Select rows from data frame based on condition and apply function

Question

I have following example of data frame

  Marked       City
1   0        New York
2   1         California

I want to select rows from data frame, where Marked is 0 and apply some function to City row. The data frame is big (more than 1M row) and function to apply is complex, so the option to use condition statement to whole data frame is bad, since i still iterate whole 1M records. Selection of rows in data frame based on condition reduces the number rows to 300k and applying function to them is much faster

Quang Hoang · Accepted Answer · 2020-12-23 17:25:26Z

1

Use loc access:

def some_func(x): return 1

mask = df.Marked == 0

df.loc[mask, 'new_col'] = df.loc[mask].apply(some_func, axis=1)

answered Dec 23, 2020 at 17:25

Quang Hoang

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1 Comment

Alex Nikitin Over a year ago

Thank you for your answer! I tried something like this and got TypeError: 'Series' objects are mutable, thus they cannot be hashed

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