Rcpp程序中的最小值和最大值

Question

I was converting an R function into Rcpp function. It is all fine but I have difficulties in using the standard max and min function. The code below:

#include <math.h>
#include <RcppArmadillo.h>
#include <algorithm>
#include <iostream>

// [[Rcpp::depends(RcppArmadillo)]]

using namespace Rcpp;
using namespace arma;
using namespace std;

double f4RandomC(double x, double a, double b) {
  double out, temp;

  temp=(log( (a*(1-x)) / (a+x) )) /log(b) ;
  out= std::min(1,temp );
  return out;
}

returns me error "no matchinf function for call min(int, &double). If possible, I would lioke to use the std:: library min function

Answer 1

Just change std::min(1,temp) to std::min(1.0,temp) :

#include <cmath>
#include <Rcpp.h>
// [[Rcpp::export]]
double f4RandomC(double x, double a, double b) {
  double out, temp;

  temp =(log( (a*(1-x)) / (a+x) )) /log(b) ;
  out = std::min(1.0,temp );
  return out;
}

I'm assuming this has to do with the template definition of std::min

template <class T> const T& min (const T& a, const T& b);

which is defined in terms of only one type ( T ), whereas you were passing it two data types ( int and double ).

Or since you are only comparing two values, you could do this a little more concisely by replacing std::min with the ternary operator ( ?: ):

double f4RandomC(double x, double a, double b) {
  double temp;

  temp =(log( (a*(1-x)) / (a+x) )) /log(b) ;
  return temp < 1 ? temp : 1;
}

I'm guessing the type deduction is a little more flexible for operator< than std::min .

Two other options with std::min :

// [[Rcpp::export]]
double f4RandomC2(double x, double a, double b) {
  double out, temp;
  int z = 1;
  temp =(log( (a*(1-x)) / (a+x) )) /log(b) ;
  out = std::min( static_cast<double>(z),temp );
  return out;
}

// [[Rcpp::export]]
double f4RandomC3(double x, double a, double b) {
  double out, temp;
  int z = 1;
  temp =(log( (a*(1-x)) / (a+x) )) /log(b) ;
  out = std::min<double>( z,temp );
  return out;
}

Although in this case it's certainly much easier to just change 1 to 1.0 than to (unnecessarily) define int z just to cast it to a double anyways later on.

You can learn a lot by reading through function / class definitions (as with most programming languages) - cplusplus.com and cppreference.com are pretty standard sources - and it will often make compiler errors seem much less cryptic.