## Three Powering Algorithms

### March 3, 2015

In mathematics, the powering operation multiplies a number by itself a given number of times. For instance, the powering operation `pow(2,3)`

multiplies 2 × 2 × 2 = 8.

Your task is to write three functions that implement the powering operation, with time complexities O(*n*), O(log *n*) and O(1) in the exponent. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

Pages: 1 2

In Haskell:

I asked a question on Stack Overflow about this O(1) solution. And the author of this post just answered me. I thought it would be a good idea to also post the answer here for who may have the same question.

Question:

I am trying to understand how to implement the power operation with time complexity O(1) in the following post.

https://programmingpraxis.com/2015/03/03/three-powering-algorithms/2/

Can anyone explain why this algorithm works? And why it is O(1)?

Answer:

First, it works because that’s the mathematical definition of logarithms. To multiply two numbers, take their logarithms, add the logarithms together, then take the anti-logarithm of the sum; in programming terms: x × y = exp ( log(x) + log(y) ). The powering operation takes the logarithm of the base, multiplies the logarithm by the exponent, then takes the anti-logarithm of the product; in programming terms, x y = exp ( log(x) × y ).

Second, it is O(1) only if you cheat. There is only a single (constant) operation involving the exponent, so the operation is constant-time only with respect to the exponent. But arithmetic takes time, in fact arithmetic on n-bit numbers takes time log(n), but we ignore that. As a practical matter, computers only provide logarithms up to some small limit, which is usually 3.4 × 10^38 for single-precision floats and 1.8 × 10^308 for double-precision floats. Within that range, the functions used internally are close enough to constant-time that we say taking logarithms and exponents is O(1). If you use a big-decimal library such as GMP, with floats that are very much larger, it will be incorrect to say that the arithmetic is done in constant time. So I guess it depends on how precise you want to be.

P.S.

Thanks very much for the post. It is very helpful.