("\\n");
}

3. Number Triangle

For n = 4, output:

1
1 2
1 2 3
1 2 3 4

int n, row, col;
printf("Enter n: ");
scanf("%d", &n);

for (row = 1; row <= n; row++) {
    for (col = 1; col <= row; col++) {
        printf("%d ", col);
    }
    printf("\\n");
}

Practical Loop Examples for Technicians

1. Average of N Readings

int n, i;
float value, sum = 0.0f;

printf("How many readings? ");
scanf("%d", &n);

for (i = 1; i <= n; i++) {
    printf("Enter reading %d: ", i);
    scanf("%f", &value);
    sum += value;
}

printf("Average = %.2f\\n", sum / n);

2. Find Maximum Reading

int n, i;
float value, max;

printf("How many readings? ");
scanf("%d", &n);

printf("Enter reading 1: ");
scanf("%f", &max);

for (i = 2; i <= n; i++) {
    printf("Enter reading %d: ", i);
    scanf("%f", &value);

    if (value > max) {
        max = value;
    }
}

printf("Maximum value = %.2f\\n", max);

Common Mistakes with Loops

• Forgetting to update the loop variable – leads to infinite loops.
• Wrong condition – loop runs one time too many or one time less (off-by-one error).
• Modifying loop variable inside the body in a confusing way.
• Using ; after for or while by mistake:

while (i <= 10);   // WRONG: extra semicolon creates empty loop
{
    printf("%d\\n", i);  // this block is not part of while
}