Effectively compare each element of a number vector with the previous element

Question

I am trying to generate intertime values ​​- given the vector say, 20, 30, 69, 89, 200, what is the difference between each pair?

The data set contains 25 m elements, so I looked at both R and RCpp for the solution - speed is important. The implementation of R was:

intertime <- function(x){

    output <- x[2:length(x)] - x[1:(length(x)-1)]
    return(output)
}

C ++ implementation:

NumericVector intertime(NumericVector timestamps) {

  //Identify size of input object
  int input_size = timestamps.size();

  //Instantiate output object
  NumericVector output(input_size-1);

  //Loop over the data
  for(int i = 1; i < input_size;++i){

    //For each entry, the output value is [entry] minus [previous entry]
    output[i-1] = (timestamps[i] - timestamps[i-1]);

  }

  //Return
  return output;
}

The implementation of C ++ is an order of magnitude faster than the implementation of R. I understand that R is optimized for vectorized operations, so the difference in speed is so small that it doesn’t shock me very much, but: can anyone think of a decent way to perform such operations in Rcpp / C ++, which is more efficient?

Answer 1

++ std::adjacent_difference , :

#include <iostream>
#include <numeric>

int main () {
  int val[] = {1,2,3,5,9,11,12};
  int result[7];

  std::adjacent_difference (val, val+7, result);
  for(auto i : result) std::cout << i << " ";
  std::cout << std::endl;
}

LIVE DEMO