1. Modularity

2. Modularity
A computer program could be written as one long
sequential piece of code.
But what happens if we want to carry out the same
process several times throughout the program?
We could copy and paste the code...
...but then what if we wanted to change the code?

3. Modularity
Example:
In the game Pacman, there are 4 ghosts. Each ghost
changes to a new and random direction when it
reaches a wall.
The movement rules for each ghost are the same. It
makes sense to develop one block of code which will
work for each ghost.

4. Subroutines
We create a subroutine to handle the movement of the
ghost. We tell the subroutine which ghost is to be moved.
SUBROUTINE move_ghost (ghost_name)
IF ghost_name at edge wall THEN
change ghost_name direction
ELSE
move ghost_name in current direction
END IF
END SUBROUTINE
This subroutine can be used for any ghost.

5. Parameters
Whenever we want to move a ghost, we call the
subroutine:
move_ghost (“Inky”)
This calls the subroutine move_ghost, passing in the
parameter (“Inky”)

6. Parameters
Notice that the code to call the subroutine move_ghost:
move_ghost (“Inky”)
Passes the parameter “Inky”, and so the variable
ghost_name will take on the value “Inky”
SUBROUTINE move_ghost (ghost_name)
IF ghost_name at edge wall THEN
change ghost_name direction

7. Parameters
The parameters which are passed into the subroutine when
it is called from another part of the program are called the
actual parameters.
“Inky” is an actual parameter.
The parameters which are used in the subroutine definition
are called the formal parameters.
ghost_name is a formal parameter.

8. Procedures and functions
There are generally two kinds of subroutine, procedures
and functions.
Procedures and functions are a series of statements which
have been given a name.
The difference between a procedure and a function is that:
A procedure produces an effect
A function produces a value

9. Procedures
The Pacman ghost subroutine move_ghost is an
example of a procedure.
It creates an effect by changing the direction of the
ghosts.

10. Functions
This subroutine is an example of a function:
FUNCTION calc_area (length, breadth)
area = length * breadth
RETURN area
END FUNCTION
It produces a value, in this case area.

11. Parameter passing by value
A parameter is passed by value if its value is not changed by the
subroutine.
pensionDetails = getPension (age)
PROCEDURE getPension(person_age)
IF person_age >65 THEN
pension_due= TRUE
END IF
RETURN pension_due
END PROCEDURE
In this case, the parameter age is not changed by the subroutine.

12. Parameter passing by reference
A parameter is passed by reference if its value is changed by the
subroutine.
increaseAge(age)
PROCEDURE increaseAge(age)
age = age + 1
END PROCEDURE
In this case, the parameter age is changed by the subroutine.
Not all languages support parameter passing by reference. Arrays are
always passed by reference.