# [R] Loops

Richard D. Morey r.d.morey at rug.nl
Sun Jan 27 12:50:22 CET 2013

```> Dear Contributors,
> I am asking help on the way how to solve a problem related to loops for
> that I always get confused with.
> I would like to perform the following procedure in a compact way.
>
> Consider that p is a matrix composed of 100 rows and three columns. I need
> to calculate the sum over some rows of each
> column separately, as follows:
>
> fa1<-(colSums(p[1:25,]))
>
> fa2<-(colSums(p[26:50,]))
>
> fa3<-(colSums(p[51:75,]))
>
> fa4<-(colSums(p[76:100,]))
>
> fa5<-(colSums(p[1:100,]))
>
>
>
> and then I need to  apply to each of them the following:
>
>
> fa1b<-c()
>
> for (i in 1:3){
>
> fa1b[i]<-(100-(100*abs(fa1[i]/sum(fa1[i])-(1/3))))
>
> }
>

I think I'd do it this way (correcting for the presumed error that Rui Barradas noted):

> dim( p ) = c(25,4,3)

> p2 = apply(p, c(2,3), sum)
> p3 = t(apply(p2, 1, function(fa) 100-(100*abs(fa/sum(fa)-(1/3))) ) )

p3 now contains all your results except the one including all the data, which is trivial to compute.

--
Richard D. Morey
Assistant Professor
Psychometrics and Statistics
Rijksuniversiteit Groningen / University of Groningen
http://drsmorey.org/research/rdmorey

```