Sorting named numeric vectors in Rcpp

Question

In a function, I want to calculate numeric values, give names to them and return a sorted NumericVector in Rcpp. I can sort the vectors (using this), but the order of the names of the values remains the same.

library(Rcpp)
x <- c(a = 1, b = 5, c = 3)
cppFunction('
NumericVector foo(NumericVector x) {
  std::sort(x.begin(), x.end());
  return(x);
}')
foo(x)
## a b c 
## 1 3 5

I want the function to return this:

## a c b 
## 1 3 5

Is it possible? How can I achieve this?

Sure. To a first approximation, you need to determine sort order, and use that to reindex the names attribute. — Dirk Eddelbuettel, Feb 17 '20 at 04:21

score 5 · Answer 1 · answered Feb 17 '20 at 14:42

Using the tip Dirk gave in his comment, I figured out that names of x is just another vector. So, I searched for sorting a vector using another vector. Using this SO answer I come up with following two solutions:

library(Rcpp)
x = c(a = 1, b = 5, c = 3, d = -3.2)

cppFunction('
NumericVector foo1(NumericVector x) {
 IntegerVector idx = seq_along(x) - 1;
 std::sort(idx.begin(), idx.end(), [&](int i, int j){return x[i] < x[j];});
 return x[idx];
}')

foo1(x)

##    d    a    c    b 
## -3.2  1.0  3.0  5.0 


cppFunction('
NumericVector foo2(NumericVector x) {
 IntegerVector idx = seq_along(x) - 1;
 //// Ordered indices based on x:
 std::sort(idx.begin(), idx.end(), [&](int i, int j){return x[i] < x[j];});
 //// Get the names of x:
 CharacterVector names_of_x = x.names();
 //// y vector is sorted x 
 NumericVector y = x[idx];
 //// Assign sorted names to y vector as names
 y.attr("names") = names_of_x[idx];
 return y;
}')

foo2(x)

##    d    a    c    b 
## -3.2  1.0  3.0  5.0

Sorting named numeric vectors in Rcpp

1 Answers1