Why does the variables passed on as arguments don't maintain their modified values?

Question

Currently I am on 'Game of Fifteen' part of pset3, and I am having problem with my implementation of the function move.

The function move calls a function legalMove which in turn calls another function searchTile.

Perhaps a look at the code snippets will make things clearer:

---

move function:

bool move(int tile)
{
    // TODO
    bool result = false;
    int tileX = 0;
    int tileY = 0;
    int emptyX = 0;
    int emptyY = 0;

    if(legalMove(tile, tileX, tileY, emptyX, emptyY))
    {
        board[tileY][tileX] = 0;
        board[emptyY][emptyX] = tile;
        result = true;
    }
    return result;
}

---

legalMove function:

bool legalMove (int tile, int tileX, int tileY, int emptyX, int emptyY)
{
    bool result = false;

    if(searchTile(tile, tileX, tileY))
    {
        if(board[tileY-1][tileX] == 0)
        {
            result = true;
            emptyY = tileY-1;
            emptyX = tileX;

        }

        else if ( board[tileY][tileX-1] == 0)
        {
            result = true;
            emptyY = tileY; 
            emptyX = tileX-1;
        }            

        else if (board[tileY+1][tileX] == 0)
        {
            result = true;
            emptyY = tileY+1;
            emptyX = tileX;
        }

        else if (board[tileY][tileX+1] == 0)
        {
            result = true;
            emptyY = tileY;
            emptyX = tileX+1;
        }
    }
    return result;
}

---

and lastly, the searchTile function:

bool searchTile(int tile, int tileX, int tileY)
{
    bool result = false;
    for (int y = 0; y<d; y++)
    {
        for (int x = 0; x<d; x++)
        {
            if(board[y][x]==tile)
            {
                result = true;
                tileX=x;
                tileY=y;
                break;
            }
        }
    }
    return result;
}

---

The problem that I am facing is that the variables that I have passed as arguments in functions, gets modified as I desired, but for some reason, as soon as the function's execution end, the variable are initialized to 0.

Below are the images of GDB for the program:

Here, it can be seen, under "Local variables" that the values if tileX and tileY have been changed as required. The current running function is searchTile.

Here, it can be seen, under "Local variables" that the values if tileX and tileY remains 2 and 1 respectively as the function returns it's result

But as soon as the program exits the function searchTile the values of tileX and tileY gets changed to 0

I would really appreciate any help.

Regards,

Uzair.

Answer 1

First I would present you some basic concepts that you are lacking (seem like) about the concept of scope of variables in c. Later I will give you idea how you can solve your problem.

A scope in any programming is a region of the program where a defined variable can have its existence and beyond that variable it cannot be accessed. There are three places where variables can be declared in C programming language −

• Inside a function or a block which is called local variables.
• Outside of all functions which is called global variables.
• In the definition of function parameters which are called formal parameters.

Formal parameters, are treated as local variables with-in a function and they take precedence over global variables. They destroy their values as soon as the function execution finishes.

For Example: tileX and tileY in move function are local variables but in function searchTile, they are formal parameters.

I would suggest you to declare your tileX etc variables outside the functions may be in the main. Any variable you want to retain value for use in other functions etc should be declared as global.

Later in C you will learn pointers and type qualifier like static etc for thes purpose but for now declare them as global.

I hope you understand. If you have still problem, comment and it solves your problem, mark the answer as accepted. Thanks and good luck.