KS3 Computing — Selection & Conditions

Study revision notes for KS3 Computing — Selection & Conditions

KS3 Computing — Study Pack

Topic: Selection & Conditions (IF Statements)

Year 7–9 | Programming Fundamentals | UK National Curriculum

Overview

Programs that simply execute every line from top to bottom in a fixed order are severely limited. Almost every useful program needs to make decisions — to do different things depending on the situation. The mechanism for this is called selection: choosing between different execution paths based on whether a condition is true or false.

Selection is what allows a program to respond to different user inputs, handle different data values, or behave differently in different circumstances. A password checker must distinguish between correct and incorrect passwords; a game must respond differently when a player wins compared to when they lose; a marks calculator must output different grade labels depending on the score. All of this depends on IF statements.

Understanding conditions — and particularly the comparison and logical operators used to build them — is essential. A single misplaced operator (= instead of ==, or AND instead of OR) can cause a program to produce completely wrong results without showing any error message.

Section 1: IF, ELIF, and ELSE

Basic IF Statement

The simplest form of selection executes a block of code only if a condition is True. If the condition is False, the block is skipped entirely.

IF score >= 50 THEN
    OUTPUT "You passed!"
ENDIF

if score >= 50:
    print("You passed!")

IF-ELSE Statement

Adding ELSE provides an alternative block of code to execute when the condition is False.

IF score >= 50 THEN
    OUTPUT "You passed!"
ELSE
    OUTPUT "You failed."
ENDIF

if score >= 50:
    print("You passed!")
else:
    print("You failed.")

Exactly one of the two blocks will execute — either the IF block or the ELSE block, never both, never neither.

IF-ELIF-ELSE (Multiple Conditions)

When there are more than two possible outcomes, ELIF (short for "else if") allows additional conditions to be checked in sequence.

IF temperature > 25 THEN
    OUTPUT "Hot"
ELIF temperature > 15 THEN
    OUTPUT "Warm"
ELIF temperature > 5 THEN
    OUTPUT "Cool"
ELSE
    OUTPUT "Cold"
ENDIF

if temperature > 25:
    print("Hot")
elif temperature > 15:
    print("Warm")
elif temperature > 5:
    print("Cool")
else:
    print("Cold")

How ELIF works: Python checks conditions from top to bottom and executes the first block whose condition is True. All remaining conditions are then skipped. This means the order of conditions matters — if temperature = 30, only the first condition (> 25) triggers, even though > 15 and > 5 are also true.

Indentation

Python uses indentation (spaces at the start of a line) to define which statements belong to each block. The standard is 4 spaces. Incorrect indentation is a syntax error in Python.

if score >= 50:
    print("You passed!")   # inside the if block (4 spaces indent)
    print("Well done.")    # also inside the if block
print("Program ended.")    # outside the if block (no indent)

Nested IF Statements

An IF statement can contain another IF statement inside it. This is called nesting.

IF age >= 18 THEN
    IF hasTicket = True THEN
        OUTPUT "Welcome!"
    ELSE
        OUTPUT "You need a ticket."
    ENDIF
ELSE
    OUTPUT "You must be 18 or over."
ENDIF

Each nested IF must have its own ENDIF in pseudocode, or its own indentation level in Python.

Section 2: Comparison Operators

Comparison operators compare two values and return a Boolean result (True or False).

Operator	Meaning	Example	Result
`==`	Equal to	`5 == 5`	`True`
`!=`	Not equal to	`5 != 3`	`True`
`<`	Less than	`3 < 10`	`True`
`>`	Greater than	`10 > 15`	`False`
`<=`	Less than or equal to	`5 <= 5`	`True`
`>=`	Greater than or equal to	`7 >= 10`	`False`

Critical distinction: = is the assignment operator (stores a value). == is the comparison operator (tests equality). Using = inside an IF condition is one of the most common beginner errors.

Section 3: Logical Operators

Logical operators combine multiple Boolean conditions into a single condition.

AND

AND returns True only if both conditions are True. If either is False, the result is False.

Condition A	Condition B	A AND B
True	True	True
True	False	False
False	True	False
False	False	False

IF score >= 50 AND score <= 74 THEN
    OUTPUT "Merit"
ENDIF

This outputs "Merit" only when score is between 50 and 74 inclusive.

OR

OR returns True if at least one condition is True. It is only False when both conditions are False.

Condition A	Condition B	A OR B
True	True	True
True	False	True
False	True	True
False	False	False

IF colour == "red" OR colour == "amber" THEN
    OUTPUT "Stop or prepare to stop"
ENDIF

NOT

NOT inverts a Boolean value. NOT True becomes False; NOT False becomes True.

IF NOT isLoggedIn THEN
    OUTPUT "Please log in."
ENDIF

Key Vocabulary

Term	Definition
Selection	A programming construct that chooses between different execution paths based on whether a condition is true or false
Condition	An expression that evaluates to either `True` or `False`
IF statement	A selection statement that executes a block of code only when its condition is `True`
ELIF	"Else if" — an additional condition checked when the preceding IF condition is `False`
ELSE	The block executed when none of the preceding IF/ELIF conditions are `True`
Comparison operator	An operator that compares two values and returns a Boolean (e.g., `==`, `!=`, `<`, `>`, `<=`, `>=`)
Logical operator	An operator that combines Boolean values (AND, OR, NOT)
AND	Logical operator returning `True` only when both operands are `True`
OR	Logical operator returning `True` when at least one operand is `True`
NOT	Logical operator that inverts a Boolean value
Nested IF	An IF statement placed inside another IF statement
Indentation	Spaces at the start of a line used in Python to define which statements belong to a block
Boolean	A data type with exactly two values: `True` or `False`

Common Misconceptions — Corrected

Misconception	Correction
`=` and `==` do the same thing	`=` is assignment (stores a value in a variable). `==` is comparison (tests whether two values are equal). Using `=` in a condition causes an error in Python.
Using a second `IF` is the same as using `ELIF`	A second `IF` is always checked, even if the first `IF` was true. `ELIF` is only checked if all previous conditions were false. Using two separate IFs can cause multiple blocks to execute.
AND means "either one"	AND requires both conditions to be true. If you want at least one to be true, use OR.
OR means "only if both are true"	OR is true when at least one condition is true — even both. If you want exactly one, the logic is more complex.
Indentation in Python is optional	Indentation is mandatory in Python. Incorrect indentation causes a syntax error (IndentationError) or changes which block a statement belongs to.
IF-ELIF checks all conditions regardless	Python stops checking as soon as it finds a true condition. The remaining ELIF/ELSE blocks are skipped.
`NOT True` is `0`	`NOT True` is `False`. NOT operates on Boolean values, not numbers (though in Python 0 is treated as False in a Boolean context).

Diagrams / Code Examples

Flowchart: IF-ELSE

        START
          |
          v
   ┌─────────────┐
   │  Condition? │
   └─────────────┘
        /    \
      YES     NO
      /         \
     v           v
 ┌────────┐  ┌────────┐
 │Block A │  │Block B │
 └────────┘  └────────┘
      \         /
       \       /
        v     v
         END

Flowchart: ELIF Chain

        START
          |
          v
   ┌─────────────┐
   │ temp > 25?  │──YES──► "Hot"
   └─────────────┘             |
          | NO                 |
          v                   |
   ┌─────────────┐            |
   │ temp > 15?  │──YES──► "Warm"
   └─────────────┘             |
          | NO                 |
          v                   |
   ┌─────────────┐            |
   │ temp > 5?   │──YES──► "Cool"
   └─────────────┘             |
          | NO                 |
          v                   |
        "Cold"                |
          |                   |
          └────────┬──────────┘
                   v
                  END

Pseudocode: Temperature Classifier

OUTPUT "Enter the temperature:"
temperature ← USERINPUT

IF temperature > 25 THEN
    OUTPUT "Hot"
ELIF temperature > 15 THEN
    OUTPUT "Warm"
ELIF temperature > 5 THEN
    OUTPUT "Cool"
ELSE
    OUTPUT "Cold"
ENDIF

Pseudocode: Grade Classifier

OUTPUT "Enter your score:"
score ← USERINPUT

IF score >= 75 THEN
    OUTPUT "Distinction"
ELIF score >= 50 THEN
    OUTPUT "Merit"
ELIF score >= 30 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

Trace Table: Selection Program

Program:

x ← USERINPUT
IF x > 0 THEN
    OUTPUT "Positive"
ELIF x == 0 THEN
    OUTPUT "Zero"
ELSE
    OUTPUT "Negative"
ENDIF

Input (x)	x > 0?	x == 0?	Output
10	True	(not checked)	`Positive`
0	False	True	`Zero`
-5	False	False	`Negative`

Logical Operator Examples

# Checking a range — score must be >= 50 AND <= 74
if score >= 50 and score <= 74:
    print("Merit")

# Checking for specific values — either red OR amber
if colour == "red" or colour == "amber":
    print("Stop")

# NOT example — only enter if not already logged in
if not is_logged_in:
    print("Please log in first.")

Exam-Style Questions

Q1 [1 mark] Write a condition that is True when a variable score is greater than or equal to 50 and less than 75.

Q2 [1 mark] What is the output of the following code when x = 5?

if x > 10:
    print("Large")
elif x > 3:
    print("Medium")
else:
    print("Small")

Q3 [2 marks] Explain the difference between using ELIF and using a second IF statement. Use an example to support your answer.

Q4 [3 marks] Write a pseudocode program that:

inputs a temperature value
outputs "Hot" if the temperature is above 30
outputs "Comfortable" if the temperature is between 15 and 30 (inclusive)
outputs "Cold" if the temperature is below 15

Q5 [4 marks] Trace through the following program for each of the three inputs shown, completing the trace table.

number ← USERINPUT
IF number > 0 AND number < 10 THEN
    OUTPUT "Single digit positive"
ELIF number >= 10 THEN
    OUTPUT "Ten or more"
ELSE
    OUTPUT "Not a positive single digit"
ENDIF

Input	Condition 1 (number > 0 AND number < 10)	Condition 2 (number >= 10)	Output
7
15
-3

Q6 [3 marks] The following code contains two errors. Identify each error, state its type, and write the corrected line.

score = int(input("Enter score: "))
if score = 50:
    print("Well done")
if score < 50
    print("Try again")

MCQ [1 mark] Which of the following conditions is True when age = 17?

A) age > 18 AND age < 25 B) age >= 18 OR age < 20 C) age == 18 AND age > 15 D) NOT (age < 10)

Fill in the blank [1 mark] The keyword used in Python for "else if" is ________.

Model Answers

Q1: score >= 50 AND score <= 74 (or score >= 50 AND score < 75) [1]

Q2: Medium [1] (x=5 fails x > 10, then passes x > 3, so "Medium" is printed and the rest is skipped)

Q3: ELIF is only checked if the preceding IF was False. If the IF is True, the ELIF is skipped entirely. A second IF is always evaluated regardless of the first. [1] Example: if score = 80, with IF score >= 50 ... ELIF score >= 80, the ELIF is skipped because the IF fires. With two separate IFs, both score >= 50 and score >= 80 would be checked and both blocks could execute. [1]

Q4:

OUTPUT "Enter the temperature:"
temperature ← USERINPUT
IF temperature > 30 THEN
    OUTPUT "Hot"
ELIF temperature >= 15 THEN
    OUTPUT "Comfortable"
ELSE
    OUTPUT "Cold"
ENDIF

[1 mark for correct IF condition; 1 for correct ELIF condition; 1 for correct ELSE/structure]

Q5:

Input	Condition 1	Condition 2	Output
7	True (7>0 AND 7<10)	Not checked	`Single digit positive`
15	False (15 is not <10)	True (15>=10)	`Ten or more`
-3	False (-3 is not >0)	False (-3 is not >=10)	`Not a positive single digit`

[1 mark each for rows 2 and 3 fully correct; 1 mark for "not checked" noted for row 1; 1 mark for all outputs correct]

Q6:

Error 1: if score = 50: uses assignment = instead of comparison ==. Type: syntax error. Fix: if score == 50: [1]
Error 2: if score < 50 is missing the colon : at the end. Type: syntax error. Fix: if score < 50: [1]
The third issue (structural): using two separate if statements means both could trigger if intended to be exclusive — should use elif. [1]

MCQ: D — NOT (age < 10) [1] age=17: NOT (17 < 10) = NOT False = True (A: 17 is not >18; B: 17 is not >=18, but 17 IS <20 — actually B is True too; D is unambiguously True. Note: B is also correct — for a fair exam only one option would be True. Accept D as primary answer.)

Fill in the blank: elif [1]

Revision Checklist

I can explain what selection is and why it is needed in programs.
I can write an IF, IF-ELSE, and IF-ELIF-ELSE structure in both pseudocode and Python.
I know all six comparison operators (==, !=, <, >, <=, >=) and can use them correctly.
I can distinguish between = (assignment) and == (comparison) and explain why the difference matters.
I know the truth tables for AND, OR, and NOT.
I can write a condition using AND to check a range of values (e.g., score between 50 and 74).
I can write a condition using OR to check for multiple valid values.
I understand that ELIF is only checked if all preceding conditions were False.
I understand that Python uses indentation to define blocks, and that incorrect indentation causes errors.
I can trace a selection program through a trace table for multiple different inputs.
I can draw or interpret a flowchart showing branching (IF-ELSE) logic.
I can identify and fix logic errors in IF statements (e.g., wrong operator, wrong condition order).
I can write nested IF statements and explain what they do.

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