Python - Create multiple lists from elementwise concatenation of two lists

Question

lst1 = ['a', 'b', 'c']
lst2 = ['1', '2']

def comb(lst1, lst2):
    for i in lst1:
            new_list = []
            for j in lst2:
                new_list.append(i + '_' + j)
    return new_list
print(comb(lst1, lst2))

Gives me:

['c_1', 'c_2']

I am hoping to get:

['a_1', 'a_2']

['b_1', 'b_2']

['c_1', 'c_2']

Could someone please point out the mistake in my code? Thanks!

@PéterLeéh That wont work. It will just create one list with all elements. Not as expected by OP. — thisisjaymehta, Aug 26 '20 at 06:52
Most probably a special case of this one: https://stackoverflow.com/questions/12935194/combinations-between-two-lists — Patrick Artner, Aug 26 '20 at 07:15

score 6 · Accepted Answer · answered Aug 26 '20 at 06:43

6

Try this

res = [[f'{x}_{y}' for y in lst2] for x in lst1]
print(res)

Output:

[['a_1', 'a_2'], ['b_1', 'b_2'], ['c_1', 'c_2']]

answered Aug 26 '20 at 06:43

deadshot

8,317
4
15
36

score 2 · Answer 2 · answered Aug 26 '20 at 06:42

Try this:

lst1 = ['a', 'b', 'c']
lst2 = ['1', '2']

def comb(lst1, lst2):
    finalList = []
    for i in lst1:
            new_list = []
            for j in lst2:
                new_list.append(i + '_' + j)
            finalList.append(new_list)
    return finalList
    
print(comb(lst1, lst2))

new_list gets empty every time first for loop is executed. So make another list to store that value before its overwritten and return that second list with all values of new_list.

Better way to do this is using list comprehension. See deadshot's [answer](https://stackoverflow.com/a/63591907/8702713) for that. — thisisjaymehta, Aug 26 '20 at 06:50

score 0 · Answer 3 · answered Aug 26 '20 at 06:43

0

Just look where new_list = [] is. You're creating it from scratch in every loop iteration. Just move it before for.

If you just want to see 3 those elements printed, change return into print.

lst1 = ['a', 'b', 'c']
lst2 = ['1', '2']

def comb(lst1, lst2):
    for i in lst1:
        new_list = []
        for j in lst2:
            new_list.append(i + '_' + j)
        print(new_list)
comb(lst1, lst2)

answered Aug 26 '20 at 06:43

Maciałek

236
2
7

1

If you replace the print with a return you get only the first result. You need to add each constructed list into another list that then gets returned - see https://stackoverflow.com/a/63591903/7505395 – Patrick Artner Aug 26 '20 at 06:59
The best solution is the one written by deadshot https://stackoverflow.com/a/63591907/10815718 . The solution with print is only direct solution of what States.the.Obvious want to get as output, – Maciałek Aug 26 '20 at 07:24

score 0 · Answer 4 · answered Aug 26 '20 at 06:47

0

try this;

lst1 = ['a', 'b', 'c']
lst2 = ['1', '2']
new_list = []
for i in lst1:
    l1=[]
    for j in lst2:
        l1.append(i + '_' + j)
    new_list.append(l1)
print(new_list)

answered Aug 26 '20 at 06:47

Juhi Dhameliya

172
2
9

score 0 · Answer 5 · answered Aug 26 '20 at 08:31

0

lst1 = ['a', 'b', 'c']
lst2 = ['1', '2']
for e1 in lst1:
    newList = []
    for e2 in lst2:
        newList.append(e1 + "_" + e2)
    print(newList)


Output:
['a_1', 'a_2']
['b_1', 'b_2']
['c_1', 'c_2']

answered Aug 26 '20 at 08:31

Batuhan Okur

11
2

Python - Create multiple lists from elementwise concatenation of two lists

5 Answers5