extern twist

[asmileguo]

Hi folks,

I learned a great deal today from experts on this board; hopefully I'll meet those kind people again...
Anyway, here's my program, spanning in three files whereas I am not sure how to properly use extern keyworld.

The main() file:

#include <cstdlib>
#include <iostream>
#include "function.h"

using namespace std;

double f(double x) {
       return g(x, array1, A);
       }

int main()
{const int A=5;
double array1[A];
double x;

for (int i=0;i<A;i++) array1[i]=i;

x=3.4;

cout<<"The f(3.4) is: "<<f(x)<<endl;

    
    system("PAUSE");
    return EXIT_SUCCESS;
}

The file "function.h":

extern const int A;
extern double *array1;

//declare the function g(.)
double g(double x, double *array, int arraydim);

The file "gfunction.cpp" that defines the g(.) function:

#include <cmath>
#include "function.h"

double g(double x, double *array, int arraydim) {
       double value=0;
       for (int i=0;i<arraydim;i++) {
           value=value+pow(array[i],i)+pow(x,2.0);
           }
           return value;
       }

whereas the error message is:

  [Linker error] undefined reference to `A' 
  [Linker error] undefined reference to `array1' 
  ld returned 1 exit status

What's wrong with my use of extern variables? My working environment is Bloodshed Dev C++ (it's so good; I love it!)

Thanks a lot...

[itsme86]

The problem is that the variables you're externing aren't global variables. Move A and array1 outside the scope of main() and it should work. Aside from that, I believe you have the concept down.

[asmileguo]

When I move the declaration of array1 outside of main(), I can only use the form:

double *array1;

Even so, when I run the program, a new Microsoft warnings window pops up and I didn't get anything on the command prompt line.

The problem is that the variables you're externing aren't global variables. Move A and array1 outside the scope of main() and it should work. Aside from that, I believe you have the concept down.

[itsme86]

Use double *array1 then, and inside main() use new to allocate memory for 5 doubles.

[asmileguo]

Done! I appreciate it!
This exercise further illustrate the use of new keyword... Thank you....

Use double *array1 then, and inside main() use new to allocate memory for 5 doubles.

[CornedBee]

You can create normal arrays globally. You just need to change the extern declaration to match the type. Of course, without N being a true global constant (i.e. put into a common header file) that's not really possible.