Using LogicalVector

Ross Bennett — written Dec 25, 2012 — source

The fact that expressions with binary logical operators such as x < 4 create a logical sugar expression (i.e. a LogicalVector type) is very powerful. This enables one to easily write simple and expressive functions with a LogicalVector as an argument. Any of the logical operators <, <=, >, >=, ==, != can be used to create a logical sugar expression. The Rcpp sugar vignette has additional examples of using binary logical operators.

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
int count_if(LogicalVector x) {
int counter = 0;
for(int i = 0; i < x.size(); i++) {
if(x[i] == TRUE) {
counter++;
}
}
return counter;
}
x <- 1:10
count_if(x < 4)
[1] 3
count_if(x != 8)
[1] 9

A simple function using just C++ and the STL to count the number of elements in a vector less than a given number could be written as follows. While this function is simple, the downside is that additional functions will have to be written for other logical operators and other types. Please see the references for the functional and algorithm headers for information regarding std::less, bind2nd, and count_if.

#include <vector>        // for std::vector
#include <functional> // for std::less and bind2nd
#include <algorithm> // for count_if

// [[Rcpp::export]]
int count_if_lt(std::vector<double> x, int n) {
return count_if(x.begin(), x.end(), bind2nd(std::less<double>(), n));
}
x <- 1:10
count_if_lt(x, 4)
[1] 3

tags: sugar  stl 

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