for or while loop to do something n times
Question:
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?
Answers:
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))
.
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()
The fundamental difference in most programming languages is that unless the unexpected happens a for
loop will always repeat n
times or until a break statement, (which may be conditional), is met then finish with a while
loop it may repeat 0 times, 1, more or even forever
, depending on a given condition which must be true at the start of each loop for it to execute 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).
It is worth noting that in Python a for
or while
statement can have break
, continue
and else
statements where:
break
– terminates the loop
continue
– moves on to the next time around the loop without executing following code this time around
else
– is executed if the loop completed without any break
statements being executed.
N.B. In the now unsupported Python 2 range
produced a list of integers but you could use xrange
to use an iterator. In Python 3 range
returns an iterator.
So the answer to your question is ‘it all depends on what you are trying to do‘!
python3 & python2
just use range()
:
for _ in range(n):
# do something n times exactly
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?
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))
.
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()
The fundamental difference in most programming languages is that unless the unexpected happens a for
loop will always repeat n
times or until a break statement, (which may be conditional), is met then finish with a while
loop it may repeat 0 times, 1, more or even forever
, depending on a given condition which must be true at the start of each loop for it to execute 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).
It is worth noting that in Python a for
or while
statement can have break
, continue
and else
statements where:
break
– terminates the loopcontinue
– moves on to the next time around the loop without executing following code this time aroundelse
– is executed if the loop completed without anybreak
statements being executed.
N.B. In the now unsupported Python 2 range
produced a list of integers but you could use xrange
to use an iterator. In Python 3 range
returns an iterator.
So the answer to your question is ‘it all depends on what you are trying to do‘!
python3 & python2
just use range()
:
for _ in range(n):
# do something n times exactly