Check whether a number is Fibonacci number or not?

Praseed Pai K.T. Praseed Pai K.T.

Praseed Pai K.T.

Specialist in Cross Domain Engineering and Enterprise Computing

发布日期: 2023年3月26日
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Sunday, May 25, 2014

(This was a Blog Entry Published in the Year 2014 in "Analysis leads to Paralysis")

I just happen to see in a site that, there is a generator to find whether some number is a Fibonacci number or not. Here is a C/C++ program which computes it 

//////////////////////////////
// IsFib.cpp
//
// A Naive C/C++ Program to check whether
// a number is Fibonacci number
//
// If 5*n*n + 4 or 5*n*n-4 is a perfect
// square, it is a Fib #
//
// g++ -o temp.exe ISFib.cpp
//

#include <stdio.h>
#include <math.h>
///////////////////////////////
//
// Checks whether a number is a Perfect
// Square
//

bool PERFECT_SQUARE( int x) 
{
     int _n = (int) sqrt(x); 
     return (_n*_n == x);  
}

////////////////////////
//
// Check If , 5*n*n + 4 or 5*n*n - 4 
// is a perferct square
//  
bool IsFib( int n ) {
   
   int c = 5*n*n;
   return PERFECT_SQUARE( c + 4 ) ||
          PERFECT_SQUARE( c - 4 );     

}      
/////////////////////////////
//
// Entrypoint
//
int main( int argc, char **argv )
{
   for( int j=0; j<200; ++j )
   {
         if ( IsFib(j) ) {
             printf("%d is a Fib #\n",j);
         }
   }

}

The console output is given below 


D:\PRASEED_PERSONAL\cpp>g++ -o temp.exe IsFib.cpp

D:\PRASEED_PERSONAL\cpp>temp
0 is a Fib #
1 is a Fib #
2 is a Fib #
3 is a Fib #
5 is a Fib #
8 is a Fib #
13 is a Fib #
21 is a Fib #
34 is a Fib #
55 is a Fib #
89 is a Fib #
144 is a Fib #

D:\PRASEED_PERSONAL\cpp>

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