st: appending to a file

[JOHN ANTONAKIS <john.antonakis@unil.ch>]

I have two files. One in long and one in wide format.

In the long file, I have observations at the group level:

group  x  y
1
1
1
1
2
2
2
2
k


In the wide file, I have data at the group level, including a variable day:

group day x y
1         1
2         1
3         2
4         2
5         3
k

How can I append the day variable from the wide file to the long file?

Best,
J.
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