That seems quite nice.
Note that there has been some related code posted. See:
http://tolstoy.newcastle.edu.au/R/help/03b/6406.html
which discusses some R idioms for list comprehensions.
Also the gsubfn package has some functionality in this direction. We
preface any function with fn$ to allow functions in its arguments
to be specified as formulas. Its more R-ish than your code and
applies to more than just list comprehensions while your code is
more faithful to list comprehensions.
> library(gsubfn)
> fn$sapply(0:11/11, ~ sin(x))
[1] 0.00000000 0.09078392 0.18081808 0.26935891 0.35567516 0.43905397
[7] 0.51880673 0.59427479 0.66483486 0.72990422 0.78894546 0.84147098
> fn$sapply(0:4, y ~ fn$sapply(0:3, x ~ x*y))
[,1] [,2] [,3] [,4] [,5]
[1,] 0 0 0 0 0
[2,] 0 1 2 3 4
[3,] 0 2 4 6 8
[4,] 0 3 6 9 12
> fn$sapply(0:4, y ~ fn$sapply(0:y, x ~ x*y))
[[1]]
[1] 0
[[2]]
[1] 0 1
[[3]]
[1] 0 2 4
[[4]]
[1] 0 3 6 9
[[5]]
[1] 0 4 8 12 16
> unlist(fn$sapply(1:4, y ~ fn$sapply(1:y, x ~ x*y)))
[1] 1 2 4 3 6 9 4 8 12 16
On Dec 9, 2007 4:41 PM, David C. Norris
<[EMAIL PROTECTED]> wrote:
> Below is code that introduces a list comprehension syntax into R,
> allowing expressions like:
>
> > .[ sin(x) ~ x <- (0:11)/11 ]
> [1] 0.00000000 0.09078392 0.18081808 0.26935891 0.35567516 0.43905397
> [7] 0.51880673 0.59427479 0.66483486 0.72990422 0.78894546 0.84147098
> > .[ .[x*y ~ x <- 0:3] ~ y <- 0:4]
> [,1] [,2] [,3] [,4] [,5]
> [1,] 0 0 0 0 0
> [2,] 0 1 2 3 4
> [3,] 0 2 4 6 8
> [4,] 0 3 6 9 12
> > .[ .[x+y ~ x <- 0:y] ~ y <- 0:4]
> [[1]]
> [1] 0
>
> [[2]]
> [1] 1 2
>
> [[3]]
> [1] 2 3 4
>
> [[4]]
> [1] 3 4 5 6
>
> [[5]]
> [1] 4 5 6 7 8
>
> > .[ x*y ~ {x <- 1:4; y<-1:x} ]
> [1] 1 2 4 3 6 9 4 8 12 16
>
> These constructions are supported by the following code.
>
> Regards,
> David
>
> ##
> ## Define syntax for list/vector/array comprehensions
> ##
>
> . <<- structure(NA, class="comprehension")
>
> comprehend <- function(expr, vars, seqs, comprehension=list()){
> if(length(vars)==0) # base case
> comprehension[[length(comprehension)+1]] <- eval(expr)
> else
> for(elt in eval(seqs[[1]])){
> assign(vars[1], elt, inherits=TRUE)
> comprehension <- comprehend(expr, vars[-1], seqs[-1], comprehension)
> }
> comprehension
> }
>
> ## Support general syntax like .[{exprs} ~ {generators}]
> "[.comprehension" <- function(x, f){
> f <- substitute(f)
> ## To allow omission of braces around a lone comprehension generator,
> ## as in 'expr ~ var <- seq' we make allowances for two shapes of f:
> ##
> ## (1) (`<-` (`~` expr
> ## var)
> ## seq)
> ## and
> ##
> ## (2) (`~` expr
> ## (`{` (`<-` var1 seq1)
> ## (`<-` var2 seq2)
> ## ...
> ## (`<-` varN <- seqN)))
> ##
> ## In the former case, we set gens <- list(var <- seq), unifying the
> ## treatment of both shapes under the latter, more general one.
> syntax.error <- "Comprehension expects 'expr ~ {x1 <- seq1; ... ; xN
> <- seqN}'."
> if(!is.call(f) || (f[[1]]!='<-' && f[[1]]!='~'))
> stop(syntax.error)
> if(is(f,'<-')){ # (1)
> lhs <- f[[2]]
> if(!is.call(lhs) || lhs[[1]] != '~')
> stop(syntax.error)
> expr <- lhs[[2]]
> var <- as.character(lhs[[3]])
> seq <- f[[3]]
> gens <- list(call('<-', var, seq))
> } else { # (2)
> expr <- f[[2]]
> gens <- as.list(f[[3]])[-1]
> if(any(lapply(gens, class) != '<-'))
> stop(syntax.error)
> }
> ## Fill list comprehension .LC
> vars <- as.character(lapply(gens, function(g) g[[2]]))
> seqs <- lapply(gens, function(g) g[[3]])
> .LC <- comprehend(expr, vars, seqs)
> ## Provided the result is rectangular, convert it to a vector or array
> ## TODO: Extend to handle .LC structures more than 2-deep.
> if(!length(.LC))
> return(.LC)
> dim1 <- dim(.LC[[1]])
> if(is.null(dim1)){
> lengths <- sapply(.LC, length)
> if(all(lengths == lengths[1])){ # rectangular
> .LC <- unlist(.LC)
> if(lengths[1] > 1) # matrix
> dim(.LC) <- c(lengths[1], length(lengths))
> } else { # ragged
> # leave .LC as a list
> }
> } else { # elements of .LC have dimension
> dim <- c(dim1, length(.LC))
> .LC <- unlist(.LC)
> dim(.LC) <- dim
> }
> .LC
> }
>
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