Outer product from matrix by row and a vector
On Aug 7, 2012, at 11:26 PM, Petr Savicky wrote:
On Wed, Aug 08, 2012 at 06:03:19AM +0000, Ingo Reinhold wrote:
Hi Jeff, David,
what I'm trying to do is speed the currently used nest for loop
a<-matrix(c(1:6),ncol=3)
b<-c(1,2,3,4)
result<-matrix(rep(0, times=dim(a)[1]*length(b)),nrow=dim(a)[1])
for(ii in 1:dim(a)[1]){
for(jj in 1:length(b)){
result[ii,jj]<-a[ii,1]+a[ii,3]/b[jj]
}
}
giving the result
[,1] [,2] [,3]
[1,] 1 3 5
[2,] 2 4 6
That is not what result equals (as Petr illustrates.)
Hi.
The printed matrix is "a". The above code yields on my computer
[,1] [,2] [,3] [,4]
[1,] 6 3.5 2.666667 2.25
[2,] 8 5.0 4.000000 3.50
Try the following
out <- a[, 1] + a[, 3] %o% (1/b)
max(abs(out - result))
[1] 4.440892e-16
Hope this helps.
It's also the case that the printed result is just the transpose of
what I earlier offered as a possible implementation of the OP's pseudo-
code.:
> identical( t( apply(a, 1, function(x) { x[1] + x[3]/ b } ) ), result)
[1] TRUE
David Winsemius, MD Alameda, CA, USA