Thanks David for the thoughts. The challenge I have with this approach is that
the criteria I have is defined by a series of tests--which I do not think I
could substitute in in place of the logical indexing.
In the combinations code I was hoping there is a step where, each new
combination is added to the current list of combinations. If this were the
case, I could put my series of tests in the code right there and then store the
combination if appropriate.
However, evalutating the code--which uses recursion--I am not sure if this
approach will work. The combinations code is listed below. Is there a simple
place(s) where I could insert my tests, operating on the current combination?
function (n, r, v = 1:n, set = TRUE, repeats.allowed = FALSE)
{
if (mode(n) != "numeric" || length(n) != 1 || n < 1 || (n%%1) !=
0)
stop("bad value of n")
if (mode(r) != "numeric" || length(r) != 1 || r < 1 || (r%%1) !=
0)
stop("bad value of r")
if (!is.atomic(v) || length(v) < n)
stop("v is either non-atomic or too short")
if ((r > n) & repeats.allowed == FALSE)
stop("r > n and repeats.allowed=FALSE")
if (set) {
v <- unique(sort(v))
if (length(v) < n)
stop("too few different elements")
}
v0 <- vector(mode(v), 0)
if (repeats.allowed)
sub <- function(n, r, v) {
if (r == 0)
v0
else if (r == 1)
matrix(v, n, 1)
else if (n == 1)
matrix(v, 1, r)
else rbind(cbind(v[1], Recall(n, r - 1, v)), Recall(n -
1, r, v[-1]))
}
else sub <- function(n, r, v) {
if (r == 0)
v0
else if (r == 1)
matrix(v, n, 1)
else if (r == n)
matrix(v, 1, n)
else rbind(cbind(v[1], Recall(n - 1, r - 1, v[-1])),
Recall(n - 1, r, v[-1]))
}
sub(n, r, v[1:n])
}
<environment: namespace:gtools>
**************************************************************************************************************************************************************
> I am working with the combinations function (available in the gtools
> package). However, rather than store all of the possible combinations I
> would like to check each combination to see if it meets a certain criteria.
> If it does, I will then store it.
>
> I have looked at the combinations code but am unsure where in the algorithm I
> would be able to operate on each combination.
Logical indexing:
> combinations(3,2,letters[1:3])[,1]=="a"
[1] TRUE TRUE FALSE
> combinations(3,2,letters[1:3])[ combinations(3,2,letters[1:3])[,1]=="a", ]
[,1] [,2]
[1,] "a" "b"
[2,] "a" "c"
--David
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