C Exercises: Check if a given number is Fibonacci number or not

Last update on March 20 2025 12:36:06 (UTC/GMT +8 hours)

26. Fibonacci Number Check Variants

Write a C program to check if a given number is a Fibonacci number or not.

In mathematics, the Fibonacci numbers, commonly denoted F_n form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is, and for n > 1. By starting with 1 and 2, the first 10 terms will be: 1, 2, 3, 5, 8, 13, 21, 34, 55, 89.

Example:
Input: n = 8
Output: Is 8 a Fibonacci number? 1

Visual Presentation:

Sample Solution:

C Code:

#include <stdio.h>
#include <math.h>

// Function to check if a number is a perfect square
int isPerfectSquare(int x) {
    int s = (int)sqrt(x); // Calculate the square root of x and truncate to an integer
    return (s * s == x); // Return true if x is a perfect square (if s*s is equal to x)
}

// Function to check if a number is a Fibonacci number
int isFibonacci(int x) {
    // Check if the given number is a perfect square of (5 * x^2 + 4) or (5 * x^2 - 4)
    return isPerfectSquare(5 * x * x + 4) || isPerfectSquare(5 * x * x - 4);
}

// Main function
int main(void) {
    // Declare and initialize the number 'n' to check if it's a Fibonacci number
    int n = 8; // The number to be checked

    // Check if 'n' is greater than 0 before proceeding
    if (n > 0) {
        // Print whether 'n' is a Fibonacci number or not by calling the 'isFibonacci' function
        printf("Is %d a Fibonacci number? %d", n, isFibonacci(n));
    }
    return 0; // End of the main function
}

Sample Output:

Is 8 a Fibonacci number? 1

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C program to determine if a given number is a Fibonacci number using mathematical properties.
• Write a C program to check Fibonacci membership by generating Fibonacci numbers until the target is reached or exceeded.
• Write a C program to verify if a number is Fibonacci by testing if one of (5*n*n+4) or (5*n*n-4) is a perfect square.
• Write a C program to use recursion to generate Fibonacci numbers and then check if the given number is in the sequence.

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