Why doesn't outer work the way I think it should (in R)?

Question

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 doe

Answer 1

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

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: 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