CODEWITHSUNDEEP
findpython
user422100
user422100

Reputation: 2179

find largest power of two less than X number?

I m doing this

def power_two(n, base = -1):
    result = 2 ** base
    if result < n:
        base += 1
        power_two(n, base)
    else:
        if result == n:
            print base
        else:
            print base - 1

what is the pythonic way to find largest power of two less than X number?

EDIT example: power_two(100) return only the power

Upvotes: 15

Views: 15879

Answers (4)

Bjamse
Bjamse

Reputation: 327

utilising the power of .format!

def maxPowOf2(n):
     return len("{0:b}".format(n))-1

This answer is similar and maybe slower than @jfs though...
translates the number into a binary string and finds the length.
Won't work properly on negative numbers though...

Upvotes: 1

Mark Byers
Mark Byers

Reputation: 839254

Find the logarithm and truncate it:

def power_two(n):
    return int(math.log(n, 2))

Upvotes: 28

jfs
jfs

Reputation: 414915

You could use bit_length():

def power_two(n):
    return n.bit_length() - 1

By definition for n != 0: 2**(n.bit_length()-1) <= abs(n) < 2**n.bit_length()

Upvotes: 23

Tony Veijalainen
Tony Veijalainen

Reputation: 5565

Two ways, first works only in Python 2.7 and maybe 3+:

import random
for number in (random.randint(0,1<<32) for _ in range(16)):
    print "%20i,%4i, %4i" % (number, number.bit_length()-1, len(bin(number))-3)

Upvotes: 6

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