python dataframe: return column name in apply lambda

Question

I want to use apply(applymap) and lambda functions to get the column name for every element in dfx (a dataframe). The lambda function is used for mapping another dataframe into dfx, and it will be rewritten afterwards.

dfx

             A     B    C
2011-01-10   123   12   123 
2011-01-10   12    32   312
2011-01-11   44    1    30.99

pseudocode

output = dfx.apply(lambda r:r.column)

I don't know how to write this part "r:r.column"

intended output

             A    B    C
2011-01-10   A    B    C 
2011-01-10   A    B    C
2011-01-11   A    B    C

Any help is more than welcome! Thanks a lot!!

When you say that the function will be rewritten afterwards, do you mean that you need access to the column's values as well, or do you just need the column names? — Will
– Will, Commented Oct 2, 2018 at 18:10

jpp · Accepted Answer · 2018-10-02 17:45:43Z

1

You can assign via pd.DataFrame.iloc:

df.iloc[:] = df.columns

print(df)

            A  B  C
2011-01-10  A  B  C
2011-01-10  A  B  C
2011-01-11  A  B  C

answered Oct 2, 2018 at 17:45

jpp

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Comments

Alex · Accepted Answer · 2018-10-02 18:32:00Z

0

When you apply a function that function takes as an argument a Series corresponding to a row. You can change this to a column by passing the kwarg axis=1 to apply. A Series does not have columns (axis 1) - only an index (axis 0). Instead you can use the original DataFrame:

dfx.apply(lambda s: dfx.columns, axis=1)

returns

            A  B  C
2011-01-10  A  B  C
2011-01-10  A  B  C
2011-01-11  A  B  C

Note: There are certainly better ways of doing this that don't "use apply and lambda functions" as desired.

edited Oct 2, 2018 at 18:32

answered Oct 2, 2018 at 17:04

Alex

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1 Comment

Yuca Over a year ago

this doesn't work as reported I get Index([ 'A', 'B', ..]) at every row

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