Problems with Viewport.Height & Width

**KABIRA16** · 10-11-2009

I'm developing some codes in XNA, and makes an application where a sprite will bounce of the edges of the screen. I have the following code

Code:

  if (Position.Y < mViewport.Y) 
  {
  mDirection.Y = -mDirection.Y; 
  }
  if (Position.X < mViewport.X) 
  {
  mDirection.X = -mDirection.X; 
  }
  if (Position.Y > mViewport.Y + mViewport.Height) 
  {
  mDirection.Y = -mDirection.Y; 
  }
  if (Position.X > mViewport.X + mViewport.Width) 
  {
  mDirection.X = -mDirection.X; 
  }

The top 2 if clauses works fine, (if I comment out the 2 lower, it will bounce from left and top edge of the screen) but the bottom 2 only works if I hard code a value instead of mViewport.Width / Height, for example. 400 and then the image will bounce back when it comes to 400, but if I use display the value it just shakes the whole picture and does not move. Why is it so? What is the problem with my mViewport.Width / Height?

**MindSpace** · 10-11-2009

See some of your IF test. I think the fact that the "<" and ">" have greater priority then a "+" and "-".

I think your code is interpreted something like runtime

if (True + mViewport.Width) // if pos.X is larger than Viewport.X
if (False + mViewport.Width) // if pos.x is less a Viewport.X

Try putting brackets around:

Code:

if (Position.X > (mViewport.X + mViewport.Width))
....

Do not know if it works, but it is my tips

**$tatic** · 10-11-2009

I think it sounds strange that <> has a higher priority than + -.

Try this:

Code:

if (Position.Y <mViewport.Y) 
{
mDirection.Y =-mDirection.Y; 
}
if (Position.X <mViewport.X) 
{
mDirection.X =-mDirection.X; 
}
if (Position.Y> mViewport.Y + mViewport.Height) 
{
Position.Y-= 5; 
mDirection.Y =-mDirection.Y; 
}
if (Position.X> mViewport.X + mViewport.Width) 
{
Position.X-= 5; 
mDirection.X =-mDirection.X; 
}

I can imagine that the problem is that when PX is larger than mV.X + mV.Width, so it stays there even if the direction is changed. Thus, it will only go into the lower block, over and over again, and change direction.

**MindSpace** · 10-11-2009

I've always been of the opinion that the + and - located on the bottom of the priority list, but I've been wrong before

But I tend to use parentheses anyway to be sure, empty when it comes to something as simple as multiplication

Code:

1 + 2 * 3 = 7 
(1 + 2) * 3 = 9 

This means that 1 + 2 * 3 is the same as 1 + (2 * 3) 
Believed therefore that 1> 2 + 3 was the same as (1> 2) + 3

**fellah** · 10-11-2009

Arithmetic comes before the comparisons. The order is as follows:

., (), [], X + +, x -, new, typeof, checked, unchecked (Primary)
+, -,:, ~, + + X, - y, (T) x (Unary, mathematical operations with only one operand)
*, /,% (Multiplicative)
+, - (Additive)
<<,>> (Shift)
<,>, <=,> =, Is, as (Relational and type testing)
==,! = (Equality)
&, ^, | (Bitwise logical)
& &, | | (Logical)
?: (Conditional)
=, *=, / =,% =, + =, -=, <<=,> =, & =, ^ =, | = (Assignment)