Permutations between two lists of unequal length

Question

I   m having trouble wrapping my head around a algorithm I   m try to implement  I have two lists and want to take particular combinations from the two lists  Here   s an example  names     a    b   numbers    1  2   the output in this case would be     a   1     b   2      b   1     a   2    I might have more names than numbers  i e  len names   gt   len numbers   Here s an example with 3 names and 2 numbers  names     a    b    c   numbers    1  2   output     a   1     b   2      b   1     a   2      a   1     c   2      c   1     a   2      b   1     c   2      c   1     b   2

User · Answer

I was looking for a list multiplied by itself with only unique combinations, which is provided as this function.

import itertools
itertools.combinations(list, n_times)

Here as an excerpt from the Python docs on itertools That might help you find what your looking for.

Combinatoric generators:

Iterator                                 | Results
-----------------------------------------+----------------------------------------
product(p, q, ... [repeat=1])            | cartesian product, equivalent to a 
                                         |   nested for-loop
-----------------------------------------+----------------------------------------
permutations(p[, r])                     | r-length tuples, all possible 
                                         |   orderings, no repeated elements
-----------------------------------------+----------------------------------------
combinations(p, r)                       | r-length tuples, in sorted order, no 
                                         |   repeated elements
-----------------------------------------+----------------------------------------
combinations_with_replacement(p, r)      | r-length tuples, in sorted order, 
                                         | with repeated elements
-----------------------------------------+----------------------------------------
product('ABCD', repeat=2)                | AA AB AC AD BA BB BC BD CA CB CC CD DA DB DC DD
permutations('ABCD', 2)                  | AB AC AD BA BC BD CA CB CD DA DB DC
combinations('ABCD', 2)                  | AB AC AD BC BD CD
combinations_with_replacement('ABCD', 2) | AA AB AC AD BB BC BD CC CD DD

User · Answer

The simplest way is to use itertools product   a     foo    melon   b    True  False  c   list itertools product a  b    gt  gt     foo   True     foo   False     melon   True     melon   False

User · Answer

Note  This answer is for the specific question asked above  If you are here from Google and just looking for a way to get a Cartesian product in Python  itertools product or a simple list comprehension may be what you are looking for - see the other answers     Suppose len list1   gt   len list2   Then what you appear to want is to take all permutations of length len list2  from list1 and match them with items from list2  In python   import itertools list1   a   b   c   list2  1 2    list zip x list2   for x in itertools permutations list1 len list2      Returns      a   1     b   2       a   1     c   2       b   1     a   2       b   1     c   2       c   1     a   2       c   1     b   2

User · Answer

You might want to try a one line list comprehension    gt  gt  gt   name number for name in  ab  for number in  12     a1    a2    b1    b2    gt  gt  gt   name number for name in  abc  for number in  12     a1    a2    b1    b2    c1    c2

User · Answer

a tiny improvement for the answer from interjay  to make the result as a flatten list    gt  gt  gt  list3    zip x list2  for x in itertools permutations list1 len list2     gt  gt  gt  import itertools  gt  gt  gt  chain   itertools chain  list3   gt  gt  gt  list4   list chain     a   1     b   2     a   1     c   2     b   1     a   2     b   1     c   2     c   1     a   2     c   1     b   2     reference from this link

User · Answer

Without itertools as a flattened list    list1 i   list2 j   for i in range len list1   for j in range len list2     or in Python 2    list1 i   list2 j   for i in xrange len list1   for j in xrange len list2

User · Answer

Answering the question  given two lists  find all possible permutations of pairs of one item from each list  and using basic Python functionality  i e   without itertools  and  hence  making it easy to replicate for other programming languages   def rec a  b  ll  size       ret          for i e in enumerate a           for j f in enumerate b               l    e f              new l   rec a i 1    b  j  b j 1    ll  size              if not new l                  ret append l              for k in new l                  l k   l   k                 ret append l k                  if len l k     size                      ll append l k      return ret  a     a   b   c   b     1   2   ll      rec a b ll  min len a  len b    print ll    Returns     a1    b2      a1    c2      a2    b1      a2    c1      b1    c2      b2    c1

User · Answer

The better answers to this only work for specific lengths of lists that are provided   Here s a version that works for any lengths of input  It also makes the algorithm clear in terms of the mathematical concepts of combination and permutation   from itertools import combinations  permutations list1     1    2   list2     A    B    C    num elements   min len list1   len list2   list1 combs   list combinations list1  num elements   list2 perms   list permutations list2  num elements   result       tuple zip perm  comb     for comb in list1 combs   for perm in list2 perms    for idx    l11  l12    l21  l22   in enumerate result     print f  idx    l11  l12   l21  l22      This outputs   0  A1 B2 1  A1 C2 2  B1 A2 3  B1 C2 4  C1 A2 5  C1 B2

User · Answer

the best way to find out all the combinations for large number of lists is   import itertools from pprint import pprint  inputdata           a    b    c          d          e    f      result   list itertools product  inputdata   pprint result    the result will be      a    d    e       a    d    f       b    d    e       b    d    f       c    d    e       c    d    f

User · Answer

May be simpler than the simplest one above    gt  gt  gt  a     foo    bar    gt  gt  gt  b    1  2  3   gt  gt  gt    x y  for x in a for y in b     for a list    foo   1     foo   2     foo   3     bar   1     bar   2     bar   3    gt  gt  gt    x y  for x in a for y in b     for a generator if you worry about memory or time complexity   lt generator object  lt genexpr gt  at 0x1048de850 gt    without any import

User · Answer

Or the KISS answer for short lists     i  j  for i in list1 for j in list2    Not as performant as itertools but you re using python so performance is already not your top concern     I like all the other answers too

[python] Permutations between two lists of unequal length

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