[R] apply on rows and columns?

[Sarah Goslee]

Hi,

On Wed, Nov 16, 2011 at 3:13 PM,  <rkevinburton at charter.net> wrote:
>
> I have the following scenario:
>
>> m <- matrix(1:4, ncol=2)
>> m
>      [,1] [,2]
> [1,]    1    3
> [2,]    2    4
>> apply(m, 2, sum)
> [1] 3 7
>> apply(m, 1, sum)
> [1] 4 6
>
> So I can apply to rows *or* columns. According to the documentation
> (?apply)
>
> MARGIN a vector giving the subscripts which the function will be applied
> over. E.g., for a matrix 1 indicates rows, 2 indicates columns, c(1, 2)
> indicates rows and columns. Where X has named dimnames, it can be a
> character vector selecting dimension names.
>
>
> But I get the following results:
>
>> apply(m, c(1,2), sum)
>      [,1] [,2]
> [1,]    1    3
> [2,]    2    4
>
> How am I to interpret this result?

I'm pretty sure R is taking the sum of m[1,1] and putting it [1,1],
and the sum of m[1,2] and putting it in [1,2] and so on. You
instructed apply() to work on rows and columns *simultaneously*,
rather than sequentially.

apply() on c(1,2) is useful if you have a matrix that's three-dimensional,
but not so much if it's two dimensional.

What are you trying to accomplish?

Sarah




-- 
Sarah Goslee
http://www.functionaldiversity.org