Creating a column of strings that need to be in numeric order

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I have a pandas data frame where the 'combined_id' column is basically a combination of the first two columns. I want to create the combined_id column such that the smaller number comes before the larger one. I know I can swap around the first two columns to be listed in order of smallest/largest, but I want the order of the first two cols to remain as they are.

what I have:

Student1         Student2       combined_id
id/USER321      id/USER329      id/USER321_USER329
id/USER123      id/USER123      id/USER123_USER123
id/USER439      id/USER122      id/USER439_USER122
id/USER999      id/USER333      id/USER999_USER333

Desired

Student1         Student2       combined_id
id/USER321      id/USER329      id/USER321_USER329
id/USER123      id/USER123      id/USER123_USER123
id/USER439      id/USER122      id/USER122_USER439
id/USER999      id/USER333      id/USER333_USER999
pandas python
2 Answers

If there are only two student columns, finding row-wise min and max will work.

import pandas as pd

df = pd.DataFrame({
    'Student1': ['id/USER321', 'id/USER123', 'id/USER439', 'id/USER999'],
    'Student2': ['id/USER319', 'id/USER123', 'id/USER122', 'id/USER333'],
})

smaller = df.min(axis=1)
larger = df.max(axis=1)
df["combined_id"] = smaller + "_" + larger

df

#     Student1    Student2            combined_id
#0  id/USER321  id/USER319  id/USER319_id/USER321
#1  id/USER123  id/USER123  id/USER123_id/USER123
#2  id/USER439  id/USER122  id/USER122_id/USER439
#3  id/USER999  id/USER333  id/USER333_id/USER999
Kota Mori0

Edit: approach is to apply a sort by row and then join the strings

#slightly changed example table
df = pd.DataFrame({
    'Student1': ['id/USER321', 'id/USER123', 'id/USER439', 'id/USER999'],
    'Student2': ['id/USER319', 'id/USER123', 'id/USER122', 'id/USER333'],
})

df['combined_id'] = df[['Student1','Student2']].apply(sorted, axis=1).str.join('_')

Output

enter image description here

mitoRibo0
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