Reacting to Player Input - Learning C# by Programming Games - page 79

if (mouse.X > 200)
spriteBatch.Draw(background, Vector2.Zero, Color.White);

In our example, we want to update the cannon barrel angle only when the player
presses the left mouse button. This means that we have to check if the state of the
left mouse button currently is pressed. This condition is given as follows:

mouse.LeftButton == ButtonState.Pressed

The == operator compares two values and returns true if they are the same, and false
otherwise. On the left hand side of this comparison operator, we find the left mouse
button state. On the right hand side, we find the state ButtonState.Pressed . So, this
condition checks whether the left button is currently pressed. We can now use it in
an if -instruction as follows in the Update method:

if (mouse.LeftButton == ButtonState.Pressed)
{

−

double opposite = mouse.Y barrelPosition.Y;

− double adjacent = mouse.X barrelPosition.X;

angle = ( float )Math.Atan2(opposite, adjacent);

}

So, only if the left mouse button is pressed, we calculate the angle and store its value
in the angle member variable. In order to see this program working, run the Painter1a
example in the solution belonging to this chapter.

6.3 Boolean Values

6.3.1 Comparison Operators

The condition in the header of an if -instruction is an expression that returns a truth
value: 'yes' or 'no'. When the outcome of the expression is 'yes', the body of the
if instruction is executed. In these conditions you are allowed to use comparison
operators. The following operators are available:

• < smaller than

• <= smaller than or equal to

• > larger than

•

>= larger than or equal to

•

== equal to

•

!= not equal to
These operators may be used between two numbers. On the left hand side and the
right hand side of these operators you may put constant values, variables, or com-
plete expressions with additions, multiplications and whatever you want, provided