[R] place values into a matrix efficiently?
carl at witthoft.com
Sun Nov 20 17:05:37 CET 2011
Excellent. That's what I was looking for.
On 11/19/11 11:16 PM, Rolf Turner wrote:
> 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
>> %for (j in 1:10) xymat[x[j],y[j]] <- some_function(x[j],y[j]) #to
>> create either false-color or 3D map .
>> % 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
> xymat <- matrix(0,nrow=10,ncol=10)
> x <- 1:10
> y <- sample.int(10,10,rep=TRUE)
> for (j in 1:10) xymat[x[j],y[j]] <- runif(1)
> gorp <- matrix(0,nrow=10,ncol=10)
> gorp[cbind(x,y)] <- runif(10)
> all.equal(xymat,gorp) # TRUE!!!
> Rolf Turner
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