Question

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

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

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

The variable combinations 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?

Answer 1

JavaScript uses IEEE-754 64-bit doubles as its number format. These cannot represent arbitrarily precise values; they become 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!