Building Errors

alkalinetri3o · #1 09-May-2007, 13:59

Hello Everyone! I'm new to GID and relatively new to C++ and was wondering if anyone could help me with a problem i'm having. In the following program im trying to pass arrays using two different methods. I just cant understand the errors im getting and how to correct them. Any help would be appreciated thanks.

CPP / C++ / C Code:

#include <iostream>
using namespace std;

double function1(double a1, double a2);
double function2(double a1, double a3);

int main()
{	
	double a1[10] = {240.5, 300.1, 189.6, 310.6, 280.7, 216.9, 199.4,160.3,177.4,192.3};
	double a2[10], a3[10];
	function1 (a1[0], a2[0]);
	function2 (a1[0], a3[0]);
	for (int c = 0; c < 10; c++)
	{
		cout << a2[c] << "     " << a3[c] << endl;
	}
	return 0;
}

double function1 (double a1, double a2)
{
	for(int i = 0; i < 10; i++)
	{
line 24:		a2[i] = a1[i];
	}
	return a2;
}

double function2 (double a1, double a3)
{
	for(int k = 0; k < 10; k++)
	{
line 33:		double *ptr = &a1[0];
line 34:   	a3[k] = *ptr;
		ptr++;
	}
	return a3;
}

and the errors i am recieving are:

Code:

whoknows.cpp(24) : error C2109: subscript requires array or pointer type
whoknows.cpp(24) : error C2109: subscript requires array or pointer type
whoknows.cpp(33) : error C2109: subscript requires array or pointer type
whoknows.cpp(34) : error C2109: subscript requires array or pointer type

davekw7x · #2 09-May-2007, 16:19

Quote:

Originally Posted by alkalinetri3o

whoknows.cpp(24) : error C2109: subscript requires array or pointer type
whoknows.cpp(24) : error C2109: subscript requires array or pointer type
whoknows.cpp(33) : error C2109: subscript requires array or pointer type
whoknows.cpp(34) : error C2109: subscript requires array or pointer type

Two important things arise:

1. You can not pass arrays as arguments to functions, and functions can not return arrays. (You can, however, pass pointers to the elements of the arrays back and forth.)

2. When you use the name of an array by itself (without the brackets []) in the argument of a function call or as the return value of a function it is treated as a pointer whose value is the address of the first element of the array.

Therefore, looking at your functions: you need to pass the addresses for the arrays and let the functions operate on the elements.

Since you don't actually use the return values of the arrays, why don't you declare them with a "void" return type? If you really want to return the address of the first element of an array, then you can make the function return type a pointer.

Like this:

CPP / C++ / C Code:

#include <iostream>
using namespace std;

double *function1(double *a1, double *a2);
double *function2(double *a1, double *a3);

int main()
{
    double a1[10] = { 240.5, 300.1, 189.6, 310.6, 280.7, 
                      216.9, 199.4, 160.3, 177.4, 192.3 };
    double a2[10], a3[10];

    function1(a1, a2);
    function2(a1, a3);

    for (int i = 0; i < 10; c++) {
        cout << a2[i] << " " << a3[i] << endl;
    }
    return 0;
}

double *function1(double *a1, double *a2)
{
    for (int i = 0; i < 10; i++) {
      a2[i] = a1[i];
    }
    return a2;
}

// Note that I don't know what the heck this is supposed to do,
// but at least it compiles now, and you have the opportunity
// to test and debug it.
//
double *function2(double *a1, double *a3)
{
    for (int k = 0; k < 10; k++) {
      double *ptr = &a1[0];
      a3[k] = *ptr;
        ptr++;
    }
    return a3;
}

Note that the functions could have been declared and defined like this:

CPP / C++ / C Code:

double *function1(double a1[], double a2[]);

And the results would have been exactly the same for your functions. It may look to you as if this notation means that you are passing arrays, but, and I hate to repeat myself, you can not pass arrays as arguments to functions. You can pass pointers and that is what happens for both notations that I showed.

Note that the notation for calling the functions and the contents of the functions would not change if you decide you like the second notation that I showed for the function header. In other words, the program itself would be exactly the same (and the results would be the same).

Regards,

Dave

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