How do I find the value of exponent in Ruby?

Question

Consider the equation below:

2 ** n = A

Let us assume A=64.

What is the easiest way to find the value of n?

I am currently using following two approaches

A= 64; n = 1; n+=1 while (A >> n) > 0; n-1

A= 64; n = 0; n+=1 until (A == ( 2 ** n));n

Is there a better approach?

Other way of stating the same problem:

2 = nth root A If I know the value of A, how do I determine the value of n?

Answer 1

log₂n = ln n / ln 2

hence

log_2_64 = Math.log(64) / Math.log(2)

Answer 2

Try this:

def exp_value(n)
  Math.log(n) / Math.log(2)
end

Answer 3

Neither of the above answers is better than your first approach:

A= 64; n = 1; n+=1 while (A >> n) > 0; n-1

Evaluating Math.log(x) takes a lot longer than doing a dozen bit-shifts, and will also give you an answer like 5.99999999999980235 to make sense of.

See this SO question for some better ideas.

Answer 4

If you care about the problems mentioned by mobrule. How about this? It is the same as your own method but use built-in function to communicate the idea explictly.

    def exp_value a 
       (1..a).to_a.detect {|n| 2**n >=a}
    end

    exp_value 64
    => 6