Re: Re: Generators, generator expressions, and loops

[Peter via Python-list]

Lovely, succinct answers.


On 17/11/2018 2:44 AM, Ian Kelly wrote:

On Fri, Nov 16, 2018 at 7:57 AM Steve Keller <keller@no.invalid> wrote:

I have looked at generators, generator expressions, and iterators and
I try to get more familiar with these.

1. How would I loop over all (with no upper bound) integers or all
powers of two, for example?

In C it would be

    for (int i = 0; ; i++) { ... }  or  for (int i = 1; ; i *= 2) { ... }

In Python, I could define a generator

     def powers():
         i = 1
         while True:
             yield(i)
             i *= 2

     for i in powers():
         ...

More elegant are generator expressions but I cannot think of a way
without giving an upper limit:

     for i in (2 ** i for i in range(1000000)):
         ...

which looks ugly.  Also, the double for-loop (and also the two loops
in the above exmaple, for + while in the generator) look unnatural,
somehow, i.e. loop over all elements which are created by a loop.

Is there a more beautyful way?

Some options:

from itertools import count

def powers():
     for i in count():
         yield 2 ** i


for i in (2 ** i for i in count()):
     ...


for i in map(lambda x: 2 ** x, count()):
     ...


from functools import partial
from operator import pow

for i in map(partial(pow, 2), count()):
     ...


Take your pick.

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