Selection: if, elif, else Statements

This lesson covers selection (also called branching or conditional statements) as required by OCR J277 Section 2.3. Selection allows a program to make decisions and execute different blocks of code depending on whether a condition is true or false.

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What Is Selection?

Selection is a programming construct that allows the program to choose between different paths of execution based on a condition. Without selection, programs would always execute the same instructions in the same order, regardless of input.

The three main selection structures in OCR pseudocode are:

1. if...then...endif — execute code if a condition is true
2. if...then...else...endif — choose between two paths
3. if...then...elseif...else...endif — choose between multiple paths

The following flowchart shows how if/elseif/else selection works:

flowchart TD
    A([Start]) --> B{Condition 1\ntrue?}
    B -- Yes --> C["Execute\nif block"]
    B -- No --> D{Condition 2\ntrue?}
    D -- Yes --> E["Execute\nelseif block"]
    D -- No --> F["Execute\nelse block"]
    C --> G([Continue])
    E --> G
    F --> G

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Simple if Statement

Executes a block of code only if the condition is true.

OCR Pseudocode:

age = int(input("Enter your age: "))
if age >= 18 then
    print("You are an adult")
endif

Python:

age = int(input("Enter your age: "))
if age >= 18:
    print("You are an adult")

If the condition is false, nothing happens — the program continues to the next line after endif.

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if...else Statement

Chooses between two paths: one if the condition is true, another if it is false.

OCR Pseudocode:

mark = int(input("Enter your mark: "))
if mark >= 50 then
    print("Pass")
else
    print("Fail")
endif

Python:

mark = int(input("Enter your mark: "))
if mark >= 50:
    print("Pass")
else:
    print("Fail")

OCR Exam Tip: In OCR pseudocode, you must include then after the condition and endif to close the block. Forgetting endif is a common mistake that will lose marks.

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if...elseif...else Statement

Chooses between multiple paths by checking conditions in order.

OCR Pseudocode:

mark = int(input("Enter your mark: "))
if mark >= 90 then
    print("Grade A*")
elseif mark >= 80 then
    print("Grade A")
elseif mark >= 70 then
    print("Grade B")
elseif mark >= 60 then
    print("Grade C")
elseif mark >= 50 then
    print("Grade D")
else
    print("Grade U")
endif

Python:

mark = int(input("Enter your mark: "))
if mark >= 90:
    print("Grade A*")
elif mark >= 80:
    print("Grade A")
elif mark >= 70:
    print("Grade B")
elif mark >= 60:
    print("Grade C")
elif mark >= 50:
    print("Grade D")
else:
    print("Grade U")

How It Works

The conditions are checked in order from top to bottom. As soon as one condition is true, that block executes and the rest are skipped. If none are true, the else block runs.

Mark	First True Condition	Output
95	mark >= 90	Grade A*
85	mark >= 80	Grade A
75	mark >= 70	Grade B
45	None	Grade U

OCR Exam Tip: Note that OCR pseudocode uses elseif (one word), while Python uses elif. Make sure you use the correct keyword for whichever language you are writing in.

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Nested Selection

You can place if statements inside other if statements. This is called nesting.

OCR Pseudocode:

age = int(input("Enter age: "))
member = input("Are you a member? (yes/no): ")

if age >= 18 then
    if member == "yes" then
        print("Adult member discount: 20% off")
    else
        print("Adult full price")
    endif
else
    print("Under 18: free entry")
endif

Python:

age = int(input("Enter age: "))
member = input("Are you a member? (yes/no): ")

if age >= 18:
    if member == "yes":
        print("Adult member discount: 20% off")
    else:
        print("Adult full price")
else:
    print("Under 18: free entry")

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Using Boolean Operators in Conditions

Conditions can be combined using AND, OR, and NOT:

OCR Pseudocode:

temperature = int(input("Enter temperature: "))
raining = input("Is it raining? (true/false): ")

if temperature > 20 AND raining == "false" then
    print("Go to the park")
elseif temperature > 20 AND raining == "true" then
    print("Visit an indoor attraction")
elseif temperature <= 20 OR raining == "true" then
    print("Stay at home")
endif

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Worked Example: Login System

OCR Pseudocode:

storedUser = "admin"
storedPass = "pass123"

username = input("Enter username: ")
password = input("Enter password: ")

if username == storedUser AND password == storedPass then
    print("Login successful")
elseif username != storedUser then
    print("Username not recognised")
else
    print("Incorrect password")
endif

Python:

stored_user = "admin"
stored_pass = "pass123"

username = input("Enter username: ")
password = input("Enter password: ")

if username == stored_user and password == stored_pass:
    print("Login successful")
elif username != stored_user:
    print("Username not recognised")
else:
    print("Incorrect password")

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Summary

• Selection allows programs to make decisions based on conditions.
• if...then...endif for a single condition.
• if...then...else...endif for two paths.
• if...then...elseif...else...endif for multiple paths.
• Conditions can be combined with AND, OR, NOT.
• Selection statements can be nested inside each other.
• OCR pseudocode uses elseif, Python uses elif.

OCR Exam Tip: When writing selection statements in the exam, always close your blocks with endif in OCR pseudocode. In Python, ensure correct indentation — Python uses indentation instead of endif to define code blocks.

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Worked Example: Calculating a BMI Category

A program takes a person's height (metres) and weight (kg), calculates their Body Mass Index (BMI), and reports which category they fall into. The BMI formula is weight divided by height squared.

OCR reference language pseudocode:

weight = real(input("Enter weight in kg: "))
height = real(input("Enter height in metres: "))
bmi = weight / (height ^ 2)

if bmi < 18.5 then
    category = "Underweight"
elseif bmi < 25 then
    category = "Healthy"
elseif bmi < 30 then
    category = "Overweight"
else
    category = "Obese"
endif

print("Your BMI is " + str(bmi) + " (" + category + ")")