How do I reset a list iterator in Python?

Question

For example, in C++ I could do the following :-

for (i = 0; i < n; i++){
    if(...){              //some condition
        i = 0;
    }
}

This will effectively reset the loop, i.e. start the loop over without introducing a second loop

For Python -

for x in a: # 'a' is a list
    if someCondition == True:
        # do something

Basically during the course of the loop the length of 'a' might change. So every time the length of 'a' changes, I want to start the loop over. How do I go about doing this?

Are values in the list being changed as you loop through them; are you just appending values to the end or inserting values into the list? — purarue
– purarue, Commented May 19, 2018 at 16:58
@SeanBreckenridge: Based on some condition, I'm appending values to the list. So every time I append a value, I want the loop to start over — Rahul Sankar
– Rahul Sankar, Commented May 19, 2018 at 17:00
@miradulo: But for that I'll need a variable to keep track of the length of my list. Is there no other way to do it? — Rahul Sankar
– Rahul Sankar, Commented May 19, 2018 at 17:01

L3viathan · Accepted Answer · 2018-05-19 16:59:20Z

9

You could define your own iterator that can be rewound:

class ListIterator:
    def __init__(self, ls):
        self.ls = ls
        self.idx = 0
    def __iter__(self):
        return self
    def rewind(self):
        self.idx = 0
    def __next__(self):
        try:
            return self.ls[self.idx]
        except IndexError:
            raise StopIteration
        finally:
            self.idx += 1

Use it like this:

li = ListIterator([1,2,3,4,5,6])
for element in li:
    ... # do something
    if some_condition:
        li.rewind()

answered May 19, 2018 at 16:59

L3viathan

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Monstah · Accepted Answer · 2018-07-26 01:37:32Z

4

Not using for, but doing by hand with a while loop:

n = len(a)
i = 0
while i < n:
  ...
  if someCondition:
    n = len(a)
    i = 0
    continue
  i+=1

edit - if you're just appending to the end, do you really need to start over? You can just move the finish line further, by comparing i to len(a) directly, or calling n=len(a) in your loop

edited Jul 26, 2018 at 1:37

answered May 19, 2018 at 17:00

Monstah

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