Algorithms

10 IST – Topic 6

What is an algorithm?

“a series of detailed instructions or steps that will solve a problem in a set amount of time”

Common examples of algorithms

Appliance Instructions

a logical set of steps to operate an appliance

Common examples of algorithms

Recipes

a logical set of steps to cook an item

Common examples of algorithms

Directions

a logical set of steps to arrive at a location

Common examples of algorithms

Repair Manual

a logical set of steps to repair an item

Advantages of using algorithms

solve problems in a logical step-by-step process avoiding vague “blundering-in-the-darkness”

finite – always has an end methods are standardised language is standardised

problems are described in the same way terminology is uniform

Representing algorithms

Pseudocodeprecise form of English that uses keywords and rules of structure

Flowchartsa pictorial method of describing algorithms using a set of symbols, connecting lines and arrows.

Pseudocode

Keyword Meaning

BEGINEND

to start a programto finish a program

INITIALISATIONEND INITIALISATION

to set any values at the start of a programto end the values section

BEGIN SUBPROGRAMEND SUBPROGRAM

to start a subprogramto finish a subprogram

IF <condition> THEN ELSEENDIF

to allow selection or choicefor another choice (may not be needed)to end the choice

CASEWHERE <condition> OTHERWISEENDCASE

to allow for many choicesto end the choices

WHILE <condition>ENDWHILE

to begin a loop at the start of a sequence

REPEATUNTIL <condition>

to begin a loop at the end of a sequence

Pseudocode Rules

Keywords are written in CAPITALS

Basic keywords come in pairs … for every BEGIN there is an END

Indenting is used to show structure

Names of subprograms are underlined

Flowcharts

Flowchart Rules

There is only ever ONE WAY into a flowchart structure and ONE WAY out.

A single flowchart should fit on one page

If larger than one page … use subprograms

Control Structures

there are 3 basic control structures to show the order in which statements are carried out:

sequencing (or steps)

selection (or choice)

repetition (or loops)

Sequencing

most common

each step is carried out in order of its position

each step is done only once.

Selection

Binary selection allows the choice between 2 possible paths if one condition is met then one path is taken, otherwise the other

path is taken

Selection

Case or Multi-way selection allows the choice between 3 or more possible paths if the condition is met (TRUE) then one path is taken, otherwise

one of the other paths is taken

Repetition carries out a particular action any number of times until a

condition is met a loop is created to return the program to a point where the

repetition starts and continues until the condition is met the loop must have a terminating (ending) condition that is…

tested at some time during each repetition updated during each repetition

Repetition

Pre-test Repetition (While Loop / Guarded Loop) the condition is tested at the start of the loop if the condition is false the first time the processes will NEVER be

carried out. Pre-test loops end when the condition is false. Uses WHILE … ENDWHILE

Repetition

Post-test Repetition (Repeat-Until Loop / Unguarded Loop) the condition is tested at the end of the loop after the loop has

been run through Post-test loops end when the condition is true Uses REPEAT … UNTIL

Repetition

For-Next Repetition The body of the loop is executed a set number of times. A counter variable is given a starting value which is incremented

each time the loop is executed until a given final value is reached