R help - Mar 2011 - Combination that adds a value

Dear all, I have three vectors "x<-1:n", "y<-1:m" and
"z<-2:(n+m)". I need to find all combinations of "x" and
"y" that add up a given value of "z", e.g.,  for
"z==3" the combinations will be "list(c(1,2),c(2,1))".

Thanks in advance for your help.

On Mon, Mar 21, 2011 at 04:08:50AM -0700, Julio Rojas
wrote:
> Dear all, I have three vectors "x<-1:n", "y<-1:m"
and "z<-2:(n+m)". I need to find all combinations of "x"
and "y" that add up a given value of "z", e.g.,  for
"z==3" the combinations will be "list(c(1,2),c(2,1))".
Dear Julio:

Try the following.

  n <- 3
  m <- 4
  x <- 1:n
  y <- 1:m
  a <- expand.grid(x=x, y=y)
  b <- subset(a, x + y == 3)
  b

    x y
  2 2 1
  4 1 2

In order to create a list of the required pairs, try
the following

  out <- vector("list", nrow(b))
  for (i in seq.int(along=out)) {
      out[[i]] <- c(b[i, 1], b[i, 2])
  }

Hope this helps.

Petr Savicky.

Thx Petr. It worked likea charm. Regards.