[R] documentation of intersect() on string vector and num vector and on duplicated elements

[David Winsemius]

On Dec 2, 2009, at 4:33 PM, Peng Yu wrote:

>> intersect(c(1,3,2),c('1','3'))   # note x is numeric and y is  
>> character
> [1] "1" "3"
>
> Apparently, intersect() treats num as string. But this is not
> documented in the help. Could somebody add it in the future version of
> R?

It is documented that intersect will return a value of that is same  
mode as its "y" argument. How could it be any more clear?

>
> Also according to the help, the argument should not have duplicated
> elements.

That's not what my help page says. It says that "conceptually" they  
won't have them, but immediately thereafter it says:
"Details
Each of union, intersect, setdiff and setequal will discard any  
duplicated values in the arguments, and they apply as.vector to their  
arguments (and so in particular coerce factors to character vectors)."

Unfortunately, R does not have a package that will ensure that users  
will read the help pages carefully.

-- 

David

> But I tried the following example, it seems that it doesn't
> matter where there are duplicated elements or not. Should the help be
> corrected? Or somebody could provide me with an example that make
> intersect() fails if there are duplicated elements?
>
>> x=c(3,1,2,1,7)
>> y=c(5,1,4,2,3,8,1)
>> x
> [1] 3 1 2 1 7
>> y
> [1] 5 1 4 2 3 8 1
>> intersect(x,y)
> [1] 3 1 2
>
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> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT