Designing Algorithms (Cambridge (CIE) IGCSE Computer Science)

What is an algorithm?

• An algorithm is precise set of rules or instructions to solve a specific problem or task

• There are three main ways to design an algorithm

  • Structure diagrams

  • Flowchart

  • Pseudocode

Structure Diagrams

What is a structure diagram?

• Structure diagrams show hierarchical top-down design in a visual form

• Each problem is divided into sub-problems and each sub-problem divided into further sub-problems

• At each level the problem is broken down into more detailed tasks that can be implemented using a single subroutine

• an example of a structure diagram for a mobile application could be:

Flowcharts

What is a flowchart?

• Flowcharts are a visual tool that uses shapes to represent different functions to describe an algorithm

• Flowcharts show the data that is input and output, the processes that take place and any decisions or repetition

• Lines are used to show the flow of control

Example

Flowchart

• The casino would like the algorithm refined so that the user also enters their first name and this is used to greet the user when they access the site

Flowchart

Pseudocode

What is pseudocode?

• Pseudocode is a text-based tool that uses short English words/statements to describe an algorithm

• Pseudocode is more structured than writing sentences in English but is very flexible

Example

• A casino would like a program that asks users to enter an age, if they are 18 or over they can enter the site, if not then they are given a suitable message

INPUT Age
IF Age >= 18
    THEN
        OUTPUT "Welcome to the site"
    ELSE
ENDIF
OUTPUT "Sorry, this site is for users 18 and over"

• The casino would like the algorithm refined so that the user also enters their first name and this is used to greet the user when they access the site

INPUT FName
INPUT Age
IF Age >= 18
    THEN
        OUTPUT "Welcome to the site", FName
    ELSE
OUTPUT "Sorry, this site is for users 18 and over"