for or while loop to do something n times

问题

In Python you have two fine ways to repeat some action more than once. One of them is while loop and the other - for loop. So let's have a look on two simple pieces of code:

for i in range(n):
    do_sth()

And the other:

i = 0
while i < n:
    do_sth()
    i += 1

My question is which of them is better. Of course, the first one, which is very common in documentation examples and various pieces of code you could find around the Internet, is much more elegant and shorter, but on the other hand it creates a completely useless list of integers just to loop over them. Isn't it a waste of memory, especially as far as big numbers of iterations are concerned?

So what do you think, which way is better?

回答1:

but on the other hand it creates a completely useless list of integers just to loop over them. Isn't it a waste of memory, especially as far as big numbers of iterations are concerned?

That is what xrange(n) is for. It avoids creating a list of numbers, and instead just provides an iterator object.

In Python 3, xrange() was renamed to range() - if you want a list, you have to specifically request it via list(range(n)).

回答2:

This is lighter weight than xrange (and the while loop) since it doesn't even need to create the int objects. It also works equally well in Python2 and Python3

from itertools import repeat
for i in repeat(None, 10):
    do_sth()

回答3:

The fundamental difference in most programming languages is that unless the unexpected happens a for loop will always repeat n times then finish with a while loop it may repeat 0 times, 1, more or even forever, depending on a given condition which is always true at the start of each loop and always false on exiting the loop, (for completeness a do ... while loop, (or repeat until), for languages that have it, always executes at least once and does not guarantee the condition on the first execution).

So the answer to your question is 'it all depends on what you are trying to do'!

来源：https://stackoverflow.com/questions/17647907/for-or-while-loop-to-do-something-n-times

标签

python

performance

loops

for-loop

while-loop