[R] assigning from multiple return values

[Gabor Grothendieck]

Just a few more examples:

# swap a and b without explicitly creating a temporary
a <- 1; b <- 2
list[a,b] <- list(b,a)

# get eigenvectors and eigenvalues
list[eval, evec] <- eigen(cbind(1,1:3,3:1))

# get today's month, day, year
require(chron)
list[Month, Day, Year] <- month.day.year(unclass(Sys.Date()))

# get first two components of linear model ignoring rest
list[Coef, Resid] <- lm(rnorm(10) ~ seq(10))





Gabor Grothendieck <ggrothendieck <at> myway.com> writes:

: 
: I think I've found a workaround that avoids the two problems in the
: replacement function approach.    With the definitions of list
: and [<-.result shown, one can write list[a,b] on the
: left side of an assignment where the right side of the assignment
: evaluates to a list of the same length (or if the list on the right
: side is shorter then the otherwise unfilled variables are set to NA and
: if the list on the right side is longer the excess entries are ignored).   
: Unlike the previous workaround using a replacement function, one
: can have a variable number of arguments to list[] and the first 
: argument to list[] no longer has to be predefined.
: 
: list <- structure(NA,class="result")
: "[<-.result" <- function(x,...,value) {
:    args <- as.list(match.call())
:    args <- args[-c(1:2,length(args))]
:    length(value) <- length(args)
:    for(i in seq(along=args))
:       eval(substitute(x <- v,list(x=args[[i]],v=value[[i]])),env=sys.frame(-
1))
:    x
: }
: 
: # it is used like this:
: 
: x <- 1:4
: fn <- function() list("zz",99)
: list[a,x[2]] <- fn()
: 
: Gabor Grothendieck <ggrothendieck <at> myway.com> writes:
: 
: : 
: : Here are two approaches assuming foo is "zz" and bar is 3.
: : 
: : FIRST
: : 
: : You could pass the return variables in the argument list and then
: : assign them in the caller's frame like this:
: : 
: : fn <- function(x,y) {
: : 	assign(as.character(substitute(x)), "zz", sys.frame(-1))
: : 	assign(as.character(substitute(y)), 3, sys.frame(-1))
: : }
: : fn(a,b)  # sets a to "zz" and b to 3
: : 
: : SECOND
: : 
: : You can make this a bit prettier, though not perfect, like this:
: : 
: : "list2<-" <- function(x,y,value) {
: : 	assign(as.character(substitute(y)), value[[2]], sys.frame(-1))
: : 	value[[1]]
: : }
: : fn <- function()list("zz",3)
: : a <- 1 # first arg must exist prior to invoking list2. Its value not 
: important.
: : list2(a,b) <- fn()
: : 
: : The two problems with list2 are:
: : 
: : 1. the first argument must exist prior to invoking list2 although its
: : actual value is immaterial since it just gets overwritten anyways.
: : 
: : 2. It only works for 2 args although you could write a list3, list4, etc.
: : 
: : Maybe someone could comment on these deficiencies.
: : 
: : Jack Tanner <ihok <at> hotmail.com> writes:
: : 
: : : 
: : : I know that if I have a function that returns multiple values, I should
: : : do return(list(foo, bar)). But what do I do on the recieving end?
: : : 
: : : fn <- function(x) {
: : :    return(list(foo, bar))
: : : }
: : : 
: : : I know that at this point I could say
: : : 
: : : values.list <- fn(x)
: : : 
: : : and then access
: : : 
: : : values.list[1]
: : : values.list[2]
: : : 
: : : But that's hideous. I'd rather be able to say something like
: : : 
: : : list(local_foo, local_bar) <- fn(x)
: : : 
: : : and have the right thing happen. I realize that it's my responsibility
: : : to not screw up and say instead
: : : 
: : : list(local_bar, local_foo)
: : : 
: : : Any suggestions?
: : : 
: : : -JT
: : :
: : 
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