# [R] Help w/ matrix calc

Adaikalavan Ramasamy ramasamy at cancer.org.uk
Thu Jul 29 15:04:29 CEST 2004

```> m <- matrix( 1:12, nc=3 )
> m
[,1] [,2] [,3]
[1,]    1    5    9
[2,]    2    6   10
[3,]    3    7   11
[4,]    4    8   12

> apply(m, 1, mean) # row means
[1] 5 6 7 8

> apply(m, 2, mean) # column means
[1]  2.5  6.5 10.5

Replace 'mean' with 'var' if you want variances instead.

On Thu, 2004-07-29 at 12:38, eesteves at ualg.pt wrote:
> Dear All,
> Help is needed! I have a matrix with frequencies of fish larvae per length
> class (var. sl) and age-group (var. median.no) obtained with
>
> >k<-table(cut(sl,(5:22)),median.no)
> >k[2:5,1:5] #to ilustrate k
>
>         4  5  6  7
> (6,7]   3  1  0  0
> (7,8]   3  0  1  0
> (8,9]   3  4  3  5
> (9,10]  3 15  7 13
>
> from this matrix I would like to obtain the mean age per length class i.e.
> vector of line means. How can I do this? If, instead of the means I wanted the
> variances, how could I do it?
> Thanks in advance,
> Eduardo Esteves
>
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