Implement and write pseudocode from a given design presented as either a program flowchart or structured English

11.1 Programming Basics

Learning Objective

Implement and write pseudocode from a given design presented as either a program flowchart or structured English.

Why Pseudocode?

• Language‑independent description of an algorithm.
• Bridges the gap between design (flowchart/structured English) and actual code.
• Helps to check logic before programming.

Key Conventions for Pseudocode

1. Upper‑case reserved words: IF, THEN, ELSE, FOR, WHILE, END IF, etc.
2. Indentation to show nesting.
3. Variable names should be meaningful and follow camelCase or snake_case.
4. Input/Output statements: READ, WRITE (or OUTPUT).
5. Mathematical expressions written as they appear in mathematics, e.g., \$total = total + price \times quantity\$.

From a Flowchart to Pseudocode

A flowchart uses symbols to represent actions, decisions, loops, and start/end points. The conversion process is:

1. Identify the start and end symbols.
2. Read the flow from top to bottom (or left to right).
3. Translate each process box to a single pseudocode statement.
4. Translate each decision diamond to an IF … THEN … ELSE … END IF construct.
5. Translate loops (e.g., “repeat until”) to WHILE or FOR structures.

Example Flowchart

Design: Compute the sum \$S = 1 + 2 + \dots + n\$ where \$n\$ is entered by the user.

Pseudocode

READ n
sum ← 0
i ← 1
WHILE i ≤ n DO
sum ← sum + i
i ← i + 1
END WHILE
WRITE "The sum is ", sum

From Structured English to Pseudocode

Structured English is a semi‑formal description using English words mixed with programming keywords. To convert:

1. Replace “Get” or “Input” with READ.
2. Replace “Display” or “Output” with WRITE (or OUTPUT).
3. Identify conditional phrases (“If … then … else …”) and map them to IF … THEN … ELSE … END IF.
4. Identify repetition phrases (“Repeat … until …” or “For each …”) and map them to WHILE or FOR loops.
5. Keep the logical order; indentation reflects nesting.

Example Structured English

Get three numbers A, B and C.
If A > B then
If A > C then
Display A as the maximum.
Else
Display C as the maximum.
End if
Else
If B > C then
Display B as the maximum.
Else
Display C as the maximum.
End if
End if

Pseudocode

READ A, B, C
IF A > B THEN
IF A > C THEN
WRITE "Maximum is ", A
ELSE
WRITE "Maximum is ", C
END IF
ELSE
IF B > C THEN
WRITE "Maximum is ", B
ELSE
WRITE "Maximum is ", C
END IF
END IF

Common Pitfalls & How to Avoid Them

• Missing END statements: Every IF, WHILE, and FOR must have a matching END IF, END WHILE, or END FOR.
• Incorrect loop condition: Ensure the loop condition matches the intended repetition (e.g., i ≤ n vs i < n).
• Variable initialization: Initialise counters and accumulators before they are used.
• Ambiguous variable names: Use descriptive names to avoid confusion when reading the pseudocode.

Summary Table

Practice Questions

1. Given a flowchart that reads an integer \$n\$, then prints all even numbers from 2 up to \$n\$, write the corresponding pseudocode.
2. Convert the following structured English into pseudocode:

   Get a list of marks.
   Set total to 0.
   For each mark in the list do
   Add mark to total.
   End for.
   Calculate average = total / number of marks.
   Display average.
   

3. Identify the error in the pseudocode below and correct it:

   READ x
   IF x > 0 THEN
   WRITE "Positive"
   ELSE IF x = 0 THEN
   WRITE "Zero"
   END IF