Special type of combination using itertools

Question

I am almost finished with a task someone gave me that at first involved easy use of the product() function from itertools. However, the person asked that it should also do something a bit different like:

li =

[[1, 2, 3],
[4, 5, 6]]

A regular product() would give something like: [1, 4], [1, 5], [1, 6], [2, 4], [2, 5], [2, 6], [3, 4] ...

What it should do is:

Do a regular product(), then, add the next item from the first element in the list and so on. A complete set of example would be:

[[1, 4, 2]
[1, 4, 3],
[1, 5, 2],
[1, 5, 3],
[2, 4, 3],
[2, 5, 3],
[2, 6, 3]]

How should I use itertools in this circumstance?

EDIT:

It might help if I explain the goal of the program: The user will enter, for example, a 5 row by 6 column list of numbers.
A normal product() will result in a 5-number combination. The person wants a 6-number combination. Where will this "6th" number come from? It would come from his choice of which row he wants.

What kind of combinations you want to get? Giving examples don't shows whole idea. — Tomasz Wysocki
– Tomasz Wysocki, Commented Aug 17, 2010 at 19:38
so, what you want is all triplets where first element is from set A, second is from set B and third from set A? — user3458
– user3458, Commented Aug 17, 2010 at 19:39
And sequences starting with last from first list (like [3,4,1]) are impossible as the next one is missing? — Tony Veijalainen
– Tony Veijalainen, Commented Aug 17, 2010 at 20:14

Odomontois · Accepted Answer · 2010-08-17 20:57:46Z

1

I wondering what is the magical computations you performing, but it look's like that's your formula:

k = int(raw_input('From What row items should be appeared again at the end?'))
res = [l for l in product(*(li+[li[k]])) if l[k]<l[len(li)] ]

edited Aug 17, 2010 at 20:57

answered Aug 17, 2010 at 20:09

Odomontois

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3 Comments

chiurox Over a year ago

Yes. This is the idea. For this particular line of code and to get the result I want as I asked in the question, yes it works. But what if I want any x rows by x+1 cols?

chiurox Over a year ago

Ohhh, I finally got the idea. That's very neat, didn't think of it this way. Just do a product() of a list with a subset of its own list! I'm hoping this would give all combinations needed.

chiurox Over a year ago

Hahaha, 'magical computations.' To be completely honest, I don't know what the person wants with this. I just took it as a chance to learn some python (never used it before) and get back in touch with some programming. Now that I think about it, it is a really trivial thing. Thanks a lot though.

Tony Veijalainen · Accepted Answer · 2010-08-17 20:42:36Z

1

Generalized for more than two sublist (map function would be the other alternative)

from pprint import pprint
for li in ([[1, 2, 3],
            [4, 5, 6]],

           [[1,  2,  3,  4],
            [5,  6,  7,  8],
            [9, 10, 11, 12]]
           ):
    triples= []
    prevlist=li[0]
    for nextlist in li[1:]:
        for spacing in range(1,len(prevlist)):
            triples.extend([[first,other,second]
                            for first,second in zip(prevlist,prevlist[spacing:])
                            for other in nextlist])

    pprint(sorted(triples))

edited Aug 17, 2010 at 20:42

answered Aug 17, 2010 at 20:09

Tony Veijalainen

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1 Comment

chiurox Over a year ago

This works for the specification I set in the question. What if I want a 5 rows by 6 cols? or any x rows by x+1 cols?

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