How to return different array in different function?

[chrishappy11]

Excuse me, I'm a beginner in C++
I want to calculate physics system so that I need to use large variable.

double multiply_matrix_RP(int u0,int u1,int u2,int u3,int u4,int u5,int u6,int u7,int u8)
{
        //u0,u1~u8 there are material array
	long double _Rp[9];
	for(int i=0;i<=8;i++)
       {
	_Rp[i]=EAI[i][0]*u0+EAI[i][1]*u1+EAI[i][2]*u2+EAI[i][3]*u3+EAI[i][4]*u4+EAI[i][5]*u5+EAI[i][6]*u6+EAI[i][7]*u7+EAI[i][8]*u8;	
	}
	return _Rp[9];
}

void CImp_simDlg::OnOK() 
{
	// TODO: Add extra validation here
	double _Rp[9];
	_Rp[9]=multiply_matrix_RP(Rp[0],Rp[1],Rp[2],Rp[3],Rp[4],Rp[5],Rp[6],Rp[7],Rp[8]);

	//CDialog::OnOK();
}

But it actually have error message when it compiler.
Please help me to solve, thank every much!!!

[Paul McKenzie]

Excuse me, I'm a beginner in C++
I want to calculate physics system so that I need to use large variable.

Arrays are not copyable, so you cannot return them. So forget about the way you wrote the code -- it will not work.

There are many ways to solve the problem, the most obvious being that you pass a pointer to the first element of the array to the function, and then let the function change the array,

double multiply_matrix_RP(double* p, int u0,int u1,int u2,int u3,int u4,int u5,int u6,int u7,int u8)
{
        //u0,u1~u8 there are material array
    for(int i=0; i < 9; ++i)
    {
        p[i]=EAI[i][0]*u0+EAI[i][1]*u1+EAI[i][2]*u2+EAI[i][3]*u3+EAI[i][4]*u4+EAI[i][5]*u5+EAI[i][6]*u6+EAI[i][7]*u7+EAI[i][8]*u8;	
     }
}
//...
double x[9];
//...
multiply_matrix_rp(x, val1, val2, val3, /*....etc... */);

Second, your code is confusing. You have _Rp, Rp, an unknown E array used in the function, etc. Where did E come from? Is _Rp and Rp supposed to be the same array? If they are, then your code will not work correctly, as you're using an uninitialized array, and doing that could cause an exception to be thrown due to invalid floating point values being used.

Regards,

Paul McKenzie

[chrishappy11]

Sorry...
In addition, There array EAI[9][9] ,Rp[9] are constant array, I declare other where.
If I return array in another function, It seem to be irregular.

[Philip Nicoletti]

Also note: you are doing things like:

double _Rp[9];
_Rp[9]=multiply_matrix_RP(Rp[0],Rp[1],Rp[2],Rp[3],Rp[4],Rp[5],Rp[6],Rp[7],Rp[8]);

But 9 is not a valid index for the _Rp array. The only valid indices are: 0,1,2,3,4,5,6,7,8

[Paul McKenzie]

Sorry...
In addition, There array EAI[9][9] ,Rp[9] are constant array, I declare other where.
If I return array in another function,

Again, you cannot return arrays. What you are doing is something else other than returning an array. Maybe your description of what you're doing is not correct. You can pass a pointer to the first element of the array, or you can wrap the array in a struct and return the struct, but you cannot return the array.

What your original code was doing was returning a single element of the array, namely element 9, and not an array. Here is a synopsis of the code you wrote:

double SomeFunction()
{
    double SomeArray[9];
    //...
    return SomeArray[9];  // This does not return an array!
}

That returns a single value, SomeArray[9]. This is what your code was doing. Not only does it not return an array, it returns an item from an invalid index in the array -- there is no element 9, as the array is indexed from 0 to 8 (just as Philip pointed out).

Also, please see what Philip pointed out -- you are accessing an invalid index in the array, causing the program to have undefined behaviour. When you access an invalid element in the array, the program may "work", may crash, or can change other values that you didn't suspect causing invalid values to be used.

If you are a beginner in C++, it is imperative you know exactly what you're doing when you write a program. If you don't know about how to handle arrays, i.e. passing and returning from functions, then you need to study that and write simpler programs giving you practice in doing this, instead of trying to write full-blown C++ programs and not know the basics of C++.

Regards,

Paul McKenzie

[ocm7]

Also one thing you should consider is the maximum size of long double, assuming its 16 bytes then you get a maximum (signed) value of (2^128)-1, or 340282366920938463463374607431768211455. This is usually more than enough, but say if your array values exceed 10k each then you will overflow. You can get 2x the space by using unsigned long double, but you lose the ability to assign negative values

[D_Drmmr]

Also one thing you should consider is the maximum size of long double, assuming its 16 bytes then you get a maximum (signed) value of (2^128)-1, or 340282366920938463463374607431768211455.

No, long double is a floating point type; it's not an integer type.

[chrishappy11]

I know how can I do.
Thank you very much~~