How to use an array in extracting values in a pandas dataframe

Question

I am very new to python and I don’t know how to proceed. I have an array and a DataFrame:

Column = np.array([3,1,3,2,4]) 
df = pd.DataFrame({
    1:[1,2,3,4,5],
    2:['A','B','C','D','E'],
    3:[6,7,8,9,0],
    4:['F','G','H','I','J']
})

  1  2  3  4
0 1  A  6  F
1 2  B  7  G
2 3  C  8  H
3 4  D  9  I
4 5  E  0  J

I would like to extract the values from the dataframe by iterating each row and use the array values to determine which column to extract the data and come up with a result [6,2,8,’D’,’J’]

6 2 8 D J

Don't remove your data. Please explain how you intend to extract the data; your selection rule doesn't seem to make any sense. Please clarify. — ifly6
– ifly6, Commented Aug 12, 2022 at 15:07

mozway · Accepted Answer · 2022-08-12 15:06:57Z

3

Use numpy indexing:

out = df.to_numpy()[np.arange(len(df)), Column-1]

NB. python indexing starts from 0 so we need to subtract 1 to Column

output: array([6, 2, 8, 'D', 'J'], dtype=object)

answered Aug 12, 2022 at 15:06

mozway

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Cameron Riddell · Accepted Answer · 2022-08-12 15:22:25Z

0

If your want to perform this same operation with pure pandas functions and label based

import pandas as pd

index = pd.MultiIndex.from_arrays([df.index, Column])

out = df.stack().loc[index]
print(out)
0  3    6
1  1    2
2  3    8
3  2    D
4  4    J
dtype: object

print(out.to_numpy())
[6 2 8 'D' 'J']

answered Aug 12, 2022 at 15:22

Cameron Riddell

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