how to use iterator in while loop statement in python

Question

Is it possible to use a generator or iterator in a while loop in Python? For example, something like:

i = iter(range(10))
while next(i):
    # your code

The point of this would be to build iteration into the while loop statement, making it similar to a for loop, with the difference being that you can now additional logic into the while statement:

i = iter(range(10))
while next(i) and {some other logic}:
    # your code

It then becomes a nice for loop/while loop hybrid.

Does anyone know how to do this?

has to be a while-loop? `for x in i` would be the natural way. — user2390182, Nov 28 '19 at 15:59
I guess you would not try your idea on `it = itertools.count(); while next(it):` — RomanPerekhrest, Nov 28 '19 at 16:03
how would you access the value the iter returned since you dont store it you just take the item and evaluaate its boolean result so what would you do in the loop since you dont have access to the item returned from the iter? — Chris Doyle, Nov 28 '19 at 16:08
What you need to elaborate is what should be a stopping factor of `while` loop. How about `it = iter([True, False, 1, 0, 1]); while next(it): print('1')` ? The answers given are simple, but what's your exact intention? — RomanPerekhrest, Nov 28 '19 at 16:10
What if we wanted to keep this general though, which is why I didn't indicate a specific additional logic. The stopping factor here can just be assumed to be exceeding the last iteration to where the iterator throws an exception. @RomanPerekhrest — Union find, Nov 28 '19 at 16:12

user2390182 · Accepted Answer · 2021-10-19T14:56:31.137

32

In Python >= 3.8, you can do the following, using assignment expressions:

i = iter(range(10))
while (x := next(i, None)) is not None and x < 5:
    print(x)

In Python < 3.8 you can use itertools.takewhile:

from itertools import takewhile

i = iter(range(10))
for x in takewhile({some logic}, i):
    # do stuff

"Some logic" here would be a 1-arg callable receciving whatever next(i) yields:

for x in takewhile(lambda e: 5 > e, i):
    print(x)
0
1
2
3
4

edited Oct 19 '21 at 14:56

answered Nov 28 '19 at 16:07

user2390182

72,016
6
67
89

1

The `None` param is important or else it will throw an `stopIteration` exception – gabriel garcia Feb 05 '21 at 14:00
1

What if the element in the collection or streams is a None? – xing Jun 01 '21 at 22:35
@xing If you have `None` elements in your collection, then you want to do `next(i)` and then catch the `StopIteration` exception – Alexei Andreev Jul 11 '23 at 14:47
1

@AlexeiAndreev You can just replace `None` by any sentinel object that will not occur in your iterator. – user2390182 Jul 11 '23 at 15:17

tobias_k · Answer 2 · 2020-05-01T19:43:29.523

There are two problems with while next(i):

Unlike a for loop, the while loop will not catch the StopIteration exception that is raised if there is no next value; you could use next(i, None) to return a "falsey" value in that case, but then the while loop will also stop whenever the iterator returns an actual falsey value
The value returned by next will be consumed and no longer available in the loop's body. (In Python 3.8+, that could be solved with an assignment expression, see other answer.)

Instead, you could use a for loop with itertools.takewhile, testing the current element from the iterable, or just any other condition. This will loop until either the iterable is exhausted, or the condition evaluates to false.

from itertools import takewhile
i = iter(range(10))
r = 0
for x in takewhile(lambda x: r < 10, i):
    print("using", x)
    r += x
print("result", r)

Output:

using 0
...
using 4
result 10

Everything's getting downvoted.. this seems like a perfectly valid question/answer. — Union find, Nov 28 '19 at 16:25

larsks · Answer 3 · 2019-11-28T16:16:53.160

5

You just need to arrange for your iterator to return a false-like value when it expires. E.g., if we reverse the range so that it counts down to 0:

>>> i = iter(range(5, -1, -1))
>>> while val := next(i):
...     print('doing something here with value', val)
...

This will result in:

doing something here with value 5
doing something here with value 4
doing something here with value 3
doing something here with value 2
doing something here with value 1

edited Nov 28 '19 at 16:16

answered Nov 28 '19 at 16:00

larsks

277,717
41
399
399

2

But this consumes `next(i)`, and if you call it again in the loop body you miss one value. Maybe with Python 3.8 assignment expressions, though. – tobias_k Nov 28 '19 at 16:02
Sure, this would be a prefect place to use assignment expressions. It wasn't clear from the question that you care about the value of the iterator. – larsks Nov 28 '19 at 16:15
Updated the example to use an assignment expression. – larsks Nov 28 '19 at 16:17

score 2 · Answer 4 · answered Dec 05 '21 at 22:01

2

a = iter(range(10))

try:
    next(a)
    while True:
        print(next(a))
except StopIteration:
    print("Stop iteration")

answered Dec 05 '21 at 22:01

kwa

49
4

1

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Dec 05 '21 at 22:17

score -3 · Answer 5 · edited Nov 28 '19 at 16:34

-3

You can do

    a = iter(range(10))

    try:
        a.next()
        while True and {True or False logic}:
            print("Bonjour")
            a.next()
    except StopIteration:
        print("a.next() Stop iteration")

edited Nov 28 '19 at 16:34

Union find

7,759
13
60
111

answered Nov 28 '19 at 16:05

TOTO

307
1
6

6

`while True and (True or False):`-- is this a joke? – Chris_Rands Nov 28 '19 at 16:05
@Chris_Rands No it's a substitute for his [some logic] – TOTO Nov 28 '19 at 16:27
This is exactly what I don't want to do. I want to put the next in the while statement. – Union find Nov 28 '19 at 16:35
1

got it (not my dv), you should also use `next(a)` in py 3 – Chris_Rands Nov 28 '19 at 16:35

how to use iterator in while loop statement in python

5 Answers5

Linked