Making Decisions

Until now, the programs in this part have mostly followed one straight path. They ask questions, perform calculations, and print results. Real programs often need to behave differently in different situations.

A ticket may cost less for a child. A study session may be classified as short or long. A login message may depend on whether the user entered the expected code.

To make those choices, a program needs expressions that answer yes-or-no questions and a way to use those answers to decide what happens next.

Comparisons Produce Boolean Values

Python has comparison operators for values:

Examples:

print(5 > 3)
print(5 == 3)
print(5 != 3)

The output is:

True
False
True

The result of a comparison is a boolean value.

= and == Are Different

One of the most common beginner mistakes is confusing assignment with equality comparison.

score = 10

assigns the value 10 to score.

score == 10

asks whether the current value of score equals 10 and produces True or False.

Read them differently:

• = means store this value under this name;
• == means are these two values equal?

Comparing Strings

Strings can be compared for equality too:

answer = "yes"
print(answer == "yes")

This is case-sensitive:

print("YES" == "yes")

is False.

This is why normalizing user input is often useful:

answer = input("Continue? ").strip().lower()
print(answer == "yes")

Now inputs such as YES, yes, and Yes all become yes before comparison.

The Basic if

A boolean expression becomes especially useful when it controls whether some code runs.

An if statement runs a block of code only when its condition is true.

minutes = 75

if minutes >= 60:
    print("That was at least an hour.")

print("Session recorded.")

The indented line belongs to the if. The final line does not.

If minutes is 75, both messages are printed. If minutes is 30, the first message is skipped and Session recorded. is still printed.

Two syntax details matter:

• the if line ends with a colon (:);
• the body is indented, normally by four spaces.

You can read the statement almost like ordinary language:

If minutes >= 60 is true, print the message.

Warning: below freezing.

Temperature checked.

else for the Alternative

Often there are two mutually exclusive cases:

age = int(input("Age: "))

if age >= 18:
    print("Adult ticket")
else:
    print("Youth ticket")

Exactly one branch runs.

The else does not need its own condition. It means “if the previous condition was false.”

elif for Several Cases

When there are more than two categories, use elif:

minutes = int(input("Minutes: "))

if minutes < 20:
    print("Short session.")
elif minutes <= 60:
    print("Solid session.")
else:
    print("Long session.")

Python checks the conditions from top to bottom. As soon as one condition is true, that branch runs and the rest are skipped.

If minutes is 45:

1. 45 < 20 is false;
2. 45 <= 60 is true;
3. the second branch runs;
4. the else is skipped.

Branch Order Matters

Consider this code:

age = 12

if age < 18:
    print("Minor")
elif age < 13:
    print("Child")

The second branch can never run. Anyone younger than 13 is already younger than 18, so the first condition catches them first.

A better order is:

if age < 13:
    print("Child")
elif age < 18:
    print("Teenager")
else:
    print("Adult")

When conditions overlap, the order determines which one wins.

Boundaries Need Deliberate Attention

Suppose a discount applies to customers aged 65 or older.

The correct comparison is:

age >= 65

not:

age > 65

The second version wrongly excludes people who are exactly 65.

Boundary values are the values directly at the edge of a rule. They deserve special tests because a single character can change the behavior:

<   versus   <=
>   versus   >=

When you write a classification, ask:

• What happens one step below the boundary?
• What happens exactly at the boundary?
• What happens one step above it?

This is the beginning of a testing habit you will develop more systematically in later parts.

Independent if Statements Are Different

These two programs do not mean the same thing.

Program A:

if score >= 70:
    print("Passed")

if score >= 90:
    print("Excellent")

Program B:

if score >= 90:
    print("Excellent")
elif score >= 70:
    print("Passed")

If score is 95, Program A prints both messages. Program B prints only Excellent.

Use separate if statements when several independent facts may all be true. Use an if/elif/else chain when you want to select one category from several alternatives.

Combining Conditions

Sometimes one decision depends on more than one yes-or-no question. Boolean expressions can be combined for those situations.

and

and is true only when both sides are true:

age = 20
has_ticket = True

if age >= 18 and has_ticket:
    print("You may enter.")

The message is printed only if the person is old enough and has a ticket.

or

or is true when at least one side is true:

is_student = True
is_senior = False

if is_student or is_senior:
    print("Discount available.")

not

not reverses a boolean value:

is_closed = False

if not is_closed:
    print("The service is open.")

You do not need complicated boolean expressions in Part 1. In fact, complicated expressions are often easier to understand when they are split into named pieces:

old_enough = age >= 18
has_permission = answer == "yes"
allowed = old_enough and has_permission

if allowed:
    print("Allowed")

Named conditions are often easier to trace and explain.

True

A Complete Decision Program

Here is a small program that combines input, conversion, arithmetic, string normalization, and conditions:

name = input("Name: ").strip()
minutes = int(input("Study minutes today: "))
energy = input("Energy level (low/okay/high): ").strip().lower()

hours = minutes / 60

print()
print(f"{name}, you studied for {hours:.1f} hours.")

if minutes < 20:
    print("A short session still counts.")
elif minutes <= 90:
    print("That is a solid block of work.")
else:
    print("That was a long session. Remember to take breaks.")

if energy == "low":
    print("Consider choosing a lighter next task.")
elif energy == "high":
    print("You may have energy for another focused task.")

Notice that the duration classification and the energy message use separate decision structures. They describe different aspects of the session, and both may produce output.

Common Pitfalls

• Writing = instead of == in a comparison.
• Forgetting the colon at the end of if, elif, or else.
• Using inconsistent indentation.
• Putting a broad condition before a narrower one in an elif chain.
• Forgetting to normalize text before comparing when case and whitespace should not matter.
• Choosing < when the requirement really says “less than or equal to,” or vice versa.
• Assuming that if a program works on one test value, all boundary values must also be correct.
• Combining so many conditions in one expression that the rule becomes hard to explain.

Check Your Understanding

1. What type of value does a comparison produce?
2. What is the difference between = and ==?
3. What determines whether the body of an if statement runs?
4. In an if/elif/else chain, how many branches can run?
5. Why does branch order matter?
6. Why should exact boundary values be tested deliberately?
7. What is the difference between two separate if statements and an if followed by elif?
8. When might one decision need and or or?