[R] apply a function on elements of a list two by two

[Patrick Hausmann]

Am 08.05.2010 15:43, schrieb Joris Meys:
> Dear all,
>
> I want to apply a function to list elements, two by two. I hoped that combn
> would help me out, but I can't get it to work. A nested for-loop works, but
> seems highly inefficient when you have large lists. Is there a more
> efficient way of approaching this?
>
> # Make some toy data
> data(iris)
> test<- vector("list",3)
> for (i in 1:3){
>      x<- levels(iris$Species)[i]
>      tmp<- dist(iris[iris$Species==x,-5])
>      test[[i]]<- tmp
> }
> names(test)<- levels(iris$Species)

# Using 'lapply' and 'split' is a little bit more flexible:
test <- lapply(split(iris[, -5], iris$Species), function(x) dist(x))

>
> # nested for loop works
> for(i in 1:2){
>      for(j in (i+1):3){
>          print(all.equal(test[[i]],test[[j]]))
>      }
> }
>
> # combn doesn't work
> combn(test,2,all.equal)

Sorry, no answer

HTH
Patrick

>
> Cheers
> Joris