I have a data frame in which its columns do not have name and I want to name them in RCPP?How can I do that? - rcpp

Question

I am very new in Rcpp. I have a data frame in which its columns do not have name and I want to name them in Rcpp. How can I do that? That is, this data frame is an input and then I want to name its columns in the first step.
Please let me know how I can do that.

Answer 1

Welcome to StackOverflow. We can modify the existing example in the RcppExamples package (which you may find helpful, just as other parts of the Rcpp documentation) to show this.
In essence, we just reassign a names attribute.
Code
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
List DataFrameExample(const DataFrame & DF) {
// access each column by name
IntegerVector a = DF["a"];
CharacterVector b = DF["b"];
DateVector c = DF["c"];
// do something
a[2] = 42;
b[1] = "foo";
c[0] = c[0] + 7; // move up a week
// create a new data frame
DataFrame NDF = DataFrame::create(Named("a")=a,
Named("b")=b,
Named("c")=c);
// and reassign names
NDF.attr("names") = CharacterVector::create("tic", "tac", "toe");
// and return old and new in list
return List::create(Named("origDataFrame") = DF,
Named("newDataFrame") = NDF);
}
/*** R
D <- data.frame(a=1:3,
b=LETTERS[1:3],
c=as.Date("2011-01-01")+0:2)
rl <- DataFrameExample(D)
print(rl)
*/
Demo
R> Rcpp::sourceCpp("~/git/stackoverflow/61616170/answer.cpp")
R> D <- data.frame(a=1:3,
+ b=LETTERS[1:3],
+ c=as.Date("2011-01-01")+0:2)
R> rl <- DataFrameExample(D)
R> print(rl)
$origDataFrame
a b c
1 1 A 2011-01-08
2 2 foo 2011-01-02
3 42 C 2011-01-03
$newDataFrame
tic tac toe
1 1 A 2011-01-08
2 2 foo 2011-01-02
3 42 C 2011-01-03
R>
If you comment the line out you get the old names.