For loop iterate over powers of 2

Question

I want to write a for loop that will iterate over the powers of 2 for each loop.

For example I would want a range like this:

2, 4, 8, 16, ... , 1024
         


        

Answer 1

You can use a generator expression so that it generates the numbers as needed and they don't waste memory:

>>> for x in (2**p for p in range(1, 11)):
...    print(x)

2
4
8
16
32
64
128
256
512
1024

In Python 2, you can use xrange instead of range to keep it as a generator and avoid creating unnecessary lists.

If you want to enter the actual stop point instead of the power to stop at, this is probably the simplest way:

from itertools import count
for x in (2**p for p in count(1)):
    if x > 1024:
        break
    print(x)

You could put it all in one line:

from itertools import count, takewhile
for x in takewhile(lambda x: x <= 1024, (2**p for p in count(1))):
    print(x)

but that's becoming silly (and not very readable).