/* ansi C */
int i, j, k;
for (i = 0; i < num; ++i)
{
    for (j = 0; j < num2; ++j)
    {
        for (k = 0; k < num3; ++k)
        {
            ...
        }
    }
}

/* now you have i,j,k all defined to some value even after the loop... */
printf("%d %d %d", i, j, k);

/* ISO C99 -- BETTER */
for (int i = 0; i < num; ++i)
{
    for (int j = 0; j < num2; ++j)
    {
        for (int k = 0; k < num3; ++k)
        {
            ...
        }
    }
}
/* i,j,k DO NOT exist outside scope of for loop YAY! */
printf("%d %d %d", i, j, k); /* undefined... */