Nested For Loops Using List Comprehension

Question

If I had two strings, 'abc' and 'def', I could get all combinations of them using two for loops:

for j in s1:
  for k in s2:
    print(j, k)

However, I would like to be able to do this using list comprehension. I’ve tried many ways, but have never managed to get it. Does anyone know how to do this?

Asked By: John Howard

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Source

Answer 1

lst = [j + k for j in s1 for k in s2]

or

lst = [(j, k) for j in s1 for k in s2]

if you want tuples.

Like in the question, for j... is the outer loop, for k... is the inner loop.

Essentially, you can have as many independent ‘for x in y’ clauses as you want in a list comprehension just by sticking one after the other.

To make it more readable, use multiple lines:

lst = [
       j + k         # result
       for j in s1   # for loop 
         for k in s2 # for loop
                     # condition   
       ]

Answered By: aaronasterling

Answer 2

Since this is essentially a Cartesian product, you can also use itertools.product. I think it’s clearer, especially when you have more input iterables.

itertools.product('abc', 'def', 'ghi')

Answered By: miles82

Answer 3

Try recursion too:

s=""
s1="abc"
s2="def"
def combinations(s,l):
    if l==0:
        print s
    else:
        combinations(s+s1[len(s1)-l],l-1)
        combinations(s+s2[len(s2)-l],l-1)

combinations(s,len(s1))

Gives you the 8 combinations:

abc
abf
aec
aef
dbc
dbf
dec
def

Answered By: Stefan Gruenwald

Answer 4

It’s just a ready-to-go version of @miles82 answer (please give credit where it’s due):

from itertools import product
list(map(list, product('abc', 'def') ))

Output:

[['a', 'd'],
 ['a', 'e'],
 ['a', 'f'],
 ['b', 'd'],
 ['b', 'e'],
 ['b', 'f'],
 ['c', 'd'],
 ['c', 'e'],
 ['c', 'f']]

In case you wondered why we need list(map(list – itertools.product returns an iterator.

Answered By: mirekphd

Nested For Loops Using List Comprehension

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