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

outer（0：5，0：6，sum）不起作用，因为 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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