I'm trying to add up a large number but it's not ing out correctly.

var searchSpace = 36;
var length = 11;
var binations = 0;

for(var i = 1; i <= length; i++) {
    binations += Math.pow(searchSpace, i);
}

The variable binations ends up being 135,382,323,952,046,190 which is not quite correct. It should be 135,382,323,952,046,196 (how is it off by 6?!) Any ideas?

I'm trying to add up a large number but it's not ing out correctly.

var searchSpace = 36;
var length = 11;
var binations = 0;

for(var i = 1; i <= length; i++) {
    binations += Math.pow(searchSpace, i);
}

The variable binations ends up being 135,382,323,952,046,190 which is not quite correct. It should be 135,382,323,952,046,196 (how is it off by 6?!) Any ideas?

• javascript
• math
• loops
• exponent

• 2 A good explanation of "Javascript doesn't have 'integers' - only floating point numbers": developer.mozilla/en/JavaScript/… – paulsm4 Commented Jun 15, 2012 at 4:21

2 Answers 2

JavaScript uses IEEE-754 64-bit doubles as its number format. These cannot represent arbitrarily precise values; they bee incorrect when you exceed a certain threshold and begin instead storing values that are close to (but not quite exactly) the actual values. According to this earlier answer, the largest value that can be stored accurately is 2⁵³, which is about 9 × 10¹⁵. Your number (which is about 1.3 × 10¹⁷) is larger than this, so (with good probability) it cannot be represented accurately.

If you want to get the exact answer in JavaScript, you will need to use a library that supports arbitrary-precision integers. A quick Google search turned up this library, but I can't vouch for how accurate it is.

Hope this helps!

All numbers in Javascript are actually floating point numbers. You're dealing with very large numbers, and doing multiple manipulations on those numbers. The inherent errors in dealing with floats pile up quickly in this case, causing the "error".

Being off by 6 on a number that large is actually pretty good. You're off by only 0.000000000000004431%, roughly