Replace an element in a pandas dataframe column if present in another dataframe column

Question

There are two dataframes given by:

train = pd.DataFrame({'Alpha': [10, 22, 10, 45, 44, 21, 62, 84, 32, 97, 38]})
test = pd.DataFrame({'Alpha': [10, 97, 32, 34, 44, 76, 49]})

If each value of test is not present in train, then the test values should be replaced with -1.

Expected output: [10, 97, 32, -1, 44, -1, -1] since 34, 76 and 49 are not present in train.

What I tried:

for x in test.Alpha:
    if x not in train.Alpha:
        test = test.Alpha.replace(x, -1)

Not working.

Quang Hoang · Accepted Answer · 2020-07-20 16:42:18Z

2

You can do with isin:

test.loc[~test.Alpha.isin(train.Alpha), 'Alpha'] = -1

Output of test:

answered Jul 20, 2020 at 16:42

Quang Hoang

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