Selection: if, elif and else

Making a decision

Everything so far has run top to bottom, every line, every time. Selection is how a program takes one path instead of another.

age = int(input("How old are you? "))

if age >= 18:
    print("You may vote.")
else:
    print("Not yet.")

The three rules

1. A colon at the end of every if, elif and else line
2. The block underneath is indented, four spaces, and the indentation is what decides

which lines belong to the branch

1. Conditions are checked top to bottom, and the first true one runs while every

other branch is skipped

mark = int(input("Enter the mark: "))

if mark >= 70:
    print("Distinction")
elif mark >= 50:
    print("Merit")
elif mark >= 40:
    print("Pass")
else:
    print("Fail")

if, elif and else are one statement

A common misunderstanding is that these are three separate statements. They are one.

# One decision, three possible outcomes. Exactly one branch runs.
if temperature > 25:
    print("Hot")
elif temperature > 15:
    print("Mild")
else:
    print("Cold")

# Three separate decisions. Several can run.
if temperature > 25:
    print("Hot")
if temperature > 15:
    print("Mild")
if temperature > 5:
    print("Not freezing")

With temperature = 30 the first version prints only "Hot". The second prints all three lines, because each if is judged independently.

Boolean logic

A condition is any expression that evaluates to True or False.

Combining conditions

age = 16
has_permission = True

if age >= 18 or has_permission:
    print("Allowed in")

if age >= 13 and age <= 19:
    print("Teenager")

if not has_permission:
    print("Permission needed")

• and is true only when both sides are true
• or is true when at least one side is true
• not reverses a condition

Chaining comparisons

Python lets you write a range the way maths does, and it is clearer:

if 13 <= age <= 19:        # same as age >= 13 and age <= 19
    print("Teenager")

Checking membership

command = input("> ").lower()

if command in ["quit", "exit", "q"]:
    print("Goodbye")

if "a" in "banana":
    print("Found one")

in saves you writing the same comparison three times with or between them.

Patterns that keep selection readable

Nested selection

An if inside another if. The inner one is only reached when the outer condition is true.

if logged_in:
    if is_admin:
        print("Admin dashboard")
    else:
        print("User dashboard")
else:
    print("Please log in")

Combining rather than nesting

The same logic, one level flatter, and usually easier to follow:

if logged_in and is_admin:
    print("Admin dashboard")
elif logged_in:
    print("User dashboard")
else:
    print("Please log in")

Validating before you act

reply = input("Enter your age: ")

if not reply.isdigit():
    print("That is not a whole number.")
else:
    age = int(reply)
    if age >= 18:
        print("You may vote.")
    else:
        print("Not yet.")

Checking the input first means int() is only ever handed something it can convert, so the program explains the problem instead of crashing.

match, for many fixed options

command = input("Enter a command: ").lower()

match command:
    case "go":
        print("You move forward.")
    case "look":
        print("You see a door.")
    case "quit":
        print("Goodbye.")
    case _:
        print("I do not understand.")

match compares one value against a list of fixed possibilities. The underscore case runs when nothing else matched, exactly like else. A long elif chain that keeps testing the same variable is usually clearer written this way.

Key terms you must be able to define

Selection

Choosing between different paths through a program depending on a condition.

Condition

An expression that evaluates to either True or False.

if

Starts a selection statement, running its block only when the condition is true.

elif

Short for else if. Tests another condition, but only if every condition above it was false.

else

Runs when no condition above it was true. It takes no condition of its own.

Boolean operator

and, or and not, used to combine or reverse conditions.

Nested selection

An if statement placed inside the block of another if statement.

Boundary value

The exact value at which a condition changes from false to true.

Flashcards

Say the definition out loud before you flip the card. Retrieving it from memory is what builds recall, reading it again is not.

Reach the top grade on this topic

• Order elif conditions from most restrictive to least, and explain why it matters
• Choose between > and >= deliberately by testing the boundary value itself
• Combine conditions with and and or instead of nesting three levels deep
• Explain why an elif chain gives one outcome while separate ifs can give several
• Validate input before converting it, so the program reports the problem rather than crashing

Mistakes that cost marks here