pandas loop column's values

Question

I have a pandas DataFrame, and one column’s values that I want to use are lists. I want to combine two elements one by one of each list, and output into another DataFrame.
For example, I have dataframe df, which contains col_a and col_b. The values of col_b are lists. I want to loop values of df.col_b, output a paired lists.

import pandas as pd

df=pd.DataFrame({'col_a':['ast1','ast2','ast3'],'col_b':[['text1','text2','text3'],['mext1','mext2','mext3'],['cext1','cext2']]})
df

    col_a   col_b
0   ast1    [text1, text2, text3]
1   ast2    [mext1, mext2, mext3]
2   ast3    [cext1, cext2]

I want this:

    col_a   col_b_1
0   ast1    [text1, text2]
1   ast1    [text1, text3]
2   ast1    [text2, text3]
3   ast2    [mext1, mext2]
4   ast2    [mext1, mext3]
5   ast2    [mext2, mext3]
6   ast3    [cext1, cext2]

akuiper · Accepted Answer · 2016-12-02 03:14:28Z

1

Assuming your col_a has unique value per row, you can use combinations from itertools to generate all two combinations of the list element:

from itertools import combinations
(df.groupby('col_a')['col_b']
   .apply(lambda x: pd.Series(list(combinations(x.iloc[0], 2))))
   .reset_index(level = 0))

#  col_a            col_b
#0  ast1    (text1, text2)
#1  ast1    (text1, text3)
#2  ast1    (text2, text3)
#0  ast2    (mext1, mext2)
#1  ast2    (mext1, mext3)
#2  ast2    (mext2, mext3)
#0  ast3    (cext1, cext2)

answered Dec 2, 2016 at 3:14

akuiper

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Comments

DYZ · Accepted Answer · 2016-12-02 03:26:40Z

You can use itertools to flatten the lists:

import itertools
series = df["col_b"].apply(lambda x: \
    pd.Series(list(itertools.combinations(x,2)))).stack()

The series must have a name to be mergeable with the "mother" dataframe:

series.name = "col_b_1"

Now, merge the two data objects and select the columns that you want:

result = df.merge(pd.DataFrame(series).reset_index(),
    left_index=True,
    right_on="level_0")[["col_a","col_b_1"]]

The result is a column of tuples; if this is not ok, .apply() function list() to it.

#   col_a         col_b_1
# 0  ast1  (text1, text2)
# 1  ast1  (text1, text3)
# 2  ast1  (text2, text3)
# 3  ast2  (mext1, mext2)
# 4  ast2  (mext1, mext3)
# 5  ast2  (mext2, mext3)
# 6  ast3  (cext1, cext2)

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