[R] efficient way to do 2-level loops

[Petr Savicky]

On Thu, Sep 06, 2012 at 10:50:00AM -0400, Jie wrote:
> Dear All,
> 
> Suppose I have two loops and would like to convert it to sapply, how
> to achive the goal?
> Below is an example, the calculation inside the loop is not essential,
> so please do not use tricks for this part.
> a <- 1:5
> b <- 1:10
> resu <- matrix(NA,nrow=5,ncol=10)
> for (i in 1:5)
> {
>    for (j in 1:5)
>     {
>     resu[i,j]=a[i]+b[j]  # will be more complicated
>     }
> }

Hi.

For this part, when the output is a matrix, try the following.

  a <- 1:5
  b <- 1:10
  f <- function(a, b) { a+b }
  resu <- outer(a, b, FUN=f)
  resu

       [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
  [1,]    2    3    4    5    6    7    8    9   10    11
  [2,]    3    4    5    6    7    8    9   10   11    12
  [3,]    4    5    6    7    8    9   10   11   12    13
  [4,]    5    6    7    8    9   10   11   12   13    14
  [5,]    6    7    8    9   10   11   12   13   14    15

Hope this helps.

Petr Savicky.