Why is the class of a vector the class of the elements of the vector and not vector itself?

Question

I don\'t understand why the class of a vector is the class of the elements of the vector and not vector itself.

vector <- c(\"la\", \"la\", \"la\")
class         


        

Answer 1

R needs to know the class of the object you are operating on to perform the appropriate method dispatch on that object. The atomic data type in R is a vector, there is no such thing as a scalar, i.e. R considers a single integer a length one vector; is.vector( 1L ).

In order to dispatch the correct method R must know the datatype. It's not much using knowing that something is a vector, when your language is implicitly vectorised and everything is designed to operate on a vector.

is.vector( list( NULL , NULL ) )
is.vector( NA )
is.vector( "a" )
is.vector( c( 1.0556 , 2L ) )

So you can take the return value of class( 1L ) which is [1] "integer" to mean, I am an atomic vector consisting of type integer.

Despite the fact that under the hood a matrix is actually just a vector with two dimension attributes, R must know it is a matrix so that it can operate row-wise or column-wise on the elements of the matrix (or individually on any single subscripted element). After subsetting, you will return a vector of the datatype of the elements in your matrix, which will allow R to dispatch the appropriate method for your data (e.g. performing sort on a character vector or a numeric vector);

/* from the underlying C code in /src/main/subset.c....*/
result = allocVector(TYPEOF(x), (R_xlen_t) nrs * (R_xlen_t) ncs)

You should also note, that before determining the class of an object, R will always check that it is a first a vector, e.g. in the case of running is.matrix(x) on some matrix, x, R checks that it is first a vector, and then it checks for dimension attributes. If the dimension attributes is a vector of INTEGER data types of LENGTH 2 it satisfies the conditions for that object being a matrix (the following code snippet is from Rinlinedfuns.h from /src/include/)

INLINE_FUN Rboolean isMatrix(SEXP s)
  495 {
  496     SEXP t;
  497     if (isVector(s)) {
  498    t = getAttrib(s, R_DimSymbol);
  499    /* You are not supposed to be able to assign a non-integer dim,
  500       although this might be possible by misuse of ATTRIB. */
  501    if (TYPEOF(t) == INTSXP && LENGTH(t) == 2)
  502        return TRUE;
  503     }
  504     return FALSE;
  505 }

#  e.g. create an array with height and width....  
a <- array( 1:4 , dim=c(2,2) )

#  this is a matrix!
class(a)
#[1] "matrix"

# And the class of the first column is an atomic vector of type integer....
class(a[,1])
[1] "integer"