C Loops - Complete Guide

#include <stdio.h>

int main() {
    int number;
    
    printf("Enter a positive number: ");
    scanf("%d", &number);
    
    // Validate input using while loop
    while (number <= 0) {
        printf("Invalid input! Number must be positive.\n");
        printf("Enter a positive number: ");
        scanf("%d", &number);
    }
    
    printf("You entered: %d\n", number);
    
    // Calculate sum of digits
    int sum = 0;
    int temp = number;
    
    while (temp != 0) {
        sum += temp % 10;  // Add last digit
        temp /= 10;        // Remove last digit
    }
    
    printf("Sum of digits: %d\n", sum);
    
    return 0;
}

#include <stdio.h>

int main() {
    // CORRECT: Loop with proper termination
    int i = 0;
    while (i < 10) {
        printf("i = %d\n", i);
        i++;  // CRITICAL: Update loop variable
    }
    
    // DANGEROUS: Infinite loop (missing update)
    /*
    int j = 0;
    while (j < 10) {
        printf("j = %d\n", j);
        // Missing j++ causes infinite loop!
    }
    */
    
    // Controlled infinite loop with break
    int counter = 0;
    while (1) {  // Infinite loop condition
        printf("Counter: %d\n", counter);
        counter++;
        
        if (counter >= 5) {
            printf("Breaking the infinite loop!\n");
            break;  // Exit loop
        }
    }
    
    return 0;
}

#include <stdio.h>

int main() {
    int score;
    char grade;
    char repeat;
    
    do {
        // Get valid score (0-100)
        do {
            printf("Enter student score (0-100): ");
            scanf("%d", &score);
            
            if(score < 0 || score > 100) {
                printf("Invalid score! Please enter between 0 and 100.\n");
            }
        } while(score < 0 || score > 100);
        
        // Determine grade
        if(score >= 90) grade = 'A';
        else if(score >= 80) grade = 'B';
        else if(score >= 70) grade = 'C';
        else if(score >= 60) grade = 'D';
        else grade = 'F';
        
        printf("Score: %d, Grade: %c\n", score, grade);
        
        // Ask if user wants to continue
        printf("Do you want to enter another score? (y/n): ");
        scanf(" %c", &repeat);  // Space before %c to skip newline
        
    } while(repeat == 'y' || repeat == 'Y');
    
    printf("Program ended.\n");
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main() {
    int secretNumber, guess, attempts = 0;
    char playAgain;
    
    // Seed random number generator
    srand(time(0));
    
    do {
        // Generate random number between 1 and 100
        secretNumber = (rand() % 100) + 1;
        attempts = 0;
        
        printf("\n=== NUMBER GUESSING GAME ===\n");
        printf("I'm thinking of a number between 1 and 100.\n");
        
        do {
            printf("Enter your guess: ");
            scanf("%d", &guess);
            attempts++;
            
            if(guess < secretNumber) {
                printf("Too low! Try again.\n");
            } else if(guess > secretNumber) {
                printf("Too high! Try again.\n");
            } else {
                printf("Congratulations! You guessed it in %d attempts.\n", attempts);
            }
            
        } while(guess != secretNumber);
        
        printf("Do you want to play again? (y/n): ");
        scanf(" %c", &playAgain);
        
    } while(playAgain == 'y' || playAgain == 'Y');
    
    printf("Thanks for playing!\n");
    return 0;
}

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

#include <stdio.h>
int main() {
    for (int i = 2; i <= 10; i += 2)
        printf("%d ", i);
    printf("\n");
    return 0;
}

#include <stdio.h>
int main() {
    int a[] = {10, 20, 30}, sum = 0;
    for (int i = 0; i < 3; i++)
        sum += a[i];
    printf("Sum: %d\n", sum);
    return 0;
}

#include <stdio.h>
int main() {
    for (int i = 0, j = 4; i < 5; i++, j--)
        printf("i=%d j=%d\n", i, j);
    return 0;
}

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

#include <stdio.h>

int main() {
    int row, col;

    for (row = 1; row <= 3; row++) {
        for (col = 1; col <= 4; col++) {
            printf("* ");
        }
        printf("\n");
    }
    return 0;
}

#include <stdio.h>

int main() {
    int a[2][2] = {{1, 2}, {3, 4}};
    int i, j, sum = 0;

    for (i = 0; i < 2; i++) {
        for (j = 0; j < 2; j++) {
            sum += a[i][j];
        }
    }
    printf("Sum = %d\n", sum);  /* 10 */
    return 0;
}