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

Question

I'm having a mental block here, and algebra not really being my thing, can you tell me how to re-write the JavaScript code below to derive the variable, c, in terms of a and b?:

a = Math.pow(b, c);
c = ?

Thanks!

Answer 1

Logarithms. You want the logarithm of a. B is the base, c is the exponent, so

log_b a = c

Answer 2

c = Math.log(a)/Math.log(b)

Answer 3

A logarithm is the inverse of an exponent, you can do, for example:

>>> x = math.pow(2,5)
>>> print x
32.0
>>> math.log(x, 2)
5.0