Conditionals and Loops

If, Elif, and Else Statements

If statements allow you to create conditions for certain code to run. This can make your programs a lot more interesting and varied.

import random

# Generates a random whole number from 1 to 10 using the random library imported above
random_number = random.randint(1, 10)

# Asks the user to input a number from 1 to 10 as a guess and turns that input into a number
guess = int(input("Guess the random number from 1 to 10: "))

if guess == random_number:
    print("You guessed correctly!")
else:
    print("You guessed incorrectly")

You can have multiple conditons before the else. For every condition after the first you use elif, short for else if.

username = input("Create a Username: ")

password = input("Create a Password: ")

if len(password) <= 8:
    print("Your password needs to be longer than 8 characters")
elif password.isupper() or password.islower():
    print("Your password needs both lowercase and uppercase letters")
elif username.lower() in password.lower():
    print("Your password must not contain your username")
else:
    print("Your password has been accepted")

Loops

Loops allow you repeat code over and over. There are 2 main types of loops, while loops and for loops.

A while loop will continue repeating code until a certain condition is no longer met. To show a while loop, we’re going to modify the number guessing game above. As it currently is, you can only guess once before the program ends. That’s boring. Using a while loop we can let the user continue guessing until they get it correct.

import random

# Generates a random whole number from 1 to 10 using the random library imported above
random_number = random.randint(1, 10)

# Asks the user to input a number from 1 to 10 as a guess and turns that input into a number
guess = int(input("Guess the random number from 1 to 10: "))

while guess != random_number:
    guess = int(input("You guessed incorrectly, try again: "))

print("You guessed correctly!")

Now the program will continue asking the user to guess the number until they guess it correctly.

A for loop lets you loop a set number of times, rather than until a condition is no longer met. Let’s create a for loop that sums the numbers from 1 to 10.

total = 0

for num in range(10):
    total += num + 1

print(total)

Let’s break this down a bit. range gives a sequence of numbers starting from 0 and ending before the given number. So in this case it gives the number sequence 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. num is being assigned one of those numbers as it goes through the loop which can then be used inside the loop as a variables. The code is saying for each number in this sequence, assign it to the variable num and then run the code inside the loop.

There are other ways to use range. Notice how we had to add 1 to num when summing the numbers, as they started at 0. We can avoid this by giving 2 numbers to range.

total = 0

for num in range(1, 11):
    total += num

print(total)

This is saying to start the sequence with the number 1 and end the sequence right before the number 11.

You can also tell range to jump numbers in a sequence. Let’s say you wanted to sum all the even numbers from 0 to 20.

total = 0

for num in range(0, 21, 2):
    total += num

print(total)

This is saying to start the sequence at 0, end it just before 21, and skip every second number. The given sequence would then be 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20.