Is there a way in python to use one column as index to change element of other string column?

Question

I have a DataFrame with two columns that look something like this:

 A B 0 '_______' [2,3] 1 '_______' [0] 2 '_______' [1,4,6]

where one column is a string with 7 "_" and the other column contains a numpy array with different lengths. My goal is to change column A using B as indexes so it looks like this:

 A B 1 '__23___' [2,3] 2 '0______' [0] 3 '_1__4_6' [1,4,6]

My code seems to work but I keep getting the error:

SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.

I don't understand how I fix this error. My code is:

for i in range(len(df)): row = df.iloc[i,:].copy() numbers = row['B'] for j in numbers: loop_string = df['A'][i] df['A'][i] = loop_string[:j] + str(j) + loop_string[j+1:]

Also the fact that I need two for loops bothers me this must be possible an other more efficient way. Can anyone help me?

Shaido · Accepted Answer · 2021-09-17 08:28:22Z

You can use apply to use a custom function on the B column:

df['A'] = df['B'].apply(lambda l: ''.join([str(i) if i in l else '_'for i in range(7)]))

The above does not consider the original value of A but instead creates an entirely new string column.

Result:

 A B 0 __23___ [2, 3] 1 0______ [0] 2 _1__4_6 [1, 4, 6]

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