Pandas DataFrame: Simple assignment - the floor of the difference of two column values to another column value, loop does not work

user_5645165 0
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How would you perform the following in Pandas?

import math
for index, row in data.iterrows():
  if row["year"] == 0:
    row["year"] = math.floor((row["death"] - row["birth"])/2)

This loop does not work but what i am trying to do is to assign the floor of the difference divided by 2 of the death and birth columns to the column year if the column year has the value 0. I know you should avoid looping in Pandas and this probably has a simple solution but i can not figure it out right now.

pandas python
2 Answers

Use numpy.where:

import numpy as np

df['newcol'] = np.where(df['year'] == 0, math.floor((df['death'] - df['birth'])/2), df['year'])

This essentially is:

np.where(condition, if True then, if False then)
user_58208140

Slice with loc:

df.loc[df['year'] == 0, 'year'] = np.floor((df.loc[df['year'] == 0, 'death'] - df.loc[df['year'] == 0, 'birth']) / 2)

Maybe a more readable solution:

mask = df['year'] == 0
df.loc[mask, 'year'] = np.floor((df.loc[mask, 'death'] - df.loc[mask, 'birth']) / 2)
user_141577500
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