Avoid two for loops in R

Question

I have a R code that can do convolution of two functions...

convolveSlow <- function(x, y) {  
nx <- length(x); ny <- length(y)  
xy <- numeric(nx          


        

Answer 1

1. For vectors, you index with [], not [[]], so use xy[ij] etc

2. Convolution doesn't vectorise easily but one common trick is to switch to compiled code. The Writing R Extensions manual uses convolution as a running example and shows several alternative; we also use it a lot in the Rcpp documentation.