matrix algebra for constructing a special matrix?

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URL: <https://stat.ethz.ch/pipermail/r-help/attachments/20121026/fb15c10c/attachment.pl>
I think f0 (from your code) and f1 give identical results

f0 <- function (m = 3, n = 4) 
{
   stopifnot(m>0, n>0)
    mn <- m * n
    k <- m + n - 1
    X <- matrix(0, mn, k)
    for (i in 1:n) {
        wr <- 1:m + (i - 1) * m
        wc <- rev(1:m + (i - 1))
        where <- cbind(wr, wc)
        X[where] <- 1
    }
    X
}

f1 <-
function (m = 3, n = 4) 
{
    stopifnot(m>0, n>0)
    whereInRow <- as.vector(outer(m:1, 0:(n - 1), `+`))
    mn <- m * n
    X <- matrix(0, mn, m + n - 1)
    X[cbind(1:mn, whereInRow)] <- 1
    X
}

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
Of Camarda, Carlo Giovanni
Sent: Friday, October 26, 2012 8:13 AM
To: r-help at stat.math.ethz.ch
Subject: [R] matrix algebra for constructing a special matrix?

Dear R-users,

would it be a better way to construct the matrix below without using any for-loop or
model.matrix? preferably with some matrix algebra?

Thanks in advance,
Carlo Giovanni Camarda

## dimensions
m <- 3
n <- 4
mn <- m*n
k <- m+n-1

## with a for-loop
X <- matrix(0, mn, k)
for(i in 1:n){
  wr <- 1:m+(i-1)*m
  wc <- rev(1:m+(i-1))
  where <- cbind(wr,wc)
  X[where] <- 1
}

## using model.matrix
ff <- factor(c(outer(m:1, seq(0,m), "+")))
X1 <- matrix(model.matrix(~-1+ff), mn, k)

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