A-Level Computer Science 9.2 Algorithms

Objective: Write pseudo-code from a flowchart

This section focuses on translating a flowchart into a step-by-step, human-readable algorithm using pseudo-code. Pseudo-code is an informal way of describing an algorithm, using a combination of natural language and structured elements.

Understanding Flowcharts and Pseudo-code

A flowchart visually represents the flow of an algorithm using symbols. Pseudo-code provides a textual equivalent, making the algorithm easier to understand and implement in any programming language.

Key Pseudo-code Constructs

• START / BEGIN: Marks the beginning of the algorithm.
• INPUT: Represents reading data from the user or a source.
• OUTPUT: Represents displaying data to the user or a destination.
• Process: Represents a calculation or operation.
• IF: Represents a conditional statement.
• ELSE: Represents the alternative block in a conditional statement.
• WHILE: Represents a loop that continues as long as a condition is true.
• ENDIF: Marks the end of an IF or ELSE block.
• ENDWHILE: Marks the end of a WHILE loop.
• RETURN: Represents the end of an algorithm or a specific section.

Example: Translating a Flowchart to Pseudo-code

Consider the following flowchart:

Here's the corresponding pseudo-code:

Step	Pseudo-code
1. START	BEGIN
2. Input	INPUT number
3. Process	result = number * 2
4. Output	OUTPUT result
5. END	END

In this example:

• The flowchart starts with a START.
• It then INPUTs a number.
• A Process is performed: the number is multiplied by 2 and the result is stored in result.
• The OUTPUT result to the user.
• Finally, the algorithm ENDs.

More Complex Example: Finding the Maximum of Two Numbers

Let's consider a flowchart for finding the maximum of two numbers:

The pseudo-code for this flowchart is as follows:

Step	Pseudo-code
1. START	BEGIN
2. Input	INPUT number1
3. Input	INPUT number2
4. Process	IF number1 > number2 THEN
5. Process	maximum = number1
6. Else	maximum = number2
7. ENDIF
8. Output	OUTPUT maximum
9. END	END

Explanation:

• The algorithm BEGINs.
• It takes two INPUTs, number1 and number2.
• An IF statement checks if number1 is greater than number2.
• If it is, number1 is assigned to maximum.
• Otherwise, number2 is assigned to maximum.
• The maximum value is then OUTPUTed.
• Finally, the algorithm ENDs.