2

Having started to learn code with C, I had always assumed that for-loops and while-loops where essentialy always equivalent (as in one could always reproduce the behaviour of one using only the other). But in python while going from a for-loop to a while-loop is always trivial, I could not find a way to achieve the reverse.

Is there any way, in python, to reproduce the behaviour of a while-loop (infinite looping) using only for-loops ?

Here is a solution that doesn't work (because of the recursion limit) using a recursive generator:

def infinite_loopy():
    yield "All work and no play makes Jack a dull boy"
    for x in infinite_loopy():
        yield x

#here starts the supposedly infinite-loop
for x in infinite_loopy():
    print(x)
Improve this question
7
  • @Aurora0001 It's not a duplicate of that question as I would like no usage of while-loops Commented Aug 24, 2016 at 12:39
  • 1
    The accepted answer of that question uses no while loops, so it's exactly what you want, yes? If not, why not? Commented Aug 24, 2016 at 12:42
  • @Kevin My bad I only checked the question. But yeah that would count as a solution. Commented Aug 24, 2016 at 12:45
  • Yeah somehow I missed the proposed duplicate and drafted an answer that was exactly identical with the accepted answer in that other question... Commented Aug 24, 2016 at 13:27
  • @Antti Haapala Does this realy count as a duplicate ? I feel like they are two different questions that append to have common answers. Commented Aug 24, 2016 at 13:33

4 Answers 4

Reset to default
5

You can do this by writing a non-yield iterator class:

class Infinite(object):
    def __iter__(self):
        return self

    def next(self): # For Python3, replace this with __next__
        return 1

# Loops forever
for i in Infinite():
    pass

(You can see it stalling on ideone if you have the patience - it's like watching paint dry).

Improve this answer
Sign up to request clarification or add additional context in comments.

3 Comments

this solution is simple and pleasant to the eyes, +1
@Lost Many thanks!
@AnttiHaapala Thanks for the correct comment. For Python3, next should be replaced with __next__ (updated in the question).
5

You can use the two-argument version of iter as follows:

for _ in iter(int, 1):
    print('All your loops are belong to us!')

The 2-argument form of iter calls the first argument as a function with no arguments. If the returned value equals (==) the second argument, StopIteration is raised, otherwise the return value is yielded. int() called without arguments returns 0 which is of course not equal to 1 thus yielding 0 forever, and we have an infinite loop.

Improve this answer

Comments

4

You can use itertools.repeat, count or cycle:

import itertools

for _ in itertools.repeat(None):
    # infinite loop

for _ in itertools.count():
    # infinite loop

for _ in itertools.cycle([None]):
    # infinite loop

All of these can be represented by functions using while loops, but the itertools module is implemented in c (in cPython), and makes no use of while loops in the source code. Similarly for jython (java), and even PyPy (python). The same is true for count and cycle.

Improve this answer

8 Comments

It's still possible that they're internally implemented using a while, no? I think that's how the OP responded to Aurora's comment just under the question.
The doc for itertools.repeat indicate that it is roughly equivalent to a function containing a while
@jadsq The source for repeat has no use of while whatsoever.
@AmiTavory See edit
@RoadieRich Oh, good answer, then (in a completely different direction).
|
-1

You can use itertools.count() in for loop.

import itertools

def infinite_loopy():
    x = "All work and no play makes Jack a dull boy"
    for x in itertools.count():
        yield x

for i in infinite_loopy():
    print("All work and no play makes Jack a dull boy")

[Reference] Looping from 1 to infinity in Python

Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.