为什么不按我认为应该的方式进行外部工作(在R中)? [英] Why doesn't outer work the way I think it should (in R)?
问题描述
由@ hadley提供关于函数中引用的函数的文章今天,我决定重新讨论一下关于 outer
函数如何工作(或不)的持久难题。为什么会失败:
Prompted by @hadley's article on functionals referenced in an answer today, I decided to revisit a persistent puzzle about how the outer
function works (or doesn't). Why does this fail:
outer(0:5, 0:6, sum) # while outer(0:5, 0:6, "+") succeeds
这表明我认为 outer
应该处理像 sum
这样的函数:
This shows how I think outer
should handle a function like sum
:
Outer <- function(x,y,fun) {
mat <- matrix(NA, length(x), length(y))
for (i in seq_along(x)) {
for (j in seq_along(y)) {mat[i,j] <- fun(x[i],y[j])} }
mat}
> Outer(0:5, 0:6, `+`)
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,] 0 1 2 3 4 5 6
[2,] 1 2 3 4 5 6 7
[3,] 2 3 4 5 6 7 8
[4,] 3 4 5 6 7 8 9
[5,] 4 5 6 7 8 9 10
[6,] 5 6 7 8 9 10 11
好吧,我没有为这个例子准确对齐我的索引,但它不会很难修复。问题是为什么像 sum
这样的函数应该能够接受两个参数并且返回一个适合矩阵元素的(原子)值,当传递给 base :: outer
function?
OK, I don't have my indices exactly aligned for that example, but it wouldn't be that hard to fix. The question is why a function like sum
that should be able to accept two arguments and return an (atomic) value suitable for a matrix element, cannot do so when passed to the base::outer
function?
因此@agstudy给出了一个更紧凑版本的 Outer
并且他更紧凑:
So @agstudy has given inspiration for a more compact version of Outer
and his is even more compact:
Outer <- function(x,y,fun) {
mat <- matrix(mapply(fun, rep(x, length(y)),
rep(y, each=length(x))),
length(x), length(y))
但问题仍然存在。因为 sin
和 cos
是vectorized这个词在这里有点含糊不清,我认为dyadic更正确通常意义上的矢量化。期望 outer
以非二元函数可以使用的方式扩展其参数是否存在根本的逻辑障碍。
However, the question remains. The term "vectorized" is somewhat ambiguous here and I think "dyadic" is more correct, since sin
and cos
are "vectorized" in the usual sense of the term. Is there a fundamental logical barrier to expecting outer
to expand its arguments in a manner that non-dyadic functions can be used.
以下是另外一个外部
- 错误,这可能与我对此问题缺乏理解相关:
And here's another outer
-error that is probably similarly connected to my lack of understanding of this issue:
> Vectorize(sum)
function (..., na.rm = FALSE) .Primitive("sum")
> outer(0:5, 0:6, function(x,y) Vectorize(sum)(x,y) )
Error in outer(0:5, 0:6, function(x, y) Vectorize(sum)(x, y)) :
dims [product 42] do not match the length of object [1]
推荐答案
sum
不是矢量化的(意思是返回一个与其两个参数长度相同的向量)。这个例子应该解释不同:
outer(0:5, 0:6, sum)
don't work because sum
is not "vectorized" (in the sense of returning a vector of the same length as its two arguments). This example should explain the difference:
sum(1:2,2:3)
8
1:2 + 2:3
[1] 3 5
c $ c> sum 使用 mapply
例如:
You can vectorize sum
using mapply
for example:
identical(outer(0:5, 0:6, function(x,y)mapply(sum,x,y)),
outer(0:5, 0:6,'+'))
TRUE
PS:通常在使用 outer
我使用 browser
在调试模式下创建我的函数:
PS: Generally before using outer
I use browser
to create my function in the debug mode:
outer(0:2, 1:3, function(x,y)browser())
Called from: FUN(X, Y, ...)
Browse[1]> x
[1] 0 1 2 0 1 2 0 1 2
Browse[1]> y
[1] 1 1 1 2 2 2 3 3 3
Browse[1]> sum(x,y)
[1] 27 ## this give an error
Browse[1]> x+y
[1] 1 2 3 2 3 4 3 4 5 ## this is vectorized
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