setting python for loop iterator during iteration

Question

How do I change a python iterator mid iteration?

For example:

for i in range(10):
    print i
    i = 5

This prints:

0
1
...
9

I would like to print:

0
5
5
5
...

EDIT: Sorry this question came out as confusing to people. I was interested in why when I try to change an iterator during a for loop the for loop ignores it on the next iteration.

Someone else has already submitted an answer that clears my confusion. range creates a list and then the for loop assigns the iterator to the next variable in the list.

If you change i in the loop it's not going to terminate. Even if it did - you are doing something conceptually wrong. — Reut Sharabani
– Reut Sharabani, Commented Aug 12, 2015 at 22:25
I realize this hence my output bring 5, 5, 5, ... as in repeating. My real code will only change the iterator inside of a condition so it can skip unnecessary data. — user2327814
– user2327814, Commented Aug 12, 2015 at 22:30
Why don't you tell us the problem you're trying to solve instead of the problem your attempted solution is causing? — TigerhawkT3
– TigerhawkT3, Commented Aug 12, 2015 at 22:30
You should tell us your actual problem, as @TigerhawkT3 said. Because while you can get the desired output that you show in your question with for x in ([0] + [5]*9): print x, or even just print '\n'.join(['0'] + ['5']*9), I don't think that's going to help with your actual problem :). — Cyphase
– Cyphase, Commented Aug 12, 2015 at 22:35
I deleted my comment that was attributing a different answer to Cody than the one he actually posted; whoops :P. — Cyphase
– Cyphase, Commented Aug 12, 2015 at 22:42

Reut Sharabani · Accepted Answer · 2015-08-12 22:33:16Z

1

Following your response what you're trying to do already exists in the form of continue. It lets you skip an iteration by checking a certain condition holds:

for i in range(10):
    if i >= 5:
        continue # go to next iteration
    print i

Assuming you're using an iterator and not a list you can skip one element at a time, so a jump to a given index is irrelevant.

answered Aug 12, 2015 at 22:33

Reut Sharabani

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Comments

Cody Braun · Accepted Answer · 2015-08-12 22:26:51Z

0

The for loop is iterating over every number in the list created by range. You can change i for a given iteration, but the for loop just goes to the next number in the list. I think a while loop would be a different story, it actually uses the current value of i

answered Aug 12, 2015 at 22:26

Cody Braun

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L.B. · Accepted Answer · 2015-08-12 22:29:09Z

0

To echo Cody Braun, see the following code and output:

for i in range(10):
    if i >= 5:
        i = 5
    print i

Output:

answered Aug 12, 2015 at 22:29

L.B.

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Comments

Jake Griffin · Accepted Answer · 2015-08-12 23:09:15Z

You could make a class that was iterable and modify the iteration in the for-loop:

>>> class MyRange:
...     def __init__(self, stop):
...         self.current = 0 
...         self.stop = stop
...     def __iter__(self):
...         return self
...     def next(self): # Python 3: def __next__(self)
...         if self.current >= self.stop:
...             raise StopIteration
...         else:
...             self.current += 1
...             return self.current - 1 
...
>>> for i in MyRange(3):
...     print i
...
0
1
2
>>> r = MyRange(10)
>>> for i in r:
...     print i
...     r.current += 1
...
0
2
4
6
8
>>> r = MyRange(10)
>>> for i in r:
...     print i
...     r.current = 5
0
5
5
5
5
5
# and so on...

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