The document discusses modularity in computer programming. It explains that programs can be broken down into smaller subprograms or modules to make the code more manageable and reusable. This is done through procedures and functions, which are subroutines that can take parameters and be called throughout the program. Parameters allow information to be passed into subroutines. Subroutines can help reduce errors and duplication by encapsulating common tasks.

1. Modularity

2. 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?
Modularity

3. Computer programs are much easier to program and
understand if we break them down into smaller blocks, or
sub programs.
Larger programs will have teams of developers working on
them so it makes for each developer to work on their
own sub-program.
Modularity means breaking large complex programs into
smaller and more manageable sub-programs.
Modularity

4. 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.
Modularity

5. We create a sub-program (or subroutine) to handle the movement of
the ghost. We tell the sub-program 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 sub-program can be used for any ghost.
Sub-programs (subroutines)

6. 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”)
Parameters

7. 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
Parameters

8. 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.
Parameters

9. 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
Procedures and functions

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

11. 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.
Functions

12. 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.
Parameter passing by value

13. 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.
Parameter passing by reference

14. Example - C
Is this a function or a procedure?
What does it do?
What are the parameters?

15. Example - C

16. Example - JavaScript
Is this a function or a procedure?
What does it do?
What are the parameters?

17. Example - JavaScript

18. Example - JavaScript
Is this a function or a procedure?
What does it do?
What are the parameters?

19. Module Libraries
Often a module of program code written for one project
can be reused in another project
Through time, companies will save modules for reuse in
other projects in a module library
This can save time and therefore money
Modules will already have been tested so the developer
should be confident that the code will work