[R] Vectorizing a "for" loop

[Thomas Lumley]

On Thu, 3 Aug 2006, Daniel Gerlanc wrote:

> Hello all,
>
> Consider the following problem:
>
> There are two vectors:
>
> rows <- c(1, 2, 3, 4, 5)
> columns <- c(10, 11, 12, 13, 14)
>
> I want to create a matrix with dimensions length(rows) x length(columns):
>
> res <- matrix(nrow = length(rows), ncol = length(columns))
>
> If "i" and "j" are the row and column indexes respectively, the values
> of the cells are abs(rows[i] - columns[j]).  The resultant matrix
> follows:
>
>     [,1] [,2] [,3] [,4] [,5]
> [1,]    9   10   11   12   13
> [2,]    8    9    10   11   12
> [3,]    7    8      9   10   11
> [4,]    6    7      8     9   10
> [5,]    5    6      7     8    9
>
> This matrix may be generated by using a simple "for" loop:
>
> for(i in 1:length(rows)){
>  for(j in 1:length(columns)){
>    res[i,j] <- abs(rows[i] - columns[j])
>  }
> }
>
> Is there a quicker, vector-based approach for doing this or a function
> included in the recommended packages that does this?
>

abs(outer(rows,columns,"-"))


 	-thomas