add array into pandas dataframe

Question

I have a pandas dataframe called A with one column called "a":

date a
2016-01-19 3
2016-01-20: 1
2016-01-21: 2

I have one array that looks like: [4,3,2]. I want to insert this array into the dataframe and give the new column the name b. How do I do that?

Expected output:

date a b
2016-01-19 3 4
2016-01-20: 1 3
2016-01-21: 2 2

If your list (or your numpy array) and your index have the same length you can do directly something like df['b'] = your_list — mgc
– mgc, Commented Jan 21, 2016 at 19:45
Then the values for b didn't end up next to a. I guess that was because of the date index. — Filip Eriksson
– Filip Eriksson, Commented Jan 21, 2016 at 19:46

Anton Protopopov · Accepted Answer · 2016-01-21 21:06:01Z

3

As @mgc pointed out in the comment you could do df['b'] = l:

import pandas as pd
from io import StringIO
data="""
    date a
    2016-01-19 3
    2016-01-20 1
    2016-01-21 2
    """
df = pd.read_csv(StringIO(data), sep='\s+')

df = df.set_index('date')
df.index = pd.to_datetime(df.index)

print(df)
            a
date         
2016-01-19  3
2016-01-20  1
2016-01-21  2

l = [4,3,2]

df['b'] = l

print(df)
            a  b
date            
2016-01-19  3  4
2016-01-20  1  3
2016-01-21  2  2

answered Jan 21, 2016 at 21:06

Anton Protopopov

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