Can't swap top element with bottom element in matrix - c

Question

I'm trying to write sliding puzzle where I'm using 3x3 matrix as board. In main function user inputs the number of tile which swaps positions with 0. Everything works except when I enter number that is located in a position above zero it does not swap positions with it. How do I correct that?
Here's the code:
#include <stdio.h>
#include <stdlib.h>
#define empty_space 0
int m,n,i,j,z,y;
void print_matrix(int matrix[3][3])
{
for ( m=0; m<3; m++){
for (n=0; n<3; n++)
{
printf("%d\t",matrix[m][n]);
} printf("\n");
}
printf("\n");
}
void swap(int *i, int *j) {
int t = *i;
*i = *j;
*j = t;
}
void slide(int a[3][3] , int t)
{
for ( i=0; i<3; i++){
for ( j=0; j<3; j++)
{
if (a[i][j]==t)
{
if (a[i+1][j]==empty_space && i+1<=2)
{
swap(&a[i+1][j],&a[i][j]);break;
}
if (a[i-1][j]==empty_space && i-1>=0)
{
swap(&a[i][j],&a[i-1][j]);break;
}
if (a[i][j+1]==empty_space && j+1<=2)
{
swap(&a[i][j],&a[i][j+1]);break;
}
if (a[i][j-1]==empty_space && j-1>=0)
{
swap(&a[i][j],&a[i][j-1]);break;
}
}
}
}
}
int goal_test (int a[3][3],int b[3][3])
{
int flag=0;
for ( z=0; z<3; z++){
for ( y=0; y<3; y++){
if(a[z][y]==b[z][y])
flag++;
}
}
if (flag==9)
return 1;
else return 0;
}
int main()
{
static int mat[3][3]={{1,2,3},{6,0,4},{7,5,8}};
int goal[3][3]={{1,0,3},{6,5,4},{7,8,2}};
print_matrix(mat);
int x;
while(goal_test(mat,goal)==0)
{
printf("enter tile to slide:\t");
scanf("%d",&x);
slide(mat, x);
print_matrix(mat);
}
return 0;
}
Here's what happens:
Any number can swap its position as long as it's not located above zero