INTRODUCTION TO ALGORITHMS

PROGRAMMING

Objectives

• Give a definition of the term algorithm

• Describe the various parts of the pseudocode

algorithm or algorithm structure.

• Understand the commands used to permit input,

output and processing of data in pseudocode.

• Write out a complete pseudocode algorithm using

sequential structures

• List the Do’s and Don’ts of writing pseudocode

What is an Algorithm?

An algorithm is a sequence of precise

instructions for solving a problem.

Pseudocode Algorithm Properties

A pseudocode algorithm must have the

following properties:

It must be:

• Precise

• Unambiguous

• Logically sequenced

• Give correct solutions in all cases

• Eventually end.

Pseudocode Algorithm Structure

Every pseudocode algorithm should have the following sections in the stated order

• Header : Algorithm’s name or title

• Declaration : A brief description of algorithm and

variables used. i.e. a statement of the purpose, as well

as stating variables.

• Body : Sequence of steps

• Terminator : An end statement

Algorithm Student data {Header}{This algorithm displays a student’s name and age on

the screen.} Name string {Declaration}Age integerStart

Display “Enter your name’Accept NameDisplay “Enter your age:” {Body}Accept AgeDisplay “Hello”, NameDisplay “You are”, Age, “years old”

Stop {Terminator}

Pseudocode Algorithm Header

• The algorithm header is usually followed by comments.

• Comments can be used to remind you of what is taking place.

• They can be used to explain or clarify any part of an algorithm.

• A comment or lack of comment has no effect on the logic of the algorithm. It is solely for documentation purposes.

• Comments are written by enclosing them in braces. A Comment extends from the left brace to the right brace, and may span one or more lines.

Declaring the Variables

• The next step is to declare the variables.

• All variables must be declared before they

can be used in the algorithm.

• The variables are followed by the data type

of the variable.

• Variable declarations must appear after the

program heading and a comment about

what the program does, but before the word

start of the main routine.

Body of the Pseudocode Algorithm

• The main executable code for the algorithm is in the body usually referred to as the main routine or simply main.

• The body consists of the main routine. The body consists of the part between Start and Stop.

• All algorithms end with a period. When we run a program it starts executing with the first statement in the main and ends when it reaches the last statement.

• The Body of the algorithm is also comprised of various structures. A structure can be sequence, a selection or a loop.

Body of the Pseudocode Algorithm

• The main routine of an algorithm or main

body of an algorithm consists of, prompt,

input, processing and output statements

that make up the various structures.

Prompt

• When developing and algorithm prompts must be made.

• Prompts are messages that appear on the screen notifying the user of what data has to be entered.

• A prompt is needed for each input statement.

• Prompt statements begin with the command PRINT followed by the string enclosed in quotes.– E.G. Print ‘Enter two numbers’

Input

• Each prompt is followed by a read.

• The command that allows the input of data is

the word Read followed by one or more

variables, used to represent the data being

entered and stored.

• When more than one variable is used a

comma is placed between each variable.

– Read num1, num2,num3

PRINTING INFORMATION OR OUTPUT

• Output can be sent to the screen when

developing an algorithm for solving a

problem.

• The command used is the word Print. Print

can be used to output the value of a variable

or to output a constant.

– Print ‘Total Revenue is ‘ TotalRevenue

PROCESSING INVOLVING CALCULATIONS

• Calculations can be done by using the

mathematical operations for addition,

subtraction, multiplication or division.

• The results of a calculation must be stored in

a variable for use.

• The result of the calculation is stored in the

variable to the left of the

PROCESSING INVOLVING CALCULATIONS

• This variable is assigned the results of the

calculation.

• Any statement, that assigns a value to a

variable, is called and assignment

statement.

• The value being assigned could be a

constant a variable a calculation or

expression.

Pseudocode Writing Do’s And Don’ts

Do’s

• Use the assignment symbol in assignment

statements not the equal sign.

• Use meaningful variable names

• Use indentation to show logic and scope

• Insert Comments to clarify meaning of blocks

of code.

Pseudocode Writing Do’s And Don’ts

Don’ts

• Do not use language specific statements in

pseudocode e.g. writeln, readln

• Do not attempt to write Pascal code before writing

the algorithm.

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