make 2d array from pandas dataframe

Question

I have a pandas dataframe over here with two columns: participant names and reaction times (note that one participant has more measures oh his RT).

    ID RT
0  foo  1
1  foo  2
2  bar  3
3  bar  4
4  foo  1
5  foo  2
6  bar  3
7  bar  4
8  bar  4

I would like to get a 2d array from this where every row contains the reaction times for one participant.

[[1,2,1,2]
[3,4,3,4,4]]

In case it's not possible to have a shape like that, the following options for obtaining a good a x b shape would be acceptable for me: fill missing elements with NaN; truncate the longer rows to the size of the shorter rows; fill the shorter rows with repeats of their mean value.

I would go for whatever is easiest to implement.

I have tried to sort this out by using groupby, and I expected it to be very easy to do this but it all gets terribly terribly messy :(

HYRY · Accepted Answer · 2013-11-07 05:10:06Z

4

import pandas as pd
import io
data = io.BytesIO("""    ID RT
0  foo  1
1  foo  2
2  bar  3
3  bar  4
4  foo  1
5  foo  2
6  bar  3
7  bar  4
8  bar  4""")

df = pd.read_csv(data, delim_whitespace=True)
df.groupby("ID").RT.apply(pd.Series.reset_index, drop=True).unstack()

output:

    0  1  2  3   4
ID                 
bar  3  4  3  4   4
foo  1  2  1  2 NaN

answered Nov 7, 2013 at 5:10

HYRY

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2 Comments

TheChymera Over a year ago

awesome! this seems to do exactly what I wanted! pity that pandas doesn't offer any more direct way for this functionality... One more thing: what if my column name of interest had a space in it ('reaction times' instead of 'RT')?

HYRY Over a year ago

try: df.groupby("ID")["reaction times"]...

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