Difference between char and character objects

Question

From the language definition:

"char": a ‘scalar’ string object (internal only) ***
"character": a vector containing character values

Users cannot easily get hold of objects of types marked with a ‘***’.

y <- "My name is hasnain" class(y) 

The class functions tells that 'y' belongs to the character class. What I have perceived is that 'y' is an object which is of char type and it belongs to character class. Does the character object also belongs to the character class?

Answer 1

The two R types char and character at the internal C side correspond to CHARSXP and STRSXP respectively. At the R level, one always deals with character objects; a single string, like:

y <- "My name is hasnain" 

is actually a character object of length 1. Internally, each element of a character is a char, but R doesn't provide (AFAIK) a direct way to extract, create and/or use a char.

Although you can't create a char/CHARSXP object with pure R, it's straightforward to get it through the R/C interface using the mkChar function, which takes a standard C string and turns it into a CHARSXP. For instance, one can create a char.c file:

#include <stdio.h> #include <stdlib.h> #include <R.h> #include <Rinternals.h> SEXP returnCHAR() {    SEXP ret = PROTECT(mkChar("Hello World!"));    UNPROTECT(1);    return ret; } 

After compiling it through R CMD SHLIB char.c, from the R side:

dyn.load("char.so")  #linux dll; extension varies across platforms x<-.Call("returnCHAR") x # <CHARSXP: "Hello World!"> typeof(x) #[1] "char" length(x) #[1] 12 

Besides typeof and length I didn't find many other R functions that acts on char objects. Even as.character doesn't work! I could neither extract a char from a standard character vector, nor insert this char into an existing character vector (assignment doesn't work).

The c function coerces to a list if an object is a char:

c(1,"a",x) #[[1]] #[1] 1 # #[[2]] #[1] "a" # #[[3]] #<CHARSXP: "Hello World!"> 

We can make use of .Internal(inspect()) (warning: inspect is an internal, not exposed function and so it might change in future releases. Don't rely on it) to have a glimpse of the internal structure of an object. As far as I know, char/CHARXSP objects are shared between string vectors to save memory. For instance:

let<-letters[1:2] .Internal(inspect(let)) #@1aff2a8 16 STRSXP g0c2 [NAM(1)] (len=2, tl=0) #  @1368c60 09 CHARSXP g0c1 [MARK,gp=0x61] [ASCII] [cached] "a" #  @16dc7c0 09 CHARSXP g0c1 [MARK,gp=0x60] [ASCII] [cached] "b" mya<-"a" .Internal(inspect(mya)) #@3068710 16 STRSXP g0c1 [NAM(3)] (len=1, tl=0) #  @1368c60 09 CHARSXP g0c1 [MARK,gp=0x61] [ASCII] [cached] "a" 

From the above output, we note two things:

• STRSXP objects are vectors of CHARSXP objects, as we mentioned;
• strings are stored in a "global pool": the "a" string is stored at the same address despite being created independently in two different objects.