Chapter 12: Stepwise refinement and structure charts

Overview of the chapter:

Current A1 course only mentioned 12.3 section. Notes on section 12.1 and 12.2 will be added after the completion of coursework.

• 12.1 Step-wise refinement
• 12.2 Modules
• 12.3 Structure charts

12.3 Structure charts

Structure charts are graphical representation of the modular structure of solutions.

A structure chart has the following functions:

• Shows hierarchy of different modules
• Shows interface (parameter passed) between modules
• Shows selection
  • When a module is called only under certain conditions
• Shows repetition
  • When a module is called more than once

Components of a structure chart

Modules

Usually subroutines.

Modules are square boxes. Yet a repetition sign and selection sign can be added to show its characteristics.

Repetition is an arrow pointing to itself. Repeating condition should be marked clearly beside.

//In Pseudocode:
PROCEDURE AskPassword ()
    WHILE user_entry <> correct_password DO
        <code...>
    ENDWHILE
ENDPROCEDURE

Selection is a diamond. Depending on conditions it calls different modules.

// In pseudocode:
PROCEDURE AskPassword():
    <...code...>

    IF User_entry = correct_password THEN
        CALL spit_out_cash()
    ELSE
        CALL call_the_police()
    ENDIF
ENDPROCEDURE

---

Parameters

Parameters pass values between components.

Use	Description	Shape
a) Normal parameter	Pass normal parameters (e.g. A number)
b) Flag Parameter	Usually a Boolean value. Sends as a signal.
c) Pass-by-reference Parameter	Variable is updated inside the module. Usually a global variable.

Parameters are passed side of the connection line, like this:

You will also be tested on converting between structure charts and code. It’s not hard.