Marc Vandemeulebroecke wrote:
Dear R users,
can someone help with these short puzzles?
1) Is there a function like outer() that evaluates a three-argument function on a threedimensional grid - or else how to define such a function, say, outer.3()? E.g., calculate (x/y)^z on (x,y,z) element of {1,2,3}x{3,4}x{4,5} and return the results in a 3-dimensional array. I would naively use outer() on two of the arguments within a for() loop for the third argument and somehow glue the array together. Is there a better way? What about outer.4(), or even outer.n(), generalizing outer() to functions with an arbitrary number of arguments?
2) Define a function dimnames.outer() such that dimnames.outer(x, y, "*") returns, for x <- 1:2, y <- 2:3, the following matrix:
y x 2 3 1 2 3 2 4 6
(Or does such such a function already exist?)
3)
How to combine puzzle 1 and puzzle 2? A function dimnames.outer.n() would be a nice little tool.
Here's what I came up with. If you need the other functions you mentioned, you can extract them from this example.
outer.3 <- function(x, y, z, FUN, ...) {
print(deparse(substitute(x))) # for question 2
n.x <- NROW(x)
n.y <- NROW(y)
n.z <- NROW(z)
nm.x <- if(is.array(x)) dimnames(x)[[1]] else names(x)
nm.y <- if(is.array(y)) dimnames(y)[[1]] else names(y)
nm.z <- if(is.array(z)) dimnames(z)[[1]] else names(z)
X <- expand.grid(x = x, y = y, z = z)
f <- FUN(X$x, X$y, X$z, ...)
array(f, dim = c(n.x, n.y, n.z),
dimnames = list(nm.x, nm.y, nm.z))
}a <- 1:3
b <- 3:4
c <- 4:5
names(a) <- a
names(b) <- b
names(c) <- c
outer.3(a, b, c, function(x, y, z) (x/y)^z)
outer.3(as.matrix(a), as.matrix(b), as.matrix(c),
function(x, y, z) (x/y)^z)4)
How can I access, within a function, the name of a variable that I have passed to the function? E.g., letting a <- 2, and subsequently calling function f(a) as defined below,
f <- function (x) { # How can I get "a" out of x? }
Use deparse(substitute(x)). See example above.
5)
Finally: Letting x <- 2, how can I transform "x+y" into "2+y" (as some suitable object), or generally "func(x,y)" into "func(2,y)"?
Use substitute(func(x, y), list(x = 2)).
Hope this is useful,
Sundar
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