How to write pseudocode

There are six basic computer operations

• A computer can receive information
• Read (information from a file)
• Get (information from the keyboard)
• A computer can put out information
• Write (information to a file)
• Display (information to the screen)
• A computer can perform arithmetic
• Use actual mathematical symbols or the words for the symbols
• Add number to total
• Total = total + number
• +, -, *, /
• Calculate, Compute also used
• A computer can assign a value to a piece of data
• 3 cases
• to give data an initial value
• Initialize, Set
• To assign a value as a result of some processing
• ‘=’
• x = 5 + y
• to keep a piece of information for later use
• Save, Store
• A computer can compare two piece of information and select one of two alternative actions
• IF condition THEN

some action

ELSE

alternative action

ENDIF

• A computer can repeat a group of actions
• WHILE condition (is true)

some action

ENDWHILE

• FOR a number of times

some action

ENDFOR

The Structure Theorem

(and the pseudocode we use to represent the control structures)

It is possible to write any computer program by using only three basic control structures: sequence, selection, repetition.

Sequence

Execution of one step after another. This is represented as a sequence of pseudocode statements:

Statement 1

Statement 2

Statement 3

Example:

Read three numbers

Add three numbers

Display total of three numbers

Selection

Presentation of a condition and the choice between two actions, the choice depending on whether the condition is true or false. This construct represents the decision making abilities of the computer to compare two pieces of information and select one of two alternative actions. In pseudocode, selection is represented by the keywords IF, THEN, ELSE and ENDIF

IF condition p is true THEN

statement(s) in true case

ELSE

statement(s) in false case

ENDIF

Example:

IF student is part_time THEN

Add one to part_time_count

ELSE

Add one to full_time_count

ENDIF

A variation – We don’t need the ELSE structure – The null ELSE

IF condition p is true THEN

statement(s) in true case

ENDIF

Repetition

Presentation of a set of instructions to be performed repeatedly, as long as a condition is true.

WHILE condition p is true

Statement(s) to execute

ENDWHILE

The condition is tested before any statements are executed. It is imperative that at lease one statement within the statement block alter the condition and eventually render it false, otherwise the logic may result in an endless loop.

Example:

Set student_total to 0

WHILE student_total < 50

Read student record

Print student name and address

Add 1 to student_total

ENDWHILE

Note: The variable student_total is initialized before the loop condition is executed

The student_total variable is incremented within the body of the loop so it will eventually stop. These are both essential feature of the WHILE construct.