Python loop to [:-1]

Question

So, I notice that calling array[:-1] is going to clone the array.

Say I have a large array with like 3000 elements in it. I don't want it to be cloned as I iterate over it! I just want to iterate to the 2nd last one.

for item in array[ :-1 ] :
  # do something with the item

So do I have to resort to a counter variable,

for c in range( 0, len( array ) - 1 ) :
  # do something with array[ c ]

or is there way to make/will array[:-1] syntax be efficient?

Answer 1

for item in itertools.islice(array, len(array) - 1):

Answer 2

Check out itertools.islice:

from itertools import islice
for item in islice(array, 0, len(array) - 1):
    # do something with item

This is about half of what you want; it eliminates the need to say array[i] but not the need to specify len(array) - 1.

For what it's worth, 3000 items is nothing to a modern computer, I wouldn't worry about the inefficiency unless your program is noticeably slow and you've profiled it to determine that this piece of code is a contributing factor.

Answer 3

For when you don't want to/can't/don't know the length of the sequence:

def allbutlast(seq):
  it = iter(seq)
  el = next(it)
  for e in it:
    yield el
    el = e

for i in allbutlast([1, 2, 3]):
  print i