Conditional lambda apply across dataframe based on list equality

Question

I have a dataframe df whos columns contain lists of strings

df =      A               B 
        ['-1']    , ['0','1','2']
     ['2','4','3'],     ['2']
      ['3','8']   ,    ['-1']

I want to get the length of all the lists except the ones that are ['-1'] for the lists that are ['-1'] I want them to be -1

Expected output:

df = A   B 
    -1,  3
     3,  1
     2, -1

I've tried

df.apply(lambda x: x.str.len() if not x == ['-1'] else -1)

and got the error ('Lengths must match to compare', (132,), (1,))

I have also tried

data_copy[colBeliefs] = data_copy[colBeliefs].apply(lambda x: x.str.len() if '-1' not in x else -1)

but this produces the wrong output where ['-1'] becomes 1 rather than -1

I'm not sure how I can apply functions to a dataframe based on the whether an entry in a dataframe is equal to a list, or whether an item is in a list.

EDIT: Output of df.head().to_dict()

{'A': {0: ['-1'],
       1: ['2','4','3'],
       2: ['3','8']},
 'B': {0: ['0','1','2'],
       1: ['2'],
       2: ['-1']}}

please include the output of df.head().to_dict() in your question — Onyambu
– Onyambu, Commented Apr 25, 2022 at 17:10
@onyambu I have included the output of df.head().to_dict(), or did you want the output of that after data_copy[colBeliefs] = data_copy[colBeliefs].apply(lambda x: x.str.len() if '-1' not in x else -1) — SemicolonExpected
– SemicolonExpected, Commented Apr 25, 2022 at 17:16

Onyambu · Accepted Answer · 2022-04-25 17:29:03Z

1

You could do:

df.applymap(lambda x: -1 if (ln:=len(x)) == 1 and x[0] == '-1' else ln)

   A  B
0 -1  3
1  3  1
2  2 -1

Edit:

If yousing python < 3.8 Use the following:

df.applymap(lambda x: -1 if len(x) == 1 and x[0] == '-1' else len(x))

edited Apr 25, 2022 at 17:29

answered Apr 25, 2022 at 17:20

Onyambu

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2 Comments

SemicolonExpected Over a year ago

I'm getting

df.applymap(lambda x: -1 if (ln:=len(x)) == 1 and x[0] == '-1' else ln)                                                       SyntaxError: invalid syntax

pointing to ln:=len(x)

Onyambu Over a year ago

@SemicolonExpected The syntax maybe because of the version you are using. do df.applymap(lambda x: -1 if len(x) == 1 and x[0] == '-1' else len(x))

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