[R] Help w/ matrix calc

[James MacDonald]

For row or column means with a matrix of any size, you will be much
better served by using
rowMeans()
colMeans()

Best,

Jim



James W. MacDonald
Affymetrix and cDNA Microarray Core
University of Michigan Cancer Center
1500 E. Medical Center Drive
7410 CCGC
Ann Arbor MI 48109
734-647-5623

>>> Adaikalavan Ramasamy <ramasamy at cancer.org.uk> 07/29/04 09:04AM >>>
> 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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