Input and Decisions

Customers keep asking Ava the same question. Is my order eligible for free delivery? She wants a program that asks for the order total, and answers on its own. For that, the program needs two new skills. Asking the user for information, and making a decision.

• Skill one: ask the user for input
• Skill two: make decisions with if
• Goal: a free delivery checker

Asking with input()

The input function pauses the program, shows a message, and waits for the user to type. Whatever they type comes back as a string, always. Even if they type a number, it is still text. To do maths, convert it, with int for whole numbers, or float for decimals.

name = input("Your name: ")
print(f"Hello, {name}!")
qty = int(input("How many plants? "))
print("Ordered:", qty)
print("Next order:", qty + 1)

Your name: Maya
Hello, Maya!
How many plants? 3
Ordered: 3
Next order: 4

Comparison operators

To decide, Python compares values. Double equals checks if two values are equal. Remember, a single equals stores a value, and a double equals asks a question. Exclamation equals means not equal. Then greater than, less than, and their or equal versions. Every comparison gives back a boolean, True or False.

if, elif and else

Now the decision. The if keyword checks a condition, and ends with a colon. The indented lines below run only when the condition is true. Use elif for more conditions, and else for everything left over. The indentation, four spaces, is how Python knows which lines belong together. It is not decoration, it is the syntax.

total = 65
if total >= 100:
    print("Free delivery + gift!")
elif total >= 50:
    print("Free delivery!")
else:
    print("Delivery costs $5")

Free delivery!

and, or, not

Combine conditions with and, or, and not. And needs both sides to be true. Or needs at least one. Not flips true to false. Here, Ava gives a student discount only to students who also order at least two plants. Notice how the code reads almost like English.

is_student = True
plants = 3
if is_student and plants >= 2:
    print("Student discount applied")
if plants > 10 or is_student:
    print("You get a free pot")
if not is_student:
    print("Join our student club")

Student discount applied
You get a free pot

Truthy and falsy

Python can treat any value as true or false. Zero, an empty string, an empty list, and None count as false. Almost everything else counts as true. So you can write, if name, to check that the text is not empty. It is a small trick, loved by Python programmers.

The free delivery checker

Time to build Ava's checker. We ask for the total, convert it, and decide. Orders of fifty or more ship free, and others pay five dollars. Run it with a small order, and it tells the customer exactly how much more to spend. That is a real, useful program, in just a few lines.

total = float(input("Order total: $"))
if total >= 50:
    print("Free delivery!")
else:
    need = 50 - total
    print(f"Add ${need:.2f} more for free delivery")

Order total: $32.5
Add $17.50 more for free delivery

Plant care advisor

Your turn. Ask the user how many days since they watered their plant. If it is seven days or more, print, Time to water. Otherwise, print, It is fine for now. Ready? Here is one solution. Convert the input to an int, compare, and print. Try it with different numbers.

days = int(input("Days since watering: "))
if days >= 7:
    print("Time to water")
else:
    print("It is fine for now")

Days since watering: 9
Time to water

Summary