Gabor, Thank you for drawing this previous work to my attention. I've attached below code that extends the list comprehension to include logical 'guard' expressions, as in
> leap.years <- .[ x ~ x <- 1900:2100 | (x %% 400 == 0 || x %% 100 != 0 && x %% 4 == 0) ] > leap.years [1] 1904 1908 1912 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 [16] 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012 2016 2020 [31] 2024 2028 2032 2036 2040 2044 2048 2052 2056 2060 2064 2068 2072 2076 2080 [46] 2084 2088 2092 2096 > I wonder, would many (most?) R users be "mathematically-trained statisticians first, and programmers second", and therefore find a mathematical notation like the list comprehension more natural than less declarative programming constructs? I would be genuinely interested in your (and others') thoughts on that question, based on your knowledge of the R user community. Regards, David Gabor Grothendieck wrote: > 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. > > >> ## Updated to include logical guards in list comprehensions ## ## Define syntax for list/vector/array comprehensions ## . <<- structure(NA, class="comprehension") comprehend <- function(expr, vars, seqs, guard, comprehension=list()){ if(length(vars)==0){ # base case of recursion if(eval(guard)) 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], guard, comprehension) } } comprehension } ## List comprehensions specified by close approximation to set-builder notation: ## ## { x+y | 0<x<9, 0<y<x, x*y<30 } ---> .[ x+y ~ {x<-0:9; y<-0:x} | x*y<30 ] ## "[.comprehension" <- function(x, f){ f <- substitute(f) ## First, we pluck out the optional guard, if it is present: if(is.call(f) && is.call(f[[3]]) && f[[3]][[1]]=='|'){ guard <- f[[3]][[3]] f[[3]] <- f[[3]][[2]] } else { guard <- TRUE } ## 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, guard) ## Provided the result is rectangular, convert it to a vector or array ## TODO: Extend to handle .LC structures more than 2-deep. ## TODO: Avoid rectangularizing nested comprehensions along guarded dimensions? 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 } ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
