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Gulzar
Gulzar

Reputation: 27946

How to subsample a dataframe?

I have dfi= (i is the index)

i     a
0     0
1     1
2     2
3     3
4     4
5     5
6     6
7     7
8     8
9     9

and dfj= (j is the index)

j     b
0     0
1     1
2     2
3     3
4     4

I want to add an a column to dfj such that the output would be dfi subsampled by a factor of 2

j     b     a
0     0     0
1     1     2
2     2     4
3     3     6
4     4     8

My attempt:

idx = pd.IndexSlice
dfj["a"] = dfi.loc[idx[::2]]

This fails because dfi.loc[idx[::2]] returns indices that don't fit for dfj. Looks like I need to divide the index, or do another workaround.

What's the correct syntax for that?

Thanks

Upvotes: 1

Views: 187

Answers (1)

Quang Hoang
Quang Hoang

Reputation: 150745

dfi.iloc[::2] or dfi.loc[idx[::2]] has different indexes than dfj, while assignment will align the indexes for you. You can pass the values instead:

dfj['a'] = dfi['a'].iloc[::2].values

Output:

   b  a
j      
0  0  0
1  1  2
2  2  4
3  3  6
4  4  8

Upvotes: 1

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