Wednesday, 2 August 2017

Calculating the square root in Javascript using the Babylonian method

Calculating the square root in Javascript is easily done using Math.sqrt(n). But it can be entertaining to use an approximation method instead and build up an algorithm to calculate the square root. We will use the Babylonian method to approximate the square root using an iterative approximation method.

The Babylonian method is described in Wikipedia here:
Babylonian method.

The following image shows how each step of a revised value for the square root is calculated.

We calculate the square root of S by using an inital guess and then revise that step by adding that guess with S divided by the guess and dividing by 2. The revised value is then used again as the new guess value. We stop iterating when the margin of error is below a given treshold. In the calculus, x is the step guess value and e is the error.

We start the calculation sample by adding in Bootstrap CSS and jQuery.


<!DOCTYPE html>
<html>

<meta charset="utf-8" />
<link data-require="bootstrap-css@*" data-semver="4.0.0-alpha.4" rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.4/css/bootstrap.min.css" />

<body>
<br />
<div class="container">
<h2>Find square root in JS with long division algorithm</h2>

<hr />
<!-- html code here -->
<h3>Estimate Square Root</h3> Enter Number:
<input id="num" value="700" size="2" type="text" />
<br />
<input id="submitButton" value="Calculate" type="submit" />
<br />
<br />
<div id="details"></div>
</div>
<script type="text/javascript">
"use strict";
var x = 25;
var guess = 9;
var marginOfError = 0.1;
var error = 0;
var counter = 0;
var htmlout = "";
$(document).ready(function() { // function code goes here function getGuess(g) { console.log(g); var newguess = (g + x / g) / 2; return newguess; }$('#submitButton').click(function() {
// JavaScript code here
console.clear();
counter = 0;
x = parseFloat($('#num').val()); guess = Math.floor(Math.random() * x + 1); error = Math.abs(guess * guess - x); while (error >= marginOfError) { guess = getGuess(guess) error = Math.abs(guess * guess - x); //console.log(guess); counter += 1; } console.log('Count is ' + counter) htmlout = "Square Root is " + guess.toFixed(2) + ". It took " + counter + " guesses";$('#details').html(htmlout);

});
});
</script>
</body>

</html>


Let us clean up the code using classes in Ecmascript 6 (ES6) and use Traceur to support the ES6 syntax.

<!DOCTYPE html>
<html>

<meta charset="utf-8" />
<link data-require="bootstrap-css@*" data-semver="4.0.0-alpha.4" rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.4/css/bootstrap.min.css" />

<body>
<br />
<div class="container">
<h2>Find square root in JS with long division algorithm</h2>

<hr />
<!-- html code here -->
<h3>Estimate Square Root</h3> Enter Number:
<input id="num" value="700" size="2" type="text" />
<br />
<input id="submitButton" value="Calculate" type="submit" />
<br />
<br />
<div id="details"></div>
</div>
<script type="text/javascript">
"use strict";

class BabylonianSquareRootSolver {

constructor(x_0, S, marginOfError = 0.1){
this.S = S;
this.x_0 = x_0;
this.marginOfError = marginOfError;
}

getRevisedSquareRoot (x_n, S){
var revisedValue = (x_n + (S/x_n)) / 2;
return revisedValue;
}

calculateSqrt(){

var counter = 0;
var error = 0;
var guess = this.x_0;

error = Math.abs(this.x_0 * this.x_0 - S);

while (error >= marginOfError) {
guess = this.getRevisedSquareRoot(guess, this.S)
error = Math.abs(guess * guess - this.S);
console.log(guess);
counter += 1;
if (counter > 10)
break;
}

var result = {
S : this.S,
error : error,
iterations : counter,
root : guess
};

return result;

}

}

var S = 1;
var guess = Math.floor(Math.random()*S);
var marginOfError = 0.1;
var error = 0;
var counter = 0;
var htmlout = "";
$(document).ready(function() { // function code goes here$('#submitButton').click(function() {
// JavaScript code here
console.clear();
counter = 0;
S = parseFloat($('#num').val()); guess = Math.floor(Math.random()*S); var bab = new BabylonianSquareRootSolver(guess, S); console.log(bab); var res = bab.calculateSqrt(); htmlout = "Square Root is approximated to " + res.root.toFixed(2) + ". It took " + res.iterations + " iterations. The error for this approximation is: " + res.error.toFixed(4);$('#details').html(htmlout);
console.log(bab.calculateSqrt());

});
});
</script>
</body>

</html>



You can test out the code yourself with the following Plunk:
Babylonian method - Plunk Here is an image of the GUI:

1 comment:

1. To allow accuracy for calculating large numbers square roots, adjust the iteration break from 10 to 100 for exmple. Look for the line:

if (counter > 10)
break;