pairwise traversal of a list or tuple

Question

a = [5, 66, 7, 8, 9, ...]

Is it possible to make an iteration instead of writing like this?

a[1] - a[0]

a[2] - a[1]

a[3] - a[2]

a[4] - a[3]

...

Thank you!

Answer 1

for a small list in python 2 or any list in python 3, you can use

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

for a larger list, you probably want

import itertools as it

[x - y for x, y in it.izip(a[1:], a)]

if you are using python 2

And I would consider writing it as a generator expression instead

(x - y for x, y in it.izip(a[1:], a))

This will avoid creating the second list in memory all at once but you will only be able to iterate over it once. If you only want to iterate over it once, then this is ideal and it's easy enough to change if you decide later that you need random or repeated access. In particular if you were going to further process it to make a list, then this last option is ideal.

update:

The fastest method by far is

import itertools as it
import operator as op

list(it.starmap(op.sub, it.izip(a[1:], a)))

---

$ python -mtimeit -s's = [1, 2]*10000' '[x - y for x, y in zip(s[1:], s)]'
100 loops, best of 3: 13.5 msec per loop

$ python -mtimeit -s'import itertools as it; s = [1, 2]*10000' '[x - y for x, y in it.izip(s[1:], s)]'
100 loops, best of 3: 8.4 msec per loop

$ python -mtimeit -s'import itertools as it; import operator as op; s = [1, 2]*10000' 'list(it.starmap(op.sub, it.izip(s[1:], s)))'
100 loops, best of 3: 6.38 msec per loop

Answer 2

Sure.

for i in range(1, len(a)):
    print a[i] - a[i-1]

I fail to see what the real problem is here. Have you read the python tutorial?

Answer 3

I acknowledge that such a simple task should not require extra functions, but programming is about abstractions, isn't it? Consecutive pairs (x0, x1), (x1, x2), (x2, x3), ..., are called pairwise combinations (see a python recipe in the itertools docs). And once you have this function in your toolset (which you should have, since Python devs cannot anticipate all your needs), it's as easy as:

a = [5, 66, 7, 8, 9]
for x, y in pairwise(a):
    print y - x