Unit 12.2 Program Design

Aims

• Know different tools that can be used to design a new program

• Know what a structure chart is used for and construct these to show the different components in a program

• Know what a state-transition diagram is used for and construct these to show the different states a program can have.

Program Design Tools

Design stage - structure of new program is defined - components, modules and their interactions, done by:

Structure Charts

Pros

• Gain overview of whole program e.g. easily identify: modules and their hierarchy, task order, different parameters passed between modules.

• easily identify parts of solutions that can be re-used elsewhere

• Additional steps/refinement, new parts become simpler and easier to work with.

Unit 9.1 Computational Thinking Skills -

• problem decomposition - breaking big problems into smaller sub-problems.

• stepwise refinement - to break down big problems into smaller single-task problems, the smaller problems can be programmed independently of others.

• Structure chart - shows small problems and how they interact with each other to form whole program.

Example

4 different modules: UpdateLoan, LoanExtend, CheckReserve, LoanReturn

Arrows show parameters passed between the modules

The structure chart shows:

UpdateLoan

• Calls either module LoanExtend or module LoanReturn

LoanExtend

• Is called with parameters LoanID and BookID

• Calls module CheckReserve to whether books been reserved with another user

• Returns TRUE if loan has been extended otherwise returns FALSE

CheckReserve

• Is called with parameter BookID

• Returns TRUE if book has been reserved otherwise returns FALSE

LoanReturn

• Is called with parameters LoanID and BookID

• Will return REAL value (fine value of overdue loan)

State-Transition Diagrams

Picture representation of program behaviour, showing various states the program can be in and the transition and triggers of them.

Start State - State the program begins, represented by incoming arrow usually labelled “Start”.

States - different situations the program can be in, each state shown as a circle, program can only be in one state at a time.

Transitions - join states together and show changes from one state to another, shown as directional arrows.

Conditions - condition or input that causes program to transition - labelled onto transition arrows.

Examples

Start State - Heaters off (incoming arrow) and one other state - Heaters off

Transition arrows show changes from one state to another, and are labelled with the conditions that trigger them.

States can be used to determine inputs and outputs (think truth table)