Check the given number belongs to Fibonacci sequence in node.js?

Question

I need to create a web server with rest service in node.js. The server have to find a given input is Fibonacci or not. If it is Fibonacci, find the next number in Fibonacci sequence.

So I just write a simple program for finding Fibonacci number, but it doesn't work.

Following are the Javascript functions I tried:

var n = 5,s, a, fib;

function isPerfectSquare(s)
{
    var a = Math.sqrt(s);
    return a * a == s;
}    
function isFibonacci(n)
{
    s = (5 * Math.pow(n, 2) + 4 || 5 * Math.pow(n, 2) - 4)
    return s;
}

//How to fix this line. I don't know how to check perfect square conditions 
if(isPerfectSquare(isFibonacci(n)) || isPerfectSquare(isFibonacci(n)))
{
    fib = Math.round(n * 1.618); // finds the next fibonacci series of given input
    console.log("The next Fibonacci number is " + fib);
}    
else
{
    console.log("The given number is not a fibonacci number");
}

What is a perfect square by your definition? Should the length of the side be a non-decimal number? — RononDex
– RononDex, Commented Dec 11, 2013 at 10:05
isPerfectSquare doesnt work at all, try with something like = function(x){ return x >=0 && !(Math.sqrt(x) % 1); }. Also, node.js is great, but doesnt work well doing numeric or heavy cpu stuffs. A tipical example is 0.1+0.3 but you could google it to see more realistic problems. — durum
– durum, Commented Dec 11, 2013 at 10:12
We can use binet's formula to find given number is fibonacci. Binet's formula is if and only if (5*n^2 +4) or (5 *n^2 -4) is perfect square or both of them. — pruzoth
– pruzoth, Commented Dec 11, 2013 at 10:12
@bduran Thanks for your suggestion. Yeah, Its interesting but I couldn't find the better stuff for beginners like me. Can you re-write this program? — pruzoth
– pruzoth, Commented Dec 11, 2013 at 12:37

Narendra Jadhav · Accepted Answer · 2019-10-15 16:00:48Z

2

Finally, I found the solution for my own Fibonacci sequence question. Thank you for your suggestions guys.

The following snippet is working for me.

function isFibonacci(n) {
  var fib,
    a = (5 * Math.pow(n, 2) + 4),
    b = (5 * Math.pow(n, 2) - 4)

  var result = Math.sqrt(a) % 1 == 0,
    res = Math.sqrt(b) % 1 == 0;

  //fixed this line
  if (result || res == true) // checks the given input is fibonacci series
  {
    fib = Math.round(n * 1.618); // finds the next fibonacci series of given input
    console.log("The next Fibonacci number is " + fib);

  } else {
    console.log(`The given number ${n} is not a fibonacci number`);
  }
}

$('#fib').on("keyup change", function() {
  isFibonacci(+this.value)
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input id="fib" type="number" min=0 placeholder="Enter numebr.." />

edited Oct 15, 2019 at 16:00

Narendra Jadhav

10.3k15 gold badges35 silver badges45 bronze badges

answered Jul 20, 2014 at 8:31

pruzoth

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