How to get x of 2^x=8000?

Question

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What's the opposite of JavaScript's Math.pow?

2^x=i

Given i, how can we calculate x using Javascript?

i is the value which user will input from form. – Spacez Ly Wang Sep 28 '12 at 05:25 — Spacez Ly Wang, Sep 28 '12 at 05:25

score 7 · Answer 1 · edited Sep 28 '12 at 05:24

7

You want to take the logarithm of 8000. JS has the Math.log function which uses base e, you want base 2 so you can write Math.log(8000) / Math.log(2) to get the logarithm of 8000 base 2, which is equal to x.

edited Sep 28 '12 at 05:24

Shad

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answered Sep 28 '12 at 05:22

Dan

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Thanks Shad, I do not know the javascripts. – Dan Sep 28 '12 at 05:24

score 0 · Answer 2 · answered Sep 28 '12 at 05:24

You need the logarithm from the Math object. It does not provide a base 2 log so do the conversion:

var x = Math.log(8000) / Math.log(2);

Reference to the javascript Math object.

In the more general case we calculate 2^x = i this way:

var i; // Some number
var x = Math.log(i) / Math.log(2);

How to get x of 2^x=8000?

2 Answers2