R «- operator

第一次看到这个 «- 赋值是在 coursera 的 assignment，这里是实现了一个 cache 了 mean 结果的 vector：

makeVector <- function(x = numeric()) {
	m <- NULL
	set <- function(y) {
		x <<- y
		m <<- NULL
	}
	get <- function() x
	setmean <- function(mean) m <<- mean
	getmean <- function() m
	list(set = set, get = get,
		setmean = setmean,
		getmean = getmean)
}

cachemean <- function(x, ...) {
	m <- x$getmean()
	if(!is.null(m)) {
		message("getting cached data")
		return(m)
	}
	data <- x$get()
	m <- mean(data, ...)
	x$setmean(m)
	m
}

对 set 而言，并不是看不到 x、m，这里 x、m 是 free variable，在 set 被定义的范围内都能看到，所以这不是可见性的问题。那这个 «- 与 <- 有啥不同呢？还是 manual 最有用：

Note that any ordinary assignments done within the function are local and temporary and are lost after exit from the function. Thus the assignment X <- qr(X) does not affect the value of the argument in the calling program.

To understand completely the rules governing the scope of R assignments the reader needs to be familiar with the notion of an evaluation frame. This is a somewhat advanced, though hardly difficult, topic and is not covered further here.

If global and permanent assignments are intended within a function, then either the “superassignment” operator, `<<-` or the function assign() can be used. See the help document for details. S-PLUS users should be aware that <<- has different semantics in R. These are discussed further in Scope.

我们可以做个小实验：

change <- function(x) {
	innerChange <- function(x0) {
		x <- x0
		x
	}
	innerChange(5)
	print(paste("after innerChange(5) (x <- 5), x =", x))
	x
}

> x <- 10
> change(x)
[1] "after innerChange(5) (x <- 5), x = 10"
[1] 10
> x
[1] 10

可见 x <- 5 出了 innerChange 就被 discard 掉了。再试试 «-：

change <- function(x) {
	innerChange <- function(x0) {
		x <<- x0
		x
	}
	innerChange(5)
	print(paste("after innerChange(5) (x <<- 5), x =", x))
	x
}

> x <- 10
> change(x)
[1] "after innerChange(5) (x <<- 5), x = 5"
[1] 5
> x
[1] 10

x <<- 5 出了 innerChange 仍然有效。注意 [1] 5 这一行是 change 的返回值。出了 change 之后，这个赋值还是失效了，x 仍然为 10，可见 «- 的作用只能跨一级 function。如果一定要修改 x 为 5 的话，在 change 这一层也要 «- 一下才行。