How to define something in a loop only once and have it remain after function ends and is called again?

Question

for pset3 my move function is the following:

int j = 0;
for(int i = 0; j < d; i++) {
    if(i == d) {
        i = 0;
        j++;
    }
    if(board[i][j] == tile) {
    printf("%i, %i", i, j);
    //swap left
        if(board[i][j] == board[xpos][ypos - 1]) {
            swap(xpos, ypos, i, j);
            board[i][j + 1] = tile;
            return true;
            printf("l");
        }

(same idea for moving right, up, or down). Here is the swap function:

void swap(int xpos, int ypos, int i, int j) {
xpos = i;
ypos = j;
board[i][j] = 0;
}

How do i make it so that the xpos and ypos are retained and arn't reset to d - 1 the next time the function runs?

Answer 1

You'll need to declare xpos and ypos as global variables, initialize them and not changing their values until you really need to.

Global variables are accessed from all the functions in your program. They never go out of scope until your program ends execution unlike local variables which might go out of scope after a code block, a loop, a condition or a function depending on where they are declared in your program.

To declare xpos and ypos as global variables, you'll need to write something like this

#include <stdio.h>

// prototypes
void foo(void);

// global variables
int xpos;
int ypos;

int main(void)
{
    // we may initialize them here
    xpos = 20;
    ypos = 10;
}

void foo(void)
{
    // and update their values here
    xpos = 10;
    ypos++;
}

Global variables can also be initialized when they're declared, but it's a good practice to declare and initialize variables only when needed.

Update: I don't really see any point of your swap() function. In fact, it doesn't do any swapping at all. I assume, you're trying to swap a tile at position (i, j) with the blank tile at position (xpos, ypos).

You don't really need to pass xpos and ypos as arguments to swap() in this case since they're already accessible from swap(). That is, you can refer to them with their names (use them) directly within the function.

To "really" swap a tile at (i, j) with the blank tile at (xpos, ypos), you may have something like this

// swap
board[xpos][ypos] = board[i][j];
board[i][j] = BLANK; // where BLANK is the value of the blank tile

// update the position of the blank tile
xpos = i;
ypos = j;

Answer 2

One approach would be to make xpos and ypos global variables whose initial value is d-1. Then they will retain their values no matter how many times you call your function.

The other option would be to make the addresses of xpos and ypos parameters of your function (they would have to be initialized outside the function).