Bounce a character around the screen using for loops & gotoxy

Greets all,

I'm following some examples of using the gotoxy function from a c book that i am reading, and can't seem to get the quiz/task. Here's how it goes, i am trying to get the character O to bounce around the screen, using a few four loops and the goto function. I am able to get the character to go diagonally accross the screen with...

Code:

---

void main(void)
{ int x,y,z;

for(x=0,y=0,z=0;z<5000;x++,y++,z++)
        {
          gotoxy(x,y);
          printf("O");
          delay(100);
          clrscr();
        }
getch();
}

---

But i want the character 'ball' to bounce around the screen. I asked my programming teacher to give me tips, but i asked him not to give me the answe completely else i won't learn anything. He said that i needed, 4 if statements, which i have tried adding into the code, but to no avail. I have now given up, and would like to find out the answer as i'm ripping my hair out.

Basically, those four if statements would detect if the ball touches the edges of the screen (one if-statement for each edge), and reverse its speed if it does.
Oh, this assumes a screen from 0-80 horizontally, and 0-25 vertically, adjust the values as needed

Code:

---

void main(void)
{
  int x,y,z;
  int vx = 1;
  int vy = 1;

  for(x=0,y=0,z=0;z<5000;x+=vx,y+=vy,z++)
  {
    if( x<0 )
    {
      vx = 1;
      x = 0;
    }
    if( x>80 )
    {
      vx = -1;
      x = 80
    }
    if( y<0 )
    {
      vy = 1;
      y = 0;
    }
    if( y > 25 )
    {
      vy = -1;
      y = 25;
    }
    gotoxy(x,y);
    printf("O");
    delay(100);
    clrscr();
  }
  getch();
}

---

Thankyou, It all seems so straight forward after you see what you were doing wrong.