Expand a Tuple Into Semi-Repeating Pairs

Question

I would like to functionally derive this input/output pattern:

>>> foo( (2, 3, 4, 5) )
[(2, 3), (3, 4), (4, 5)]

The idea is that the second element of the previous tuple is repeated as the first element of the next tuple. I can't get it any other way than using iterative paradigms.

For what it's worth, I'm trying to answer this question, and I also have to demonstrate functional Python at a meetup next month. So help me kill two birds with one stone please, and thanks!

score 4 · Accepted Answer · answered Feb 25 '12 at 20:33

4

>>> f = (2, 3, 4, 5)
>>> zip(f[:-1], f[1:])
[(2, 3), (3, 4), (4, 5)]

Or, from the docs:

>>> 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)
... 
>>> tuple(pairwise(f))
((2, 3), (3, 4), (4, 5))

answered Feb 25 '12 at 20:33

senderle

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Hey! You already answered the other guy's question! Shoo! `;)` – yurisich Feb 25 '12 at 20:36
@Droogans, lol, sorry -- I didn't realize it was the same one. – senderle Feb 25 '12 at 20:37
@Droogans, but looking at your comments, I think I see what you're getting at, and I think it's more interesting than my answer anyway (which was more educational than efficient). You should post it anyway. – senderle Feb 25 '12 at 20:42
Wow...JTLYK, I wrote out a module that *does what their question asked*, but when checking the results, realized that **they posted the incorrect formula**. Oh well, at least I learned a lot. – yurisich Feb 26 '12 at 16:00

Óscar López · Answer 2 · 2012-02-25T22:56:04.863

How about a recursive (and functional!) solution:

def foo(t):
    if len(t) < 2:
        return []
    return [(t[0], t[1])] + foo(t[1:])

Here's another way to solve it, a bit more efficient since it doesn't create temporary slices of t as the above solution:

def foo(t):
    limit = len(t) - 2
    def bar(i):
        if i > limit:
            return []
        return [(t[i], t[i+1])] + bar(i+1)
    return bar(0)

Expand a Tuple Into Semi-Repeating Pairs

2 Answers2