Interweave groups in Pandas

Question

I have a DataFrame that I want "intereaved" row-wise by groups.

For example, this DataFrame:

Group	Score
A	10
A	9
A	8
B	7
B	6
B	5

The desired result would be grabbing the first of A, and the first of B, then the second of A, then the second of B, etc.

Group	Score
A	10
B	7
A	9
B	6
A	8
B	5

Any ideas?

Asked By: David Davó

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Source

Answer 1

You can use the cumcount of each Group as a sorting key :

out = df.sort_values("Group", key=lambda _: df.groupby("Group").cumcount())

Or better, as suggested by @mozway, you can use one of these variants:

out = df.sort_values(by="Group", key=lambda s: s.groupby(s).cumcount())

out = df.iloc[np.argsort(df.groupby("Group").cumcount())]

Output :

print(out)

  Group  Score
0     A     10
3     B      7
1     A      9
4     B      6
2     A      8
5     B      5

Answered By: Timeless

Answer 2

Solution 1

Another possible, which calculates the number of elements by group, n, and then, by reshaping and flattening the index of the dataframe with a convenient order (order F), gets the proper index to reindex (with iloc) and obtain the wanted output:

n = len(df) // df['Group'].nunique()
df.iloc[df.index.values.reshape(-1, n).flatten(order='F')]

Solution 2

Yet another possible solution, which is based on list comprehension. This solution may not be as efficient as the previous one, because there is a groupby:

g = df.groupby('Group')
df.iloc[[index for y in zip(*[x.index for _, x in g]) for index in y]]

Output

  Group  Score
0     A     10
3     B      7
1     A      9
4     B      6
2     A      8
5     B      5

Answered By: PaulS

Interweave groups in Pandas

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