Loop a list to output each element twice with different prefix

Question

A simple list, that I want to loop through to print each element twice. Each time to add a different prefix. The output will be appended into a new list.

List1 = ["9016","6416","9613"]

The ideal result is:

['AB9016', 'CD9016', 'AB6416', 'CD6416', AB9613', 'CD9613']

I have tried below and but the output is:

new_list = []

for x in List1:
    for _ in [0,1]:
        new_list.append("AB" + x)
        new_list.append("CD" + x)

print (new_list)

['AB9016', 'CD9016', 'AB9016', 'CD9016', 'AB6416', 'CD6416', 'AB6416', 'CD6416', 'AB9613', 'CD9613', 'AB9613', 'CD9613']

And I can’t use:

new_list.append("AB" + x).append("CD" + x)

What’s the proper way to do it? Thank you.

Asked By: Mark K

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Source

Answer 1

The fastest method is to use the list comprehension. We are using 2 list comprehension to create the desired_list. Note that I also used the f string so I could easily ad the ‘ABandCD` prefix.

list1 = ["9016","6416","9613"]
desired_list = [f'AB{x}' for x in list1] + [f'CD{x}' for x in list1]
print(desired_list)

Answered By: Aleksander Ikleiw

Answer 2

The problem is caused by the inner loop: the two appends will be called twice. Fixed code:

new_list = []

for x in List1:
    new_list.append("AB" + x)
    new_list.append("CD" + x)

Regarding chaining append calls: It would work if append returned the list (with the new item appended to it), but this is not the case, the append method returns None (doc).

Answered By: kol

Answer 3

Also could try an easy comprehension:

List1 = ["9016","6416","9613"]
result = [j+i for i in List1 for j in ('AB','CD')]
# ['AB9016', 'CD9016', 'AB6416', 'CD6416', 'AB9613', 'CD9613']

Answered By: jizhihaoSAMA

Answer 4

I would use itertools.product for that task following way

import itertools
list1 = ["9016","6416","9613"]
prefixes = ["AB","CD"]
result = [x+y for y,x in itertools.product(list1,prefixes)]
print(result)

Output:

['AB9016', 'CD9016', 'AB6416', 'CD6416', 'AB9613', 'CD9613']

Answered By: Daweo

Answer 5

Here is a solution using itertools.product:

from itertools import product

lst1 = ['9016', '6416', '9613']
lst2 = ['AB', 'CD']

result = list(map(''.join, map(reversed, product(lst1, lst2))))

Answered By: Riccardo Bucco

Answer 6

we can use sum too:

In [25]: sum([[f'AB{i}',f'CD{i}'] for i in List1],[])
Out[25]: ['AB9016', 'CD9016', 'AB6416', 'CD6416', 'AB9613', 'CD9613']

Answered By: Akhilesh_IN

Loop a list to output each element twice with different prefix

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