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

## C Programming Mathematics: Exercise-26 with Solution

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: **

**C Programming Code Editor:**

**Improve this sample solution and post your code through Disqus.**

**Previous:** Write a C program to print all prime factors of a given number.

**Next:** Write a C program to multiply two numbers using bitwise operators.

**What is the difficulty level of this exercise?**

Test your Programming skills with w3resource's quiz.

**It will be nice if you may share this link in any developer community or anywhere else, from where other developers may find this content. Thanks.**

https://www.w3resource.com/c-programming-exercises/math/c-math-exercise-26.php

**Weekly Trends and Language Statistics**- Weekly Trends and Language Statistics