Chapter 20 Attributes

20.1 Attributes related functions

The following member functions are used to access the attributes of Rcpp objects.

attr( name )

accesses the attribute specified by the character string “name” and gets and sets the value.

List L;
L.attr("class") = "my_class";

attributeNames()

Returns a list of the attributes the object has. Since the return type of this function is std::vector<std::string>, if you want to convert it to CharacterVector use wrap() function.

CharacterVector ch = wrap(x.attributeNames());

hasAttribute( name )

If this object has an attribute with the name specified by the string “name”, it returns true.

bool b = x.hasAttribute("name");

// Creating a List object
NumericVector   v1 = {1,2,3,4,5};
CharacterVector v2 = {"A","B","C"};
List L = List::create(v1, v2);

// Setting element names
L.attr("names") = CharacterVector::create("x", "y");

// Creating a new attribute and set its value
L.attr("new_attribute") = "new_value";

// Changing the class name of this object to "new_class"
L.attr("class") = "new_class";

// Outputting a list of the names of the attributes of this object
CharacterVector ch = wrap(L.attributeNames());
Rcout << ch << "\n"; // "names" "new_attribute" "class"

// Check if this object has the attribute "new_attribute".
bool b = L.hasAttribute("new_attribute");
Rcout << b << "\n"; // 1

20.2 Access functions for common attributes

Dedicated access functions may be prepared for frequently used attributes such as element names.

// Element name, the following two sentences are synonymous
x.attr("names");
x.names();

The code example below shows how to access common attributes.

Vector v
v.attr("names"); // Element names
v.names();       // Element names

Matrix m;
m.ncol();  // Number of columns
m.nrow();  // Number of rows
m.attr(“dim”) = NumericVector::create( nrows, ncols );
m.attr(“dimnames”) = List::create( row_names, col_names );

DataFrame df;
df.attr(“names”);     // column names
df.attr(“row.names”); // row names

List L;
L.names(); // Element names