# [R] apply and table

peter dalgaard pdalgd at gmail.com
Sun May 19 19:27:22 CEST 2013

```On May 19, 2013, at 16:22 , Jinsong Zhao wrote:

> Hi there,
>
> I have the following code:
>
> z <- matrix(c("A", "A", "B", "B", "C", "C", "A", "B", "C"), ncol = 3)
> apply(z, 2, table, c("A", "B", "C"))
>
> which give correct results.
>
> However, the following code:
>
> apply(z[,1,drop=FALSE], 2, table, c("A", "B", "C"))
>
> which does not give what I expect. I have been thought it should give the same result as:
>
> apply(z, 2, table, c("A", "B", "C"))[[1]]
>
> What's the difference? Does apply not apply to column vector?

To clue the casual reader in, the former gives:

> apply(z, 2, table, c("A", "B", "C"))
[[1]]

A B C
A 1 1 0
B 0 0 1

[[2]]

A B C
B 1 0 0
C 0 1 1

[[3]]

A B C
A 1 0 0
B 0 1 0
C 0 0 1

whereas the latter gives the first of the tables strung out as a 6x1 matrix.

This is a generic awkwardness of apply(). It tries to simplify the result (similar to sapply), so if the result for all columns have the same length (say, k), it converts them to a (k x C) matrix. If the results are incommensurable, it gives up and returns a list.

So if we modify the code to always give a 3x3 matrix, the following happens:

> ABC <- LETTERS[1:3]
> apply(z, 2, function(x) table(factor(x, levels=ABC), ABC))
[,1] [,2] [,3]
[1,]    1    0    1
[2,]    0    1    0
[3,]    0    0    0
[4,]    1    0    0
[5,]    0    0    1
[6,]    0    1    0
[7,]    0    0    0
[8,]    1    0    0
[9,]    0    1    1

You can't turn simplification off in apply(), but a passable workaround is

> tapply(z, col(z), function(x) table(factor(x, levels=ABC), ABC))
\$`1`
ABC
A B C
A 1 1 0
B 0 0 1
C 0 0 0

\$`2`
ABC
A B C
A 0 0 0
B 1 0 0
C 0 1 1

\$`3`
ABC
A B C
A 1 0 0
B 0 1 0
C 0 0 1

>
> Another question: how to output the table in squared matrix (or data frame)? For example:
>
> > table(c("C", "B", "B"), c("A", "B", "C"))
>
>    A B C
>  B 0 1 1
>  C 1 0 0
>
> I hope to get the result something like:
>
>    A B C
>  A 0 0 0
>  B 0 1 1
>  C 1 0 0
>
> Is there a way that can output that?
>
> Any suggestions will be really appreciated. Thanks in advance.

--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

```