How to loop through all but the last item of a list?


Using a for loop, how can I loop through all except the last item in a list? I would like to loop through a list checking each item against the one following it. Can I do this without using indices?

Asked By: David Sykes



for x in y[:-1]

If y is a generator, then the above will not work.

Answered By: freespace

If you want to get all the elements in the sequence pair wise, use this approach (the pairwise function is from the examples in the itertools module).

from itertools import tee, izip, chain

def pairwise(seq):
    a,b = tee(seq)
    return izip(a,b)

for current_item, next_item in pairwise(y):
    if compare(current_item, next_item):
        # do what you have to do

If you need to compare the last value to some special value, chain that value to the end

for current, next_item in pairwise(chain(y, [None])):
Answered By: Ants Aasma

if you meant comparing nth item with n+1 th item in the list you could also do with

>>> for i in range(len(list[:-1])):
...     print list[i]>list[i+1]

note there is no hard coding going on there. This should be ok unless you feel otherwise.

Answered By: Perpetualcoder

the easiest way to compare the sequence item with the following:

for i, j in zip(a, a[1:]):
     # compare i (the current) to j (the following)
Answered By: SilentGhost

To compare each item with the next one in an iterator without instantiating a list:

import itertools
it = (x for x in range(10))
data1, data2 = itertools.tee(it)
for a, b in itertools.izip(data1, data2):
  print a, b
Answered By: odwl

This answers what the OP should have asked, i.e. traverse a list comparing consecutive elements (excellent SilentGhost answer), yet generalized for any group (n-gram): 2, 3, … n:

zip(*(l[start:] for start in range(0, n)))


l = range(0, 4)  # [0, 1, 2, 3]

list(zip(*(l[start:] for start in range(0, 2)))) # == [(0, 1), (1, 2), (2, 3)]
list(zip(*(l[start:] for start in range(0, 3)))) # == [(0, 1, 2), (1, 2, 3)]
list(zip(*(l[start:] for start in range(0, 4)))) # == [(0, 1, 2, 3)]
list(zip(*(l[start:] for start in range(0, 5)))) # == []


  • l[start:] generates a a list/generator starting from index start
  • *list or *generator: passes all elements to the enclosing function zip as if it was written zip(elem1, elem2, ...)


AFAIK, this code is as lazy as it can be. Not tested.

Answered By: juanmirocks
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