redefine length.character in R

Question

Since length is a generic method, why can't I do

length.character <- nchar

? It seems that strings are treated special in R. Is there a reason for that? Would you discourage defining functions like head.character and tail.character?

Answer 1

My 2c:

Strings are not treated specially in R. If length did the same thing as nchar, then you would get unexpected results if you tried to compute length(c("foo", "bazz")). Or to put it another way, would you expect the length of a numeric vector to return the number of digits in each element of the vector or the length of the vector itself?

Also creating this method might side-effect other functions which expect the normal string behavior.

Answer 2

If you look at the help page for InternalMethods (mentioned in the details portion of the help page for length) it states that

For efficiency, internal dispatch only occurs on objects, that is those for which ‘is.object’ returns true.

Vectors are not objects in the same sense as other objects are, so the method dispatch is not being done on any basic vectors (not just character). if you really want to use this type of dispatch you need a defined object, e.g.:

> tmp <- state.name
> class(tmp) <- 'mynewclass'
> length.mynewclass <- nchar
> length(tmp)
 [1]  7  6  7  8 10  8 11  8  7  7  6  5  8  7  4  6  8  9  5  8 13  8  9 11  8
[26]  7  8  6 13 10 10  8 14 12  4  8  6 12 12 14 12  9  5  4  7  8 10 13  9  7
> 

Answer 3

Now I found a reason not to define head.character: it changes the way how head works. For example:

head.character <- function(s,n) if(n<0) substr(s,1,nchar(s)+n) else substr(s,1,n)
test <- c("abc", "bcd", "cde")
head("abc", 2) # works fine
head(test,2) 

Without the definition of head, the last line would return c("abc", "bcd"). Now, with head.character defined, this function is applied to each element of the list and returns c("ab", "bc", "cd").

But I have a strhead and a strtail function now.. :-)