Concatenate strings along the off diagonals

Question

Setup

import pandas as pd
from string import ascii_uppercase

df = pd.DataFrame(np.array(list(ascii_uppercase[:25])).reshape(5, 5))

df

   0  1  2  3  4
0  A  B  C  D  E
1  F  G  H  I  J
2  K  L  M  N  O
3  P  Q  R  S  T
4  U  V  W  X  Y

Question
How do I concatenate the strings along the off diagonals?

Expected Result

0        A
1       FB
2      KGC
3     PLHD
4    UQMIE
5     VRNJ
6      WSO
7       XT
8        Y
dtype: object

What I Tried

df.unstack().groupby(sum).sum()

This works fine. But @Zero's answer is far faster.

@pirRSquared much similar to https://stackoverflow.com/questions/6313308/get-all-the-diagonals-in-a-matrix-list-of-lists-in-python — Bharath M Shetty, Oct 06 '17 at 09:01

Zero · Accepted Answer · 2017-10-06T04:48:44.030

3

You could do

In [1766]: arr = df.values[::-1, :] # or np.flipud(df.values)

In [1767]: N = arr.shape[0]

In [1768]: [''.join(arr.diagonal(i)) for i in range(-N+1, N)]
Out[1768]: ['A', 'FB', 'KGC', 'PLHD', 'UQMIE', 'VRNJ', 'WSO', 'XT', 'Y']

In [1769]: pd.Series([''.join(arr.diagonal(i)) for i in range(-N+1, N)])
Out[1769]:
0        A
1       FB
2      KGC
3     PLHD
4    UQMIE
5     VRNJ
6      WSO
7       XT
8        Y
dtype: object

You may also do arr.diagonal(i).sum() but ''.join is more explicit.

edited Oct 06 '17 at 04:48

answered Oct 06 '17 at 04:34

Zero

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This is beautiful! Could you do `arr = df.values[::-1, :]` and avoid the vertical flip for every iteration through the comprehension? – piRSquared Oct 06 '17 at 04:44

Concatenate strings along the off diagonals

1 Answers1