How to use pointers in this example?

[Chris Winton]

Hi im not sure to how to use a pointer for this script, i am trying to call the value from the previous from the previous function and use it in the main function as shown;

void prioriestimate ()
{
int T = 1;	
 int i = 0;
 int j = 0;
 int k = 0;
double A[6][6] =    {{1, 0, 0, T, 0, 0},
		     {0, 1, 0, 0, T, 0},
		     {0, 0, 1, 0, 0, T},
		     {0, 0, 0, 1, 0, 0},
		     {0, 0, 0, 0, 1, 0},
		     {0, 0, 0, 0, 0, 1}};
    
double A_t[6][6] =  {{1, 0, 0, 0, 0, 0},
		     {0, 1, 0, 0, 0, 0},
		     {0, 0, 1, 0, 0, 0},
		     {T, 0, 0, 1, 0, 0},
		     {0, T, 0, 0, 1, 0},
		     {0, 0, T, 0, 1, 0}};
 
double P_k[6][6] =  {{1, 0, 0, 0, 0, 0},
		     {0, 1, 0, 0, 0, 0},
		     {0, 0, 1, 0, 0, 0},
		     {0, 0, 0, 1, 0, 0},
		     {0, 0, 0, 0, 1, 0},
		     {0, 0, 0, 0, 1, 0}}; 

{

 double a[6][6];
 double a_p[6][6]; 
       for(i = 0; i < 2; i++) 
           for( j = 0; j < 4; j++)
               for( k = 0; k < 3; k++)  
                   a[i][j] +=  A[i][k] * A_t[k][j];



	for(i = 0; i < 2; i++) 
           for( j = 0; j < 4; j++)
               for( k = 0; k < 3; k++)  
                   a_p[i][j] +=  a[i][k] * P_k[k][j];

}
}
int main(double Q[6][6], int i, int j, int k)
{
Pk = a_p + Q;
        for(i = 0; i < 2; i++) 
           for( j = 0; j < 4; j++)
               for( k = 0; k < 3; k++)  
		Pk[i][j] += a_p[i][k] + Q[k][j]
return;
}
}

Any help would be grateful

[Savage]

int main(double Q[6][6], int i, int j, int k)

Main can't have parameters like these ones.It's either parameterless or it got two parameters which it recievies from the command line.Secondly, those variables(Pk,a_ p,A,A_t..) are local and exist only inside

void prioriestimate ()

function. If you want to return variable from a function you either need to change return type to the type of variable you want to return(in case you want to return a single variable only) or pass pointers to the type of variables you want to return in parameters list. Also you actually need to call the function you want from main.

For example:

int returnByType()
{
    int a =5;
    return a; //you can only return a single variable
}

void returnByPointerReference(int *a, int *b,int *c)
{
    int x = 5;
    
    *a = x;
    *b = x*x;
    *c = x+10;
}

int main() 
{
   int a,b,c,d;

   d = returnByType(); //calling the function which returns value by its return type from main

   returnByPointerReference(&a,&b,&c); //return trough pointers, note the usage of address-of operator to pass the address of variable, and not its contents

 return 0;
}

Additionaly read this.

Regards.

[vivek nandan]

You can use following conventions,
X[i][j] is equivalent to
*(X+i*col+j) , where i varies for row and j varies for column.

[Chris Winton]

cheers thanks for the response, helps alot.