Software Engineering for Students A Programming Approach

106 Chapter 8 ■ Functional decomposition
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check boundaries
if bomb is outside window
then
destroy bomb
endif
if laser is outside window
then
destroy laser
endif
This completes some of the detail. The next item that has some complexity is the
method collides. It has two parameters, the objects to be tested for impact. It
returns either true or false. It uses the x and y coordinates of the objects,
together with their heights and widths. We use the notation a.x to mean the x
coordinate of object a. (The joy of pseudo-code is that you can use any notation
that is convenient.)
collides(a, b)
if a.x > b.x
and a.y < b.y + b.height
and a.x + a.width < b.x + b.width
and a.y + a.width > b.y
then
return true
else
return false
endif
This illustrates how we have used the tool of abstraction to hide this complexity
within a method.
There is still some way to go with this particular design, but this much gives the flavor
of the design and of the method. Notice how the design is refined again and again. Notice
how, throughout, we can check the design without having to take account of a lot of
unnecessary detail. Notice the informality of the statements used; we can simply make up
new actions as necessary, but still within the discipline of the control structures – sequence,
if and while, supplemented with methods.
SELF-TEST QUESTION
8.1 Write method move alien assuming it is at window coordinates x
and y.
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