Tutorial 3¶

This tutorial covers Statements, boolean logic, and Control Flow (if, else, elif statements).

1. What are Statements?¶

"Statements" are basically assertions made in your code that evaluate to either True or False (they cannot be both at the same time). Control flow statements inherently work on this simple principle: they evaluate the statement written, and if it evaluates to True, a specific code block gets executed.

cars = ['audi', 'bmw', 'subaru', 'toyota']

for car in cars: 
    if car == 'bmw':
        print(car.upper())
    else:
        print(car.title())

Assignment vs. Comparison¶

It is crucial not to mix these up:

• Assignment Operator (=): Used to assign a value to an object/variable.
• Comparison Operator (==): Used to compare the values stored in two objects. It is case-sensitive when comparing strings!
• Not Equal Operator (!=): Checks if a value is not equal to another.

x = y  # Assignment
x == y # Comparison

2. Boolean Values & Logical Operators¶

Expressions that evaluate to True or False are called boolean expressions. Variables holding these are boolean variables. They are the fundamental building blocks of while loops and if statements.

Using and & or¶

You can evaluate multiple conditions simultaneously using and (both must be true) or or (at least one must be true).

# The "and" operator
age_0 = 22
age_1 = 18
print(age_0 >= 21 and age_1 >= 21) # Output: False

age_1 = 22
print(age_0 >= 21 and age_1 >= 21) # Output: True

# The "or" operator
age_0 = 22
age_1 = 18
print(age_0 >= 21 or age_1 >= 21)  # Output: True

Checking if a Value is in a List¶

Use the in and not in keywords to verify if a specific item exists inside a list.

requested_toppings = ['mushrooms', 'onions', 'pineapple']

print('mushrooms' in requested_toppings) # Output: True
print('pepperoni' in requested_toppings) # Output: False

# Using "not in"
banned_users = ['andrew', 'carolina', 'david']
user = 'marie'

if user not in banned_users:
    print(f"{user.title()}, you can post a response if you wish.")

3. Control Flow Statements¶

The if Statement¶

Syntax: if some_condition: The colon (:) tells the interpreter that the next indented line is the start of the block. If the condition is false, the program skips the block entirely.

age = 19
if age >= 18:
    print("You are old enough to vote!")
print("thank you")

The if-else Statement¶

This adds a catch-all block. If the if statement handles a specific pool of cases, else handles everything else.

age = 17
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")
else:
    print("Sorry, you are too young to vote.")
    print("Please register to vote as soon as you turn 18!")

Dev Tip: Typecasting & User Input You can compose functions together! input("Prompt") takes text from the user. You can wrap it in a typecasting function like int() to instantly convert that text into an integer.

age = int(input("Please enter your age: "))

if age < 18:
    print("You are under age. Find your dad and go away.")
else:
    print("You are eligible to vote, congrats.")

(Note: As you level up as a dev, prioritize readability. Code is read many more times than it is written! You can actually run import this in Python to read the "Zen of Python" guidelines, though avoid putting random imports in the middle of your production code).

The if-elif-else Statement¶

When you have multiple mutually exclusive conditions, use elif (else-if). The interpreter checks them top-to-bottom. As soon as one evaluates to True, it executes that block and exits the entire control flow chain.

age = 12

if age < 4:
    price = 0
elif age < 18:
    price = 25
elif age < 65:
    price = 40
else:
    price = 20

print(f"Your admission cost is ₹{price}.")

Note: The else block at the end is entirely optional. If your if and elif statements cover all necessary logic, you can omit it.

Multiple if Statements¶

Use if-elif when only one valid case should execute. If your mental model allows for multiple valid cases to trigger simultaneously, use a series of independent if statements.

requested_toppings = ['mushrooms', 'extra cheese']

# Every single one of these will be checked, regardless of whether 
# the previous ones were True or False.
if 'mushrooms' in requested_toppings:
    print("Adding mushrooms.")

if 'pepperoni' in requested_toppings:
    print("Adding pepperoni.")

if 'extra cheese' in requested_toppings:
    print("Adding extra cheese.")

print("\nFinished making your pizza!")