[R] RES: applying function to multiple columns of a matrix

[Filipe Leme Botelho]

Hi Frederico. I would keep the data as it is, create two small vectors referring to the ranges and use a mapply (as a sapply but with multiple variables) for the function. Hope the example below is helpful, although as usual someone out there will have a better solution for it.

> dta <- c()
> for (i in 1:12) dta <- cbind(dta,matrix(i,5,1))
> dta
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
[1,]    1    2    3    4    5    6    7    8    9    10    11    12
[2,]    1    2    3    4    5    6    7    8    9    10    11    12
[3,]    1    2    3    4    5    6    7    8    9    10    11    12
[4,]    1    2    3    4    5    6    7    8    9    10    11    12
[5,]    1    2    3    4    5    6    7    8    9    10    11    12
>
>
> rng.a <- seq(1,10,by=3)
> rng.b <- seq(3,12,by=3)
> rng.a
[1]  1  4  7 10
> rng.b
[1]  3  6  9 12
> mapply(x=rng.a, y=rng.b, function(x,y) sum(dta[,c(x:y)]))
[1]  30  75 120 165
>

Cheers,
Filipe

-----Mensagem original-----
De: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Em nome de Federico Calboli
Enviada em: terça-feira, 12 de julho de 2011 10:07
Para: r-help
Assunto: [R] applying function to multiple columns of a matrix

Hi,

I want to apply a function to a matrix, taking the columns 3 by 3. I could use a for loop:

for(i in 1:3){ # here I assume my data matrix has 9 columns
j = i*3
set = my.data[,c(j-2,j-1,j)]
my.function(set)
}

which looks cumbersome and possibly slow. I was hoping there is some function in the apply()/lapply() families that could take 3 columns at a time. 

I though of turning mydata in a list, then using lapply()

new.data = list(my.data[,1:3], my.data[,4:6], my.data[,7:9])
lapply(new.data, my.function)

but that might incur in too much memory penalty and does have the issue of requiring a for loop to create the list (not all my data is conveniently of 9 columns only).

Any suggestion would be much appreciated.

Bw

Federico


--
Federico C. F. Calboli
Department of Epidemiology and Biostatistics
Imperial College, St. Mary's Campus
Norfolk Place, London W2 1PG

Tel +44 (0)20 75941602   Fax +44 (0)20 75943193

f.calboli [.a.t] imperial.ac.uk
f.calboli [.a.t] gmail.com

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