当提供来自 R 的向量时，C 中的 For 循环不返回预期的 output

Question

So I have a functioning loop below that I can compile and execute happily in C++.

#include <iostream >
using std::cout;
using std::endl;
int main()
{
    int x[] = { 1, 2, 3, 10, 3, 2 };
    int j = 6;
    double highest = 0;
    double position = 0;
    for (int i = 0; i < j; i++) {
        if (x[i] > highest) {
            highest = x[i];
            position = i;
        }
    }
    cout << position;
    cout << endl;
    cout << highest;
    return 0;
}

However when I try to turn this into a function that I can call from R, the position and highest values dont seem to update from the 'for' loop. I am sure it has something to do with pointers but cant for the life of me figure it. Any help would be appreciated.

Note that we cant use RCPP for this task and have to use extern "C" {}

#include <R.h>
extern "C" {

void whichmax(double* x, int* len)
{
    double highest = 0;
    double position = 0;
    for (int i = 0; i < *len; i++) {
        if (x[i] > highest) {
            highest = x[i];
            position = i;
        }
    }
    Rprintf("please work %f \n", highest);
    Rprintf("please work %f \n", position);
}
}

Code used in R to run the function

dyn.load("whichmax.dll")

x <- c(1,2,2,8,4,8)

.C("whichmax",x, length(x))

dyn.unload("whichmax.dll")

Thanks in advance for an assistance!

Answer 1

Figured it out.

The problem was actually in the R code, I had to force the input 'len' value to be an integer and it all ran just fine.

dyn.load("whichmax.dll")

x <- c(1,2,2,8,4,8)

.C("whichmax",x, as.integer(length(x)))

dyn.unload("whichmax.dll")