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Asked: 2026-05-16T02:27:04+00:00

I’m using reshape in R to compute aggregate statistics over columns of a data.frame.

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I’m using reshape in R to compute aggregate statistics over columns of a data.frame. Here’s my data.frame:

> df
  a a b b ID
1 1 1 1 1  1
2 2 3 2 3  2
3 3 5 3 5  3

which is just a little test data.frame to try and understand the reshape package. I melt, and then cast, to try and find the mean of the as and the bs:

> melt(df, id = "ID") -> df.m
> cast(df.m, ID ~ variable, fun = mean)
  ID a b
1  1 1 1
2  2 2 2
3  3 3 3

Argh! What? Was hoping the mean of c(2,3) was 2.5 and so on. What’s going on? Here’s a thing:

> df.m
   ID variable value
1   1        a     1
2   2        a     2
3   3        a     3
4   1        a     1
5   2        a     2
6   3        a     3
7   1        b     1
8   2        b     2
9   3        b     3
10  1        b     1
11  2        b     2
12  3        b     3

what’s going on? Where did both my 5s go? Do I have a very basic misunderstanding going on here? If so: what is it?

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