CODEWITHSUNDEEP
pythonpython-2.7
MrMordja
MrMordja

Reputation: 13

How can you manipulate the iterator in a for loop in python?

If I'm trying to move through a very small range, and want to refer to a variable created in an earlier step of the iteration, how can I reference it?

If I use a for loop with a range of say 2,6, and on the first iteration I create two variables a[i] & b[i], is it possible to reference a3 on the 4th or 5th iteration? Or something like doSomething(a[i-1],a[i])

Apologies for the terrible psuedo-code.

Upvotes: 0

Views: 1459

Answers (1)

abarnert
abarnert

Reputation: 365707

You can't "manipulate the iterator". But you can do two simple things.

First, since you're apparently iterating over sequences (like range), you can use indices. For example:

for i, value in enumerate(my_sequence):
    if 3 <= i <= 4:
        old_value = my_sequence[i-2]
        do_something(old_value, value)

Second, you can just remember the old values:

lastlast = last = None
for value in my_sequence:
    if lastlast is not None:
        do_something(lastlast, value)
    lastlast, last = last, value

If you want to make the second one more readable, you can pretty easily write a function that takes any iterable and iterates over overlapping groups of 3 values, so you can just do this:

for lastlast, last, value in triplets(my_sequence):
    do_something(lastlast, value)

So, how do you write that triplets function?

def triplets(iterable):
    a, b, c = itertools.tee(iterable, 3)
    next(b, None)
    next(c, None)
    next(c, None)
    return itertools.izip(a, b, c)

Or, if you only care about sequences, not iterables in general, and are willing to waste some space to maybe save a little time and definitely save a bit of code:

def triplets(sequence):
    return zip(sequence, sequence[1:], sequence[2:])

Upvotes: 1

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