finding the power of 2 of a number

Question

What is the most optimized way to find the power of 2 of a number. For example : if number is 64 then I want 6 as result.

P.S. : I have read many posts about how to check if the number can be represented as a power of 2 or not but they don't give the actual power as result.

"the most optimized way" Not sure you really need the most optimized way. Usually I would advice to use the logarithm. — NoDataDumpNoContribution, Jun 06 '19 at 10:41

score 3 · Accepted Answer · answered Jun 06 '19 at 10:42

3

The two logarithm of a number is the power it'd have to have to be that number:

>>> import math
>>> math.log2(64)
6.0

answered Jun 06 '19 at 10:42

MatsLindh

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score 3 · Answer 2 · answered Jun 06 '19 at 10:45

3

It's kind of hack. Convert it to binary number. If input is going to be a multiple of 2, then this would not be a problem

>> bin(2).count('0') -1
1
>>> bin(64).count('0') -1
6
>>> bin(1024).count('0') -1
10
>>> bin(1024)
'0b10000000000'

answered Jun 06 '19 at 10:45

sam

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score 1 · Answer 3 · answered Jun 06 '19 at 10:42

1

You just need to compute the logarithm of the number in base 2. Following are the two ways to do so

Method 1

import numpy as np

number = 64

power = np.log2(64)
# 6.0

Method 2

import math

math.log2(number)
# 6.0

answered Jun 06 '19 at 10:42

Sheldore

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hiro protagonist · Answer 4 · 2019-06-06T10:49:34.500

this is an option (you are looking for the logarithm in base 2):

from math import log2


def integer_log2(n):
    if not bin(n).count("1") == 1:
        raise ValueError
    return int(log2(n))

in binary representation (bin(n)) a number an only be an integer power o 2 it it contains exactly one 1. the function presented here will raise a ValueError if this is not the case.

integer_log2(64))
# 6

as you'd need the binary representation anyway, you could then also just count the number of 0s to get the log:

def integer_log2(n):
    b = bin(n)
    if not b.count("1") == 1:
        raise ValueError
    return b.count("0") - 1

finding the power of 2 of a number

4 Answers4