Identify if list has consecutive elements that are equal

Question

I'm trying to identify if a large list has consecutive elements that are the same.

So let's say:

lst = [1, 2, 3, 4, 5, 5, 6]

And in this case, I would return true, since there are two consecutive elements lst[4] and lst[5], are the same value.

I know this could probably be done with some sort of combination of loops, but I was wondering if there were a more efficient way to do this?

Answer 1

You can use itertools.groupby() and a generator expression within any() ^*:

>>> from itertools import groupby
>>> any(sum(1 for _ in g) > 1 for _, g in groupby(lst))
True

Or as a more Pythonic way you can use zip(), in order to check if at least there are two equal consecutive items in your list:

>>> any(i==j for i,j in zip(lst, lst[1:])) # In python-2.x,in order to avoid creating a 'list' of all pairs instead of an iterator use itertools.izip()
True

Note: The first approach is good when you want to check if there are more than 2 consecutive equal items, otherwise, in this case the second one takes the cake!

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^{* Using sum(1 for _ in g) instead of len(list(g)) is very optimized in terms of memory use (not reading the whole list in memory at once) but the latter is slightly faster.}

Answer 2

You can use a simple any condition:

lst = [1, 2, 3, 4, 5, 5, 6]
any(lst[i]==lst[i+1] for i in range(len(lst)-1))
#outputs:
True

any return True if any of the iterable elements are True