user3032689 - 2 months ago 30

R Question

Assume you have an undefined nr of lists as possible arguments for a function, for example the following 3 can be picked (this example is as simple as possible, so vectors are stored in lists):

`a <- list(c(1,2,3,4,5))`

b <- list(c(3,6,7,2,1))

c <- list(c(3,9,8))

If I want to calculate the intersection of all three lists, this can be done as follows:

`Map(intersect,c,Map(intersect,a,b))`

# or equivalent:

mapply(intersect,c,mapply(intersect,a,b,SIMPLIFY=F))

# [1] 3

But how can I change the nr of arguments to be undefined? I read about

`...`

`...`

`intersectio <- function(...){`

Map(function(...){

intersect(...)

})

}

Q: But that doesn't work of course, because intersect must be applied recursively. Is there any way to achieve this in R?

Q2: Here is an updated example with a nested list structure. How can it be done in this case, i.e. intersect every sublist of the parent list with the associated sublist (same index) of the other parent lists?

`a <- list(list(c(1,2,3,4,5)),list(c(3,6,7,2,1)),list(c(3,9,8)))`

b <- list(list(c(1,2)),list(c(3,6,9,11,12)),list(c(3)))

c <- list(list(c(1,9)),list(c(65,23,12)),list(c(14,15)))

Answer

As @Roland suggested, you can use `Reduce`

to solve your problem. In the case of flat lists (as in the first version of the question), you can use the following:

```
Reduce(intersect, c(a, b, c))
```

In the case of nested lists (as in the updated question), you can just have to wrap that inside a `mapply`

call:

```
mapply(function(...) Reduce(intersect, c(...)), a, b, c)
```