Nested for loop in 'lists' and 'iterators'

Question

I have these two lists

l1 = [1,2,3]
l2 = [10,20,30]

I use this nested for loop to access all possible pairs:

for a in l1:
    for b in l2:
        print(a,b)

I get the following expected result:

But if I convert l1 and l2 to iterators, I get a different result and I can't access to all of the pairs:

l1=iter(l1)
l2=iter(l2)

for a in l1:
    for b in l2:
        print(a,b)

1 10
1 20
1 30

I can't understand the difference between these two code snippets. Why the iterators generate this result?

Once you've consumed the iterator, its done. No more values for you. After the first for b in l2: the well is dry. — tdelaney
– tdelaney, Commented Jun 28, 2020 at 7:23
I know this doesn't look like a duplicate, but that's the closest match I can easily find for this common problem. Sequence iterators created by iter have the same relevant behaviour as file iterators created by open. — Karl Knechtel
– Karl Knechtel, Commented Jun 28, 2020 at 7:24
@KarlKnechtel -Actually... files can be rewound (e.g., file.seek(0)). They are a counter example. iter() itself doesn't care which type of iterator you get. list_iterator can't be rewound, file can. — tdelaney
– tdelaney, Commented Jun 28, 2020 at 7:26
They can, but the point is about the specific style of iteration in play. — Karl Knechtel
– Karl Knechtel, Commented Jun 28, 2020 at 7:27
@KarlKnechtel- Yes this is because reader objects are iterators, just like file object returned by open(). — Mahsa
– Mahsa, Commented Jun 28, 2020 at 8:03

rdas · Accepted Answer · 2020-06-28 07:22:34Z

3

l2=iter(l2)

This creates an iterator out of l2. An iterator can only be iterated through once. After that, the interator is finished. If you wanted to iterate over l2 again, you'd need to create another iterator from it.

for a in l1:
    for b in l2:
        print(a,b)

This will try to iterate over l2 - once for each item in l1. If there are more than 1 items in l1, that won't work - because the l2 iterator is already exhausted after the first iteration.

So you need something like this:

l1 = iter(l1)

for a in l1:
    l2_iter = iter(l2)
    for b in l2_iter:
        print(a,b)

Which would do what you expect.

answered Jun 28, 2020 at 7:22

rdas

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