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pairwise combination of two lists

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  pairwise combination of two lists Yingjie Lin <Yingjie.Lin@mssm.edu> - 2011-08-17 16:22 -0400
    Re: pairwise combination of two lists Mel <mwilson@the-wire.com> - 2011-08-17 17:15 -0400
      Re: pairwise combination of two lists Mel <mwilson@the-wire.com> - 2011-08-17 17:18 -0400
    Re: pairwise combination of two lists Marc Christiansen <usenet@solar-empire.de> - 2011-08-17 23:43 +0200
    Re: pairwise combination of two lists Luis M. González <luismgz@gmail.com> - 2011-08-17 16:27 -0700
    Re: pairwise combination of two lists Paul Rubin <no.email@nospam.invalid> - 2011-08-18 00:18 -0700
    Re: pairwise combination of two lists Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2011-08-18 09:30 +0200
    Re: pairwise combination of two lists SigmundV <sigmundv@gmail.com> - 2011-08-18 14:51 -0700

#11714 — pairwise combination of two lists

Hi Python users,

I have two lists: 

li1 = ['a', 'b']
li2 = ['1', '2']

and I wish to obtain a list like this

li3 = ['a1', 'a2', 'b1', 'b2']

Is there a handy and efficient function to do this, especially when li1 and li2 are long lists.
I found zip() but it only gives [('a', '1'), ('b', '2')],  not exactly what I am looking for.

Thank you.


- Yingjie

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#11715

Yingjie Lin wrote:
> I have two lists:
> 
> li1 = ['a', 'b']
> li2 = ['1', '2']
> 
> and I wish to obtain a list like this
> 
> li3 = ['a1', 'a2', 'b1', 'b2']
> 
> Is there a handy and efficient function to do this, especially when li1
> and li2 are long lists.
> I found zip() but it only gives [('a', '1'), ('b', '2')],  not exactly
> what I am looking for.

This seems to do it :

mwilson@tecumseth:~$ python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import itertools
>>> li1 = ['a', 'b']
>>> li2 = ['1', '2']
>>> map (lambda (x,y):x+y, list (itertools.product (li1, li2)))
['a1', 'a2', 'b1', 'b2']


	Mel.

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#11716

Mel wrote:

> Yingjie Lin wrote:
>> I have two lists:
>> 
>> li1 = ['a', 'b']
>> li2 = ['1', '2']
>> 
>> and I wish to obtain a list like this
>> 
>> li3 = ['a1', 'a2', 'b1', 'b2']
[ ... ]
> This seems to do it :
> 
> mwilson@tecumseth:~$ python
> Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56)
> [GCC 4.4.3] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> import itertools
>>>> li1 = ['a', 'b']
>>>> li2 = ['1', '2']
>>>> map (lambda (x,y):x+y, list (itertools.product (li1, li2)))
> ['a1', 'a2', 'b1', 'b2']


I have doubts about this in Python3, since tuple unpacking in a argument 
list isn't done there, and I don't think sum works on strings.  Some other 
function can probably be made to work.

	Mel.

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#11719

Yingjie Lin <Yingjie.Lin@mssm.edu> wrote:
> Hi Python users,
> 
> I have two lists: 
> 
> li1 = ['a', 'b']
> li2 = ['1', '2']
> 
> and I wish to obtain a list like this
> 
> li3 = ['a1', 'a2', 'b1', 'b2']
> 
> Is there a handy and efficient function to do this, especially when
> li1 and li2 are long lists.

Depending on your needs, we can offer you three solutions:

For our customers who want it all at once, but without any unneccessary
waste, the list expression:
[a + b for a, b in itertools.product(li1, li2)]

or if you don't need the whole list at once (especially interesting for
our customers with large lists), the generator expression (genexp):
(a + b for a, b in itertools.product(li1, li2))

and if you don't like the throwaway genexp and want something more
ecofriedly, we can present you a memory efficient, reusable solution, the
generator:
def combiner(li1, li2):
    for a, b in itertools.product(li1, li2):
        yield a + b

;)

HTH, Marc

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#11723

This is the easiest and most pythonic way (IMHO):

>>>> l3 = [i+e for i in li1 for e in li2]
>>>> l3
['a1', 'a2', 'b1', 'b2']

Regards,
Luis

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#11739

Yingjie Lin <Yingjie.Lin@mssm.edu> writes:
> li1 = ['a', 'b']
> li2 = ['1', '2']
>
> and I wish to obtain a list like this
> li3 = ['a1', 'a2', 'b1', 'b2']

from itertools import *

li3 = list(chain.from_iterable(izip(li1,li2)))

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#11740

Yingjie Lin <Yingjie.Lin@mssm.edu> writes:

> I have two lists: 
>
> li1 = ['a', 'b']
> li2 = ['1', '2']
>
> and I wish to obtain a list like this
>
> li3 = ['a1', 'a2', 'b1', 'b2']
>
> Is there a handy and efficient function to do this, especially when
> li1 and li2 are long lists.

It's not difficult to write your own:

def product(l1,l2):
    for x1 in l1:
        for x2 in l2:
            yield x1+x2

use it like: for p in product(l1,l2) ... The call to product() produces
a generator, the cross product is never built in full, it should work on
long lists.

-- Alain.

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#11810

On Aug 17, 9:22 pm, Yingjie Lin <Yingjie....@mssm.edu> wrote:
> I found zip() but it only gives [('a', '1'), ('b', '2')],  not exactly what I am looking for.

Yet, if you feed the zip into a list comprehension you get what you
want:

li3 = [''.join(l) for l in zip(li1,li2)]


Sigmund

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