Passing an element of an array of structs to a function

As others pointed out, what you need cannot be done in that way. One way to resolve the issue is to transform the matrix before passing it to the method:

using namespace std;

struct grid{ 
    int p; 
    int s; 
};

void create(int a[9][9])
{ 
    a[0][2] = 4; 
}

void ExtractS(struct grid a[9][9], int s[9][9])
{
    for (int i = 0; i < 9; i++)
        for (int j = 0; j < 9; j++)
            s[i][j] = a[i][j].s;
}

int main() 
{
    struct grid a[9][9]; 
    int s[9][9];
    ExtractS(a, s);
    create(s);
}

But this opens more questions than it answers:

• Data would have to be returned back into original matrix of structures, or otherwise change made by create is lost
• This method is very inefficient because it copies all the data

Maybe you should revise the design and make create function receive matrix of grid elements.

Initial Note: void create(int a[9][9]) turns in fact into
void create(int a[][9]) or void create(int (*a)[9])

You may want void create(int (&a)[9][9]).

As you have struct grid { int p; int s; }; and struct grid a[9][9];

You should initialize like this:

void create(grid (&g)[9][9])
{
    g[0][2].s = 4;
}

or change your struct as

struct grid {
    int p[9][9];
    int s[9][9];
};

And then have

void create(grid &g)
{
    g.s[0][2] = 4;
}

or

void create(int (&a)[9][9])
{
    a[0][2] = 4;
}

that you call respectively create(g) and create(g.s).

You basically have three choices, switch to two matrices, put the dimensions in the struct, or have create() take an array of struct:

Two arrays:

void create(int a[9][9]) 
{ 
  a[0][2] = 4; 
}

int main() 
{
  int p[9][9];
  int s[9][9]; 
  create(s); 
}

-- or -- put the dimensions in the struct:

struct grid{ 
  int p[9][9]; 
  int s[9][9]; 
};

void create(int a[9][9])
{ 
  a[0][2] = 4; 
}

int main() 
{
  struct grid a; 
  create(a.s); 
}

-- or -- Pass an array of struct

struct grid{ 
  int p; 
  int s; 
};

void create(struct grid[9][9] * a) 
{ 
  (*a)[0][2].s = 4; 
}

int main() 
{
  struct grid a[9][9]; 
  create(&a); 
}

You cannot cherry pick elements out of an array or matrix of structs

You are passing an integer to a function which expects a pointer to 2D array as an argument,

The prototype for the function to accept the member variable s of structure grid as argument should be as follows,

void create(int a); 

Also there is an unwanted semicolon in your program in the function definition.

void create(int a[9][9]); <-- No need
{ 
    a[0][2] = 4; 
}`
`