Loops

Loops repeat a code block until a condition is met, enabling repetitive tasks like processing data, generating sequences, or waiting for valid input. They are essential for automation and efficiency.

1. while Loop

Purpose: Repeats code while a condition is true, checked before each iteration.
Why Necessary: Ideal for indefinite loops where the number of iterations is unknown, like validating user input.
Syntax:

while (condition) {
    // code
}

Example: Print numbers from 1 to 5.

#include <stdio.h>

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

Output:

2. do-while Loop

Purpose: Repeats code at least once, checking the condition after each iteration.
Why Necessary: Ensures the loop body executes at least once, useful for menu-driven programs or input validation.
Syntax:

do {
    // code
} while (condition);

Example: Prompt user until valid input.

#include <stdio.h>

int main() {
    int num;
    do {
        printf("Enter a positive number: ");
        scanf("%d", &num);
    } while (num <= 0);
    printf("You entered: %d\n", num);
    return 0;
}

Output (example with input 7):

Enter a positive number: 7
You entered: 7

3. for Loop

Purpose: Repeats code for a fixed number of iterations, combining initialization, condition, and increment/decrement.
Why Necessary: Compact and efficient for counted loops, ideal for known iteration counts (e.g., array traversal or sequence generation).
Syntax:

for (initialization; condition; increment/decrement) {
    // code
}

Components:
Initialization: Sets the loop variable (e.g., int i = 0).
Condition: Checked before each iteration (e.g., i < n).
Increment/Decrement: Updates the loop variable (e.g., i++).
Example 1: Sum of first 10 numbers.

#include <stdio.h>

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

Output: Sum: 55 - Example 2: Print array elements.

#include <stdio.h>

int main() {
    int arr[] = {10, 20, 30, 40, 50};
    int n = 5;
    for (int i = 0; i < n; i++) {
        printf("Element %d: %d\n", i, arr[i]);
    }
    return 0;
}

Output:

Element 0: 10
Element 1: 20
Element 2: 30
Element 3: 40
Element 4: 50

Example 3: Countdown with decrement.

#include <stdio.h>

int main() {
    for (int i = 10; i >= 1; i--) {
        printf("%d\n", i);
    }
    printf("Blast off!\n");
    return 0;
}

Output:

10
9
8
7
6
5
4
3
2
1
Blast off!

4. Nested Loops

Purpose: Loops inside loops to handle multi-dimensional tasks.
Why Necessary: Enables processing of patterns, matrices, or combinations, such as generating tables or 2D arrays.
Example: Print a 3x3 grid.

#include <stdio.h>

int main() {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            printf("%d ", j);
        }
        printf("\n");
    }
    return 0;
}

Output:

1 2 3 
1 2 3 
1 2 3