Write pseudocode statements for: expressions involving any of the arithmetic or logical operators input from the keyboard and output to the console

11.1 Programming Basics

Learning Objectives

• Read arithmetic or logical expressions from the keyboard.
• Evaluate the expressions using the correct operators, functions and precedence rules.
• Display the result on the console.
• Write simple selection (IF‑THEN‑ELSE) and loop (FOR/WHILE) structures when required.

Pseudocode Syntax Conventions (Cambridge style)

• Input: READ variable – prompts the user and stores the value. Only READ (or READ …  with a prompt) is accepted in the exam.
• Output: WRITE expression – displays the evaluated expression. Only WRITE is accepted.
• Assignment: variable ← expression
• Selection: IF condition THEN … END IF (or ELSE branch when required)
• Looping: FOR i ← start TO end DO … END FOR or WHILE condition DO … END WHILE
• Comments: # comment text
• Keywords (READ, WRITE, IF, THEN, END IF, FOR, WHILE, etc.) are case‑insensitive; the syllabus recommends writing them in UPPER‑CASE for clarity.
• Indentation and a single space around operators improve readability and are expected in exam scripts.

Variable & Constant Declaration (optional)

Explicit declarations are not required in the exam, but they help the examiner see the intended data type.

# Declaration (optional)
INTEGER a, b, n, i
REAL    total, average, C, F
CONST   PI ← 3.14159

Constants are introduced with CONST and never altered after the initial assignment.

Operator Summary

Quick Reference – Operator Precedence (exam tip)

When in doubt, enclose the intended part of an expression in parentheses – this avoids ambiguity and loss of marks.

Type‑Conversion Notes

• Division of two INTEGER values yields a REAL result (e.g., 5 / 2 = 2.5).
• To obtain an integer result, wrap the expression with INT() (truncates toward zero).
• Mixed‑type arithmetic (INTEGER + REAL) is automatically promoted to REAL.
• Be aware of overflow/under‑flow when converting a very large REAL to INTEGER – the result is undefined in the exam environment.

Built‑in Functions (syllabus‑approved only)

Only the functions listed above are guaranteed to be available in the Cambridge exam environment.

Common Pitfalls

• Using = for assignment – in pseudocode = is a relational operator; assignment uses the left‑arrow ←.
• Forgetting parentheses when mixing operators of different precedence.
• Assuming integer division – remember that / always produces a real result.
• Mixing data types without considering promotion (e.g., adding an INTEGER to a REAL automatically yields a REAL).
• Omitting the CONST keyword when defining a constant.
• Using functions not listed in the syllabus (e.g., LOG, EXP) – these are not permitted in the exam.

Worked Examples (Cambridge‑style)

Example 1 – Simple Arithmetic (no function)

# Sum, product and remainder of two integers
READ a
READ b
sum       ← a + b
product   ← a * b
remainder ← a mod b
WRITE "Sum = " + sum
WRITE "Product = " + product
WRITE "Remainder = " + remainder

Example 2 – Logical test with relational operators

# Eligibility based on age and test score
READ age
READ score
eligible ← (age ≥ 18) AND (score ≥ 70)
WRITE "Eligibility: " + eligible

Example 3 – Selection (IF‑THEN‑ELSE) with a relational test

# Pass/fail decision based on average mark
READ m1
READ m2
READ m3
average ← (m1 + m2 + m3) / 3          # real division
IF average ≥ 50 THEN
result ← "PASS"
ELSE
result ← "FAIL"
END IF
WRITE "Average = " + average
WRITE "Result  = " + result

Example 4 – Loop – factorial using a FOR loop

# Compute n! (n is a positive integer)
READ n
fact ← 1
FOR i ← 1 TO n DO
fact ← fact * i
END FOR
WRITE "Factorial of " + n + " = " + fact

Example 5 – Unit conversion (Celsius ↔ Fahrenheit)

# Celsius to Fahrenheit conversion
CONST FA ← 9 / 5          # 9/5 = 1.8 (real constant)
READ C                    # temperature in Celsius (REAL)
F ← FA * C + 32
WRITE "Celsius    = " + C
WRITE "Fahrenheit = " + F

Example 6 – Using a built‑in function (distance from origin)

# Pythagoras theorem
READ x
READ y
distance ← SQRT( POWER(x,2) + POWER(y,2) )
WRITE "Distance from origin = " + distance

Practice Questions

1. Multiple of 5 or 7
   

   Write pseudocode that reads an integer n and outputs TRUE if n is a multiple of 5 or 7, otherwise FALSE.

   
   Mark‑scheme hint: use the modulo operator and an OR logical test.

2. Modular arithmetic with squares
   

   Write pseudocode that reads two numbers x and y, computes (x² + y²) mod 10, and displays the result.

   
   Mark‑scheme hint: use POWER(x,2) and mod.

3. Temperature conversion
   

   Write pseudocode that reads a temperature in Celsius, converts it to Fahrenheit using F = (9/5)·C + 32, and prints both values.

   
   Mark‑scheme hint: declare a constant for 9/5 (or write the fraction directly).

4. Factorial (loop)
   

   Write pseudocode that reads a positive integer n, calculates n! using a loop, and writes the result.

   
   Mark‑scheme hint: a FOR loop from 1 to n is the simplest approach.

5. Pass/Fail using IF‑THEN‑ELSE
   

   Write pseudocode that reads three exam marks, computes the average, and writes “PASS” if the average is at least 50, otherwise writes “FAIL”.

   
   Mark‑scheme hint: use the relational operator ≥ inside an IF statement.