Examples to compute the cumulative sum of a vector

2012-12-22-vector-cumulative-sum.cpp

/**
 * @title Using Sugar Function cumsum()
 * @author Ross Bennett
 * @license GPL (>= 2)
 * @tags sugar cumsum
 * @summary Demonstrates different ways to compute the cumulative
 *   sum of a vector and illustrates the use of sugar function cumsum().
 */
 
 #include <Rcpp.h>
 #include <numeric>		// for std::partial_sum
 using namespace Rcpp;
 
 /**
 * The traditional way to compute the cumulative sum of a vector is with a
 * for loop. This is demonstrated with the function cumsum1().
 */
 
 // [[Rcpp::export]]
 NumericVector cumsum1(NumericVector x){
 	// initialize an accumulator variable
 	double acc = 0;
 	
 	// initialize the result vector
 	NumericVector res(x.size());
 	
 	for(int i = 0; i < x.size(); i++){
 		acc += x[i];
 		res[i] = acc;
 	}
 	return res;
 }
 
 /**
 * The C++ standard template library (STL) has the partial_sum() function
 * that computes the cumulative sum of a vector. This is demonstrated with
 * the function cumsum2().
 */
 
 // [[Rcpp::export]]
 NumericVector cumsum2(NumericVector x){
 	// initialize the result vector
 	NumericVector res(x.size());
 	std::partial_sum(x.begin(), x.end(), res.begin());
 	return res;
 }
 
 /**
 * With Rcpp sugar, there is a cumsum() function which makes writing 
 * this function in C++ very similar to using the cumsum function in R.
 */
 
 // [[Rcpp::export]]
 NumericVector cumsum_sug(NumericVector x){
 	// initialize the result vector
 	NumericVector res = cumsum(x);
 	return res;
 }
 
 /*** R
 x <- 1:10
 cumsum1(x)
 cumsum2(x)
 cumsum_sug(x)
 # cumsum function in base R
 cumsum(x)
 */