📘 Section 2 — Loops

Learn how to repeat actions with while, for, and do-while loops.

🎯 What You’ll Learn

Why loops are essential in programming
Three types of loops: while, for, do-while
How to trace loops step by step
How to use loops with conditions and counters

1. Why Loops?

A loop is a sequence of instructions that keeps repeating until a certain condition is reached.

Loop concept

Imagine you need to print numbers from 1 to 1000. Without loops:

printf("number1: %d\n", 1);
printf("number2: %d\n", 2);
printf("number3: %d\n", 3);
// ... 997 more lines!

Now assume we need to print from 1 to 1000! Loops let you do this in just a few lines.

2. While Loop

while (condition) {
    /* code */
}

The condition is checked before each iteration. The loop repeats as long as the condition is true.

3. For Loop

for (initialization; condition; update) {
    /* code */
}

Part	Purpose
Initialization	Initializing a new variable to use inside the loop
Condition	Setting the condition to exit the loop
Update	Updating the variable after each iteration until condition is met

4. Do-While Loop

do {
    /* code */
} while (condition);

The condition is checked after each iteration, so the body runs at least once.

🧪 Exercises

Exercise 1: Print 1–20 with While Loop

Task: Print the integers from 1 to 20 using a while loop and the counter variable x. Print only 5 integers per line.

💡 Hint

Use the modulus operator % to detect every 5th number
When x % 5 == 0, print a newline

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int x = 1;
    while (x <= 20) {
        printf("%d ", x);
        if (x % 5 == 0) {
            printf("\n");
        }
        x++;
    }
    return 0;
}

Output:

2 3 4 5 
7 8 9 10 
12 13 14 15 
17 18 19 20 

Exercise 2: Trace the Switch in a Loop

Task: What will the output of the following code be?

for (int k = 7; k <= 16; k++)
    switch (k % 10) {
        case 0: printf(", ");       break;
        case 1: printf("OFTEN ");   break;
        case 2:
        case 8: printf("IS ");      break;
        case 3: printf("NOT ");     break;
        case 4:
        case 9: printf("DONE ");    break;
        case 5: printf("WELL");     break;
        case 6: printf(".");        break;
        case 7: ("WHAT ");          break;
        default: printf("Bad number. ");
    }
printf("\n");

🟢 Click to Show Answer

Let’s trace each value of k:

k	k % 10	Output
7	7	nothing
8	8	`IS`
9	9	`DONE`
10	0	`,`
11	1	`OFTEN`
12	2	`IS`
13	3	`NOT`
14	4	`DONE`
15	5	`WELL`
16	6	`.`

Final output:

IS DONE , OFTEN IS NOT DONE WELL.

💡 Notice: case 7 has ("WHAT "); but no printf, so it does not print anything. Cases 2 & 8 share the same code, and cases 4 & 9 also share code.

Exercise 3: Divisible by 5, Not by 9

Task: Write a C program to print all the integer numbers that are divisible by 5 and not divisible by 9 in the range 1 to 100.

💡 Hint

Divisible by 5: i % 5 == 0
NOT divisible by 9: i % 9 != 0
Combine with &&

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    for (int i = 1; i <= 100; i++) {
        if (i % 5 == 0 && i % 9 != 0) {
            printf("%d ", i);
        }
    }
    return 0;
}

Output:

5 10 15 20 25 30 35 40 50 55 60 65 70 75 80 85 95 100

Notice that 45 and 90 are missing — they’re divisible by both 5 and 9.

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