cart2pol and external code interface

I have octave running just fine on my ubuntu 16.xx machine. I also ran the
octave demo program for calling octave from C++ (external code interface)
and this did the Matrix algebra just fine.
  Now I need to run "cart2pol"  but I get "  but I get the errors below:
Any ideas appreciated
Fritz
============ERROR MESSAGES =============================
error: ‘cart2pol’ was not declared in this scope
   cart2pol(1,1);
   ^~~~~~~~
..../comp2: line 2: ./octpolar: No such file or directory
======== COMPLETE CODE ===============================
/////////////////////////////////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <complex.h>
#include <iostream>
#include<math.h>
#include <iostream>

#include <octave/oct.h>
int
main (void)
{
  //float theta,rho;
  cart2pol(1,1);
  //std::cout << theta;
  //std::cout << rho;
  return 0;
}


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