[R] Build Design Matrix with avoiding loops
Martyn Byng
Martyn.Byng at nag.co.uk
Fri Jun 4 17:30:26 CEST 2010
Hi,
Something like
x = as.factor(rep(1:k,rep(n/k,k))
X = model.matrix(~x-1)
Might be what you are looking for
Martyn
-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Gildas Mazo
Sent: 04 June 2010 16:19
To: r-help at r-project.org
Subject: [R] Build Design Matrix with avoiding loops
Dear R users,
I'd like to build a simple design matrix in a efficient way. I naively
wrote the code below.
####
n = 15
k = 3
nbPerGrp = c(5,5,5)
xT <- list()
for (i in 1:k){
xT[[i]] <- rep(0, k)
xT[[i]][i] <- 1
}
X <- matrix(nrow = n, ncol = k) #design matrix
for (i in 1:nbPerGrp[1]){
X[i,] <- xT[[1]]
}
for (i in 1:k-1){
for (j in nbPerGrp[i]+1:nbPerGrp[i+1]){
X[j,] <- xT[[i]]
}}
for (i in 1:nbPerGrp[k]){
X[n - nbPerGrp[k] + i, ] <- xT[[k]]
}
X # That's I wanna get.
####
But as soon as n, k increase it takes too much time because of the
loops.
Then my question is how can I get such a design matrix X without too
much loops ? Which function I should look at ?
Thanks in advance for responding me,
Gildas
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