# R-alpha: tapply() strangeness

Kurt Hornik Kurt.Hornik@ci.tuwien.ac.at
Mon, 25 Aug 1997 09:10:20 +0200

```Here's something related to last week's apply() problem:

R> x <- matrix(1:20, nc = 4)
R> x
[,1] [,2] [,3] [,4]
[1,]    1    6   11   16
[2,]    2    7   12   17
[3,]    3    8   13   18
[4,]    4    9   14   19
[5,]    5   10   15   20

R> tapply(x, row(x), table)
[1] Numeric,4 Numeric,4 Numeric,4 Numeric,4 Numeric,4

???

In S,

> tapply(x, row(x), table)
\$"1":
1 6 11 16
1 1  1  1

\$"2":
2 7 12 17
1 1  1  1

\$"3":
3 8 13 18
1 1  1  1

\$"4":
4 9 14 19
1 1  1  1

\$"5":
5 10 15 20
1  1  1  1

which is (apart from labels) the same as

R> lapply(split(x, row(x)), table)
\$1
1  6 11 16
1  1  1  1

\$2
2  7 12 17
1  1  1  1

\$3
3  8 13 18
1  1  1  1

\$4
4  9 14 19
1  1  1  1

\$5
5 10 15 20
1  1  1  1

-k
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```