Example: Fibonacci Series in a Given Range in C

// Fibonacci Series in a Given Range
#include <stdio.h>

int main() {
    // Step 1: Declare variables for the range and Fibonacci numbers
    int start_range, end_range;
    int num1 = 0; // First Fibonacci number
    int num2 = 1; // Second Fibonacci number
    int next_num; // To store the next Fibonacci number

    // Step 2: Prompt the user for the range
    printf("Enter the starting number of the range: ");
    scanf("%d", &start_range);
    printf("Enter the ending number of the range: ");
    scanf("%d", &end_range);

    // Step 3: Input validation for the range
    if (start_range > end_range || start_range < 0) {
        printf("Invalid range. Start range must be non-negative and less than or equal to end range.\n");
        return 1; // Indicate an error
    }

    printf("Fibonacci series within range [%d, %d]:\n", start_range, end_range);

    // Step 4: Handle initial Fibonacci numbers (0 and 1) if they fall within the range
    // Check for 0
    if (num1 >= start_range && num1 <= end_range) {
        printf("%d ", num1);
    }
    // Check for 1, ensuring it's not a duplicate if 0 was just printed and 1 is also valid
    // This condition also handles cases where start_range is 1 and 0 was not printed
    if (num2 >= start_range && num2 <= end_range && num2 != num1) {
        printf("%d ", num2);
    }

    // Step 5: Generate subsequent Fibonacci numbers and check against the range
    while (1) { // Infinite loop, will break when condition met
        next_num = num1 + num2;

        // If the next number exceeds the end range, break the loop
        if (next_num > end_range) {
            break;
        }

        // If the next number is within the specified range, print it
        if (next_num >= start_range && next_num <= end_range) {
            printf("%d ", next_num);
        }

        // Update num1 and num2 for the next iteration
        num1 = num2;
        num2 = next_num;
    }

    printf("\n"); // New line for cleaner output

    return 0;
}