some descriptions

drag.cpp

#include <Rcpp.h>
#include <cmath>  // ::ceil
using namespace Rcpp;


//' linear inter/extropolation.
//' combination of stats::approx and Hmisc::Hmisc.
//' @para x 
//' @para y
//' @para xx where interpolation is to take place.
//' @return yy interpolated 
//' @details right continous, and flat left extrms and extropolated right extrme.
//' @example
//' x <- sort(rnorm(10))
//' y <- rnorm(10)
//' xx <- seq(2*min(x), 2*max(x), len=50)
//' yy <- approx_rcpp(x, y, xx)
//' ggplot(data.frame(x, y), aes(x,y))+geom_line(col="red") + geom_point(data = data.frame(xx, yy), aes(x=xx, y=yy)) 

// [[Rcpp::export]]
NumericVector approx_rcpp(NumericVector& x, NumericVector& y, NumericVector& xx){
  int n = x.size(), k = xx.size();
  NumericVector yy(k);
  for (int i = 0; i < k; i++){ // flat left extrms 
    if (xx[i] < x[0]){
      yy[i] = NA_REAL; 
      continue;
    }
    if (xx[i] > x[n-1]){ // extropolated right extrms 
      yy[i] = NA_REAL;
      continue;
    }
    for (int j = 1; j < n ; j++){
      if ( xx[i] <= x[j]){
          yy[i] = (y[j] - y[j-1])/(x[j] - x[j-1]) * (xx[i] - x[j-1]) + y[j-1]; 
        break;
      }
    }
  }
  return yy;
}