convert decimal number to fraction in javascript or closest fraction

Unless you are willing to work on developing something yourself then I would suggest using a library that someone has already put effort into, like fraction.js

Javascript

var frac = new Fraction(0.3435);

console.log(frac.toString());

Output

687/2000

On jsFiddle

You can use brute force test on different denominators and retain the result that has least error.

The algorithm below is an example of how you might go about this, but, suffers from being inefficient and limited to searching for denominators up to 10000.

function find_rational( value, maxdenom ) {
  console.clear();
  console.log( "Looking up: " + value );
  let best = { numerator: 1, denominator: 1, error: Math.abs(value - 1) }
  if ( !maxdenom ) maxdenom = 10000;
  for ( let denominator = 1; best.error > 0 && denominator <= maxdenom; denominator++ ) {
    let numerator = Math.round( value * denominator );
    let error = Math.abs( value - numerator / denominator );
    if ( error >= best.error ) continue;
    best.numerator = numerator;
    best.denominator = denominator;
    best.error = error;
    console.log( "Intermediate result: "
                   + best.numerator + "/" + best.denominator
                   + " (" + ( best.numerator/best.denominator)
                   + " error " + best.error + " )" );
  }
  console.log( "Final result: " + JSON.stringify( best ) );
  return best;
}
  
function calc() {
    const value = parseFloat( $("#myInput").val() );
    if ( isNaN(value) ) {
        $( "#myResult" ).val( "NaN" );
        return;
    }
    const rational = find_rational( value, 10000 );
    $("#myResult").val( rational.numerator
                        + " / " + rational.denominator
                        + " ( Error: " + rational.error + " )" );
}

calc();

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<P>
Enter a decimal number:<BR/>
<INPUT type="text" name="myInput" id="myInput" value=".3435" onkeyup="calc()"/><BR/>
</P>

<P>
Resulting Rational:<BR/>
<INPUT name="myResult" id="myResult" value=""/><BR/>
</P>

The above determines the .3435 as a fraction is 687 / 2000.

Also, had you gave it PI (e.g. 3.1415926) it produces good looking fractions like 22/7 and 355/113.

Your first 2 steps are reasonable.

But what you should do is for the numerator and denominator calculate the Greatest Common Divisor (GCD) and then divide the numerator and denominator with that divisor to get the fraction you want.

GCD is rather easy to calculate. Here is Euclid's algorithm:

var gcd = function(a, b) {
  if (!b) return a;

  return gcd(b, a % b);
};

Edit

I've added a fully working JSFiddle.

convert decimal number to fraction in javascript or closest fraction

Tags:

Javascript

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