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} Ageinteger Start Display “Enter your name’ Accept Name Display “Enter your age:” {Body} Accept Age Display “Hello”, Name Display “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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