[S] answers to trying to avoid looping

[Pamela Goodman <pgoodman@nwifc.wa.gov>]

Thanks to Gardar Johannesson and Steve McKinney^2 for their suggestions. I
have not tested them for my needs yet, but, they will likely do very well
for me.  


### from Gardar Johannesson

foo <- function(x) {
  xr <- row(x)
  xc <- col(x)
  ind <- xr <= xc
  x[,xr[ind]] * x[,xc[ind]]
}

x is the matrix.  the function returns a matrix with each column storing
the product of some two columns in the matrix x.


### from Steve McKinney [1]
This function by Bill Venables might
do the trick, try it out.
You can just add the indices such as
1 1
2 2
3 3
etc.
to the list produced by subsets().


subsets <- function(n, r, v = 1:n)
  if(r <= 0) vector(mode(v), 0) else
     if(r >= n) v[1:n] else {
    rbind(cbind(v[1], Recall(n-1, r-1, v[-1])),
                Recall(n-1, r, v[-1]))
}


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

To avoid extra computations, you could run subsets
for some large value of n, save the results, and
just access the saved results to get the index
info you need.


### from Steve McKinney [2]

Here's another version of Bill Venable's
code for subset calculations that saves
intermediate results for speedier processing.


Subsets <- function(n, r, v = 1:n, frame = 1)
  if(r <= 0) vector(mode(v), 0) else
  if(r >= n) v[1:n] else {
    if(r > 1) {
      i1 <- paste("#", n - 1, r - 1)
      i2 <- paste("#", n - 1, r)
      if(!exists(i1))
        assign(i1,
               as.vector(Recall(n - 1, r - 1,
                  1:(n - 1))), frame = frame)
      if(!exists(i2))
        assign(i2, as.vector(Recall(n - 1, r,
                  1:(n - 1))), frame = frame)
      rbind(cbind(v[1],
            matrix(v[-1][get(i1)], ncol=r-1)),
            matrix(v[-1][get(i2)], ncol=r))
    } else matrix(v[1:n], ncol = 1)
  }

nextSubset <- function(n, r, v = 1:n) {
  nam <- paste(n, r)
  if(exists(nam, frame = 0)) {
    ind <- get(nam)
    s <- seq(along = ind)[ind]
    if(s[1] == n - r + 1) {
      remove(nam, frame = 0)
      return(NULL)
    }
    k <- max(s[s < (n-r+1):n])
    t <- sum(ind[k:n])
    ind[k:n] <- c(F, rep(T, t),
                  rep(F, n - k - t))
  } else {
    ind <- c(rep(T, r), rep(F, n-r))
  }
  assign(nam, ind, frame = 0)
  v[1:n][ind]
}


Pam

###################################
Pam Goodman
Biometrician
Northwest Indian Fisheries Commission
6730 Martin Way E
Olympia, WA 98516-5540
Phone: (360)438-1181 ext 380
Fax: (360)753-8659
http://www.nwifc.wa.gov/
###################################
-----------------------------------------------------------------------
This message was distributed by s-news@wubios.wustl.edu.  To unsubscribe
send e-mail to s-news-request@wubios.wustl.edu with the BODY of the
message:  unsubscribe s-news