Set list to subset of pandas dataframe

Question

I wish to set a list of lists in a column (say "B") for a subset of rows. Suppose my dataframe (df) looks like below:

import pandas as pd
import numpy as np
np.random.seed(42)

df = pd.DataFrame({"A": np.random.randn(5)})
idx = df["A"] < 0
mylist = np.random.randint(0, 5, (idx.sum(), 3)).tolist()

df["B"] = None

First attempy was to do this which chucked an error: df.loc[idx, "B"] = mylist. The next attempt df.loc[idx, "B"] = pd.Series(mylist) ran but the values in the wrong place. The final thing I got to work was:

df.loc[idx, "B"] = pd.Series(mylist, index=df.index[idx])

My question is, is this the only way of doing this? Feels like there might be a simpler way to achieve the same effect?

In my opinion your solution is nice.

jezrael
– jezrael

2020-12-01 06:52:56 +00:00
Commented Dec 1, 2020 at 6:52 — jezrael
– jezrael, Commented Dec 1, 2020 at 6:52

meTchaikovsky · Accepted Answer · 2020-12-01 06:46:46Z

1

You can use at method, for example

my_list = [np.random.rand(5).tolist() for _ in range(6)]

test_df = pd.DataFrame({'A':np.random.randint(-1,1,6)})
idx = np.where(test_df['A'] < 0)[0]
test_df['B'] = np.random.rand(len(my_list)).astype('object')

for ind in idx:test_df.at[ind,'B'] = my_list[ind]

answered Dec 1, 2020 at 6:46

meTchaikovsky

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1 Comment

sachinruk Over a year ago

I'm not 100% sure but I feel like at should be a last resort. I've +1ed for the answer regardless. Thanks!

Sunny · Accepted Answer · 2020-12-06 08:50:50Z

0

This way should work fine:

df['B'].loc[idx]=mylist

answered Dec 6, 2020 at 8:50

Sunny

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