Replace Value Nested List Comprehension

Question

I am struggling to figure out how to mix nested list comprehension with a replacing Here is an example of the nested for loops that I am trying to convert

array1 = [[1,2,3],[4,5,6],[7,8,9]]
array2 = [['a','b','c'],[7,4,1]]

for i in array1: 
    value=i[0] 
    for val2 in array2[1]: 
        if value==val2: 
            #convert i to array2[0][array2[1].index(val2)]

I've tried this but it just converts everything to ['a','b','c']

In [34]: [[x if x == y else array2[0] for y in array2[1]] for x in array1]

Out[34]:
[[['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']],
 [['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']],
 [['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]]

The result I am expecting is

In[35]:array1
Out[35]: 
[['c',2,3],['b',5,6],['a',8,9]]

If list comprehension isn't the best way solve, this, I would appreciate that answer also. I could do this, but I don't think it's the most efficient way.

for ii in range(len(array1))
    value = array1[ii]
    ...
            array1[ii] = array2[0][array2[1].index(val2)]

Laggs · Accepted Answer · 2020-12-09 21:41:20Z

1

A little bit of readability. The nested list comp is excessive IMO. array2 is really the wrong data structure for what it is doing, so my first inclination is to make it a dictionary/mapping, as that is how you are using it here.

array1 = [[1,2,3],[4,5,6],[7,8,9]]
array2 = [['a','b','c'],[7,4,1]]

conversion = {}
letter_list, num_list = array2
for num, letter in zip(num_list, letter_list):
    conversion[num] = letter

output = [
    [conversion.get(num, num) for num in num_list] for num_list in array1 
]

# [['c', 2, 3], ['b', 5, 6], ['a', 8, 9]]

edited Dec 9, 2020 at 21:41

answered Dec 9, 2020 at 21:36

Laggs

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1 Comment

DisappointedByUnaccountableMod Over a year ago

I agree this is much better than a one-liner. Well done!

Z Li · Accepted Answer · 2020-12-09 21:38:06Z

0

This should get what you want in one line:

[[[x]+z[1:] for z in array1 if z[0] == y][0] for x, y in zip(*array2)]

[['a', 8, 9], ['b', 5, 6], ['c', 2, 3]]

for loop version:

output = []
for i, x in enumerate(array2[0]):
    for y in array1:
        if y[0] == array2[1][i]:
            output.append([x]+y[1:])
output

answered Dec 9, 2020 at 21:38

Z Li

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6 Comments

DisappointedByUnaccountableMod Over a year ago

Yeah the one-liner is like totally unmaintainable - an incomprehensible comprehension.

Z Li Over a year ago

Op wanted the list comprehension version...@barny

DisappointedByUnaccountableMod Over a year ago

Some people want to e.g. juggle knives, doesn’t make it a good idea. (Wasn’t my downvote, BTW)

Z Li Over a year ago

@barny I agree with you. I know the pain to maintain "fancy" codes

theastronomist Over a year ago

I like your method. I agree, the loop version might be best. I recommend swapping the order of the loops so the order of output matches array1

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