PCC·05 · Chapter record

if Statements

Chapter 5 — teaching your programs to make decisions

Done

Five sessions on branching logic: writing conditional tests, combining them with and/or/in, if/else, if-elif-else chains, and putting if statements to work inside loops over lists. Each one explains the concept, lists the key terms, then gives you exercises — try them yourself before revealing the code.

5.1

Equality & Comparison Tests

Concept

At the heart of every if statement is a conditional test — an expression that evaluates to True or False.
== checks equality and != checks inequality — a single = assigns a value, a double == asks a question.
Equality is case-sensitive: 'Audi' == 'audi' is False. Convert with .lower() before comparing if case shouldn't matter — it doesn't change the original variable.
The usual numeric comparisons work too: < <= > >=.

Key Terms

conditional test: an expression that evaluates to True or False
==: checks whether two values are equal
.lower(): converts a string to lowercase, for case-insensitive comparisons

Exercise

Activity 1

Write at least five conditional tests using ==, !=, <, <=, >, and >=. Predict True or False before you run each one.

Activity 2

Store a name with mixed capitalization, then write a test that compares it to a lowercase version using .lower().

5.2

Combining Conditions

Concept

and requires both tests to be True for the overall expression to be True.
or passes if either test is True — it only fails when both tests fail.
Use in to check whether a value is inside a list — 'mushrooms' in toppings.
Use not in to check that a value is missing from a list — handy for things like banned-user checks.

Key Terms

and: True only when both conditions are True
or: True when at least one condition is True
in / not in: checks whether a value is (or isn't) inside a list

Exercise

Activity 3

Pick two numbers and write both an and test and an or test that combine two comparisons on them.

Activity 4

Make a list of your favorite fruits, then write one test confirming a fruit is in the list and another confirming a fruit is not in the list.

5.3

if / else

Concept

if condition: runs the indented block only when the condition is True — if it's False, nothing happens.
Add else: to give Python something to do when the test fails — exactly one of the two blocks always runs.
Just like for loops, indentation decides what belongs to each block.

Key Terms

if: runs a block only when its condition is True
else: runs when the if condition was False

Exercise

Activity 5

Store a color as a string. If it's "green", print one message; otherwise print a different message — using if/else.

Activity 6

Store an age. Using if/else, print "You can drive" if it's 16 or older, and how many years remain otherwise.

5.4

The if-elif-else Chain

Concept

elif adds another test that only runs if every test above it failed — Python checks top to bottom and stops at the first one that passes.
else at the end catches everything that matched none of the tests above — it's optional.
Chain as many elif blocks as you need.
It's often cleaner to have the chain just set a variable, then print once after the chain, instead of repeating print() in every branch.

Key Terms

elif: an additional test, checked only if earlier tests failed
chain: an if / elif / … / else sequence — only one branch runs

Exercise

Activity 7

Give an alien a color — green, yellow, or red — and use if-elif-else to print how many points it's worth (5, 10, or 15).

Activity 8

Extend the admission-price example with a fourth bracket: seniors 65+ pay a discounted rate.

5.5

if Statements with Lists

Concept

Put an if inside a for loop to treat one special item differently from the rest as you loop.
An empty list is falsy in an if test; a list with at least one item is truthy — handy for checking "is there anything here?" before looping.
Check one list against another with in — e.g. confirm each requested topping is actually available before adding it to the pizza.

Key Terms

truthy / falsy: a non-empty list is True in an if test; an empty list is False

Exercise

Final challenge

Check a requested username (case-insensitively) against a list of already-registered usernames. If it's taken, ask for a different one; if the registered list is empty, print a welcome message instead.