Making Decisions

Step 1: Asking yes/no questions

A comparison asks a question about two values. The answer is always True or False.

print(7 > 3)
print(7 == 3)
print(7 != 3)
print(5 <= 5)

True
False
True
True

Be careful: = stores a value, and == compares two values. They are not the same.

Step 2: if and else

An if runs some code only when its condition is True. else runs when it is not.

temperature = 31

if temperature > 30:
    print("It's hot. Drink water!")
else:
    print("Nice weather.")

It's hot. Drink water!

Two things to notice:

1. The if line ends with a colon (:).
2. The lines that belong inside are indented by 4 spaces. That is how Python knows where the block starts and ends.

Step 3: More than two choices with elif

elif means “else if”. Python checks the conditions from top to bottom and runs the first one that is true.

score = 82

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Keep practicing!")

Grade: B

Why the order matters: 82 is also greater than 70, but Python stops at the first true condition. If you wrote the >= 70 check first, everyone above 70 would get a C.

Step 4: and, or, not, and the remainder

Combine conditions with and, or and not.

age = 15
has_ticket = True

if age >= 13 and has_ticket:
    print("Come in!")

if age < 5 or age > 100:
    print("Please ask a grown-up.")

print(not has_ticket)

Come in!
False

• and needs both sides to be true.
• or needs at least one side to be true.
• not flips True to False and False to True.

The % operator gives the remainder of a division. It is the standard way to test for even numbers.

print(7 % 2)
print(8 % 2)

1
0

If number % 2 == 0, the number divides evenly by 2, so it is even.

Step 5: Putting it together

Here is a short program that makes a decision from the user’s input.

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

if age >= 18:
    print("You can vote in many countries.")
elif age >= 13:
    print("You're a teenager.")
else:
    print("You're still a kid. Enjoy it!")

How old are you? 15
You're a teenager.

Tip: When your program does the wrong thing, test it with the “edge” values: the numbers right on either side of each rule, like 12 and 13.