Read other items in a python list while iterating through it

Question

Possible Duplicate:
Python: Looping through all but the last item of a list

Is there a better way of iterating through a list when you also need the next item (or any other arbitrary item) in the list? I use this, but maybe someone can do better...

values = [1, 3, 6, 7 ,9]
diffs = []
for i in range(len(values)):
    try: diffs.append(values[i+1] - values[i])
    except: pass

print diffs

gives:

[2, 3, 1, 2]

Answer 1

You could use

for pos, item in enumerate(values):
    try:
        diffs.append(values[pos+1] - item)
    except IndexError:
        pass

although in your case (since you're just looking for the next value), you could also simply use

for item,nextitem in zip(values, values[1:]):
    diffs.append(nextitem-item)

which can also be expressed as a list comprehension:

diffs = [nextitem-item for item,nextitem in zip(values, values[1:])]

Answer 2

>>> values = range(10)
>>> values
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> zip(values[0:],values[1:])
[(0, 1), (1, 2), (2, 3), (3, 4), (4, 5), (5, 6), (6, 7), (7, 8), (8, 9)]

Answer 3

for i, j in zip(values, values[1:]):
     j - i

Answer 4

diff = [values[i+1] - values[i] for i in range(len(values)-1)]

Answer 5

what about with the aid of pairwise recipe from itertools document

from itertools import tee, izip

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return izip(a, b)
def main():
    values = [1, 3, 6, 7 ,9]
    diffs = [post - prior for prior, post in pairwise(values)]
    print diffs


if __name__ == "__main__":
    main()

output
[2, 3, 1, 2]

Answer 6

[y - x for x,y in zip(L,L[1:])]