place values into a matrix efficiently?

On 20/11/11 16:27, Carl Witthoft wrote:

This question attacked me as I was thinking about matrix value updates.

I probably will never need to do this, but wanted to ask if there are
efficient methods to perform the for-loop in the following sequence.

%xymat<-matrix(rep(0,100) nr=10,nc=10) # empty matrix
%x<-1:10
%y<-sample.int(10,10,rep=T)

%for (j in 1:10) xymat[x[j],y[j]] <- some_function(x[j],y[j]) #to
create either false-color or 3D map .
plot(0:1,0:1,t='n')
% rasterImage(xymat/max(xymat),0,0,1,1,interp=F)

This will produce a raster image of the original data(x vs y) that
looks like plot(x,y) .

Anyway, I just seem to be blanking: is there some vectorized way to
place values, or even a constant value, into all elements of xymat
whose row,col coordinates match ordered pairs in x and y ?

Yes. Use ``xymat[m]'' where m is a 2-column matrix of row and column
indices.

E.g.:

xymat <- matrix(0,nrow=10,ncol=10)
x <- 1:10
y <- sample.int(10,10,rep=TRUE)
set.seed(42)
for (j in 1:10) xymat[x[j],y[j]] <- runif(1)

gorp <- matrix(0,nrow=10,ncol=10)
set.seed(42)
gorp[cbind(x,y)] <- runif(10)
all.equal(xymat,gorp) # TRUE!!!

cheers,

Rolf Turner