Chapter 11 | Programming

11.1 Programming Basics

Input Processing & Output

Input: Entering data into a system

Pseudocode example: INPUT MyVar

Process: Manipulate or change data in some way, e.g. performing a calculation or finding a result

Pseudocode example: MyNum ← MyNum + 1

Output: Send data out from the system, display or print data

Pseudocode example: OUTPUT “Hello World”

Variables & Constants

Variables

A variable is a storage location with an identifier. The value of the variable can change.

DECLARE <identifier> : <data type>

Variables can be expressed in identifier tables, which include:

Variable name
Data type
Example
Explanations & descriptions
The scope (Local or Global)

Constants:

A constant is a named storage location. The value of the constant can not change.

CONSTANT <identifier> = <value>

11.2 Constructs

Logic statements

IF THEN ELSE ENDIF

Provides a selection of 2 possible pathways

IF <condition> THEN

<statement(s)>

ELSE

<statement(s)>

ENDIF

A condition consists of at least one logic proposition. Logic propositions use comparison & relational operators.

Operator	Comparison
=	Equal to
<	Less than
>	Greater than
<=	Less than or equal to
>=	Greater than or equal to
<>	Unequal to

Conditions are either true or false
Nested if statements consist of two if statements inside each other

CASE Statement

Selection of multiple pathways

Each condition can be:

A single value
Single values separated by commas
A range

CASE OF <expression>

OTHERWISE <statement(s)> //optional; used for error trapping

ENDCASE

Loops

Count-controlled (FOR) loop

Iterates a set number of times

FOR <control variable> ← s TO e STEP i // STEP is optional

<statement(s)>

ENDFOR

Post-conditional (REPEAT...UNTIL) loop

A post-conditional loop must run at least once. The condition is evaluated at the end.Aas long as the condition evaluates to false, the loop runs again. The loop will terminate once the condition evaluates to true.

REPEAT

<statement(s)>

UNTIL <condition>

Pre-conditional (WHILE) loop

A pre-conditional loop doesn’t always run. This evaluates the condition at the start. The loop will execute the statement as long as the condition evaluates to true, and will terminate once the condition evaluates to false.

WHILE <condition>

<statement(s)>

ENDWHILE

11.3 Structured Programming

Modular programming

Modular programming: The process of subdividing a computer program into separate sub-programs.

Modules: Seperate software component

Benefits:

Less code written
Single procedure can be reused, eliminating need to retype
Programs can be designed more easily as small teams deal with each module
Allows programmers to collaborate on an application

A subroutine is a set of instructions that performs a specific task as part of a larger program. Defined away from the program, and called with an identifier when needed. 2 types; procedures & functions.

Procedures

A subroutine that doesn’t return a value

PROCEDURE <identifier> (<parameter1>: <dataType> …)

<statement(s)>

ENDPROCEDURE

CALL <identifier>

May be given data - using variables called parameters passed in as values called arguments

Functions

A subroutine that returns a value

FUNCTION <identifier> (<parameter1>: <dataType> …) RETURNS dataType

<statement(s)>

ENDFUNCTION

call ← identifier()

Example functions:

X DIV Y: Integer division. How many times X divides into Y
X MOD Y: The remainder when X divides by Y

Passing parameters

By Value: At call time, a copy of the current value of the variable is passed into the subroutine. The value of the variable in the calling program is not affected by what happens in the subroutine

Inside heading: BYVAL <identifier1> : <dataType>

By Reference: A pointer to the memory location of the variable is passed into the procedure. Any changes applied in the subroutine will affect the variable’s contents in the calling program.

Inside heading: BYREF <identifier1> : <dataType>