Learning Python: Day 18

Using int() to accept numerical input

with the input() function, Python interprets everything as a string

>>> age = input(“How old are you? “)

How old are you? 21

>>> age

’21’

when we ask for the value of age, Python returns the string ’21’
you’ll get an error if you use the input as a number

>>> age = input(“How old are you? “)

How old are you? 21

>>> age >= 18

TypeError ‘>=’ not supported between instances of ‘str’ and ‘int’

string ’21’ can’t be compared to the numerical value 18
can be resolved by using int() function

>>> age = input(“How old are you? “)

How old are you? 21

>>> age = int(age)

>>> age >= 18

True

after entering 21, we convert it from a string to a numerical representation
this allows Python to run the conditional test to compares ages
real-world example testing if someone is tall enough to ride a roller coaster

rollercoaster.py

height = input(“How tall are you, in inches? “)

height = int(height)

if height >= 48:

….print(“\nYou’re tall enough to ride!”)

else:

….print(“\nYou’ll be able to ride when you’re a little older.”}

can compare height to 48 because height = int(height)

How tall are you, in inches? 71

You’re tall enough to ride!

when you use a numerical input to do calculations and comparisons, convert the input value to a numerical representation first

The modulo operator

modulo operator (%) – a useful tool for working with numerical information that divides one number by another number and returns the remainder

>>> 4 % 3

>>> 5 % 3

>>> 6 % 3

>>> 7 % 3

modulo operator only tells you the remainder, which can be useful for determining if a number is even or odd

even_or_odd.py

number = input(“Enter a number, and I’ll tell you if it’s even or odd: “)

number = int(number)

if number % 2 == 0

….print(f”\nThe number {number} is even.”)

else:

….print(f”\nThe number {number} is odd.”)

since even numbers are always divisible by 2, if the module of a number and two is zero, it’s even
otherwise, it’s odd

Enter a number, and I’ll tell you if it’s even or odd: 42

The number 42 is even.

Introducing while loops

for loop executes a block of code for each item in a collection
while loop runs as long as a certain condition is true

The while loop in action

you can use a while loop to count up through a series of numbers

counting.py

current_number = 1

while current_number <= 5:

….print(current_number)

….current_number += 1

first line assigns value to 1
second line sets a while loop to run while value is less than or equal to 5
third line inside the loop will print the value current_number
fourth line uses the += operator which is shorthand for current_number = current_number + 1

most programs use while loops
a game will keep running unless you ask it to quit

Letting the user choose when to quit

parrot.py will keep running as long as the user wants by putting most of the program inside a while loop
quit value = stop a program running

parrot.py

prompt = “\nTell me something, and I will repeat it back to you:”

prompt += “\nEnter ‘quit’ to end the program. “

message = “”

while message != ‘quit’:

….message = input(prompt)

….print(message)

as long as the value of message is not quit, the while loop continues to run
first time through the loop, message is an empty string
at message = input(prompt), user is prompted to enter their input
input is assigned to message and printed
condition is reevaluated, as long as quit is not entered, the user is prompted again to enter their input
‘quit’ will cause Python to stop executing the while loop

Tell me something, and I will repeat it back to you:

Enter ‘quit’ to end the program. Hello everyone!

Hello everyone!

Tell me something, and I will repeat it back to you:

Enter ‘quit’ to end the program. Hello again.

Hello again.

Tell me something, and I will repeat it back to you:

Enter ‘quit’ to end the program. quit

quit

a simple if test will prevent Python from printing quit

prompt = “\nTell me something, and I will repeat it back to you:”

prompt += “\nEnter ‘quit’ to end the program. “

message = “”

while message != ‘quit’:

….message = input(prompt)

….if message != ‘quit’:

……..print(message)

now program does a check before displaying the message

Tell me something, and I will repeat it back to you:

Enter ‘quit’ to end the program. Hello everyone!

Hello everyone!

Tell me something, and I will repeat it back to you:

Enter ‘quit’ to end the program. Hello again.

Hello again.

Tell me something, and I will repeat it back to you:

Enter ‘quit’ to end the program. quit

Using a flag

imagine a game that needs to end if any one of the multiple conditions is met
trying to test all these conditions in one while statement becomes complicated
flag – acts as a signal to the program
can write a program to run while the flag is set to True and stop running when any of several events sets the value of the flag to False
while statement needs to check only one condition: whether the flag is currently True
let’s add a flag called active to parrot.py, which will monitor whether or not the program should continue running

prompt = “\n Tell me something, and I will repeat it back to you:”

prompt += “\nEnter ‘quit’ to end the program. “

active = True

while active:

….message = input(prompt)

if message == ‘quit’:

….active = False

else:

….print(message)

can add more tests (such as elif statements) for events that should cause active to become False

Using break to exit a loop

break statement – used to exit a while loop immediately without running any remaining code in the loop, regardless of the results of any conditional test
it directs the flow of your program; can be used to control which lines of code are executed and which aren’t, the program only executes code that you want it to, when you want it to
for example, a program asking users about places they’ve visited
can stop the while loop by calling break as soon as the user enters the ‘quit’ value

cities.py

prompt = “\nPlease enter the name of a city you have visited:”

prompt += “\n(Enter ‘quit” when you are finished.) “

while True:

….city = input(prompt)

….if city == ‘quit’:

……..break

….else:

……..print(f”I’d love to go to {city.title()}!”)

loop that starts with while True will run forever unless it reaches a break statement

Please enter the name of a city you have visited:

(Enter ‘quit’ when you are finished.) New York

I’d love to go to New York!

Please enter the name of a city you have visited:

(Enter ‘quit’ when you are finished.) San Francisco

I’d love to go to San Francisco!

Please enter the name of a city you have visited:

(Enter ‘quit’ when you are finished.) quit

can use the break statement in any of Python’s loops, such as a for loop to quit working through a list or dictionary

Using continue in a loop

continue statement – can be used to return to the beginning of the loop, based on the result of a conditional test rather than breaking out of a loop without executing the rest of its code
for example, a loop that counts from 1 to 10 but prints only odd numbers

counting.py

current_number = 0

while current_number < 10:

….current_number += 1

….if current_number % 2 == 0:

……..continue

….print(current_number)

if modulo is 0, the continue statement tells Python to ignore the rest of the loop and return to the beginning
if the current number is odd, the rest of the loop is executed and Python prints the current number

Avoiding Infinite Loops

every while loop needs a way to stop running so it won’t continue running forever

counting.py

x = 1

while x <= 5:

….print(x)

….x += 1

if you omit the line x += 1, the loop will run forever

# This loop runs forever!

x = 1

while x <= 5:

….print(x)

print a series of 1s

–snip–

infinite while loops will occur from time to time
press CTRL-C or close the terminal window
test every while loop to ensure it stops when expected to
one part of the program should make the loop’s condition False or cause it to reach a break statement

Using a while loop with lists and dictionaries

will need to use lists and dictionaries with our while loops to keep track of many users and pieces of information
shouldn’t modify a list inside a for loop because Python will have trouble keeping track of the items in the list
use a while loop to modify a list as you work through it

Moving items from one list to another

can use a while loop to pull users from the list of unconfirmed users as we verify them and then add them to a separate list of confirmed users

confirmed_users.py

# Start with users that need to be verified,

# and an empty list to hold confirmed users.

unconfirmed_users – [‘alice’, ‘brian’, ‘candace’]

confirmed_users = []

# Verify each user until there are no more unconfirmed users.

# Move each verified user into the list of confirmed users.

while unconfirmed_users:

….current_user = unconfirmed_users.pop()

….print(f”Verifying user: {current_user.title()}”)

….confirmed_users.append(current_user)

# Display all confirmed users.

print(“\nThe following users have been confirmed:”)

for confirmed_user in confirmed_users:

….print(confirmed_user.title())

when the list of unconfirmed users is empty, the loop stops and the list of confirmed users is printed

Verifying user: Candace

Verifying user: Brian

Verifying user: Alice

The following users have been confirmed:

Candace

Brian

Alice

Removing all instances of specific values from a list

remove() function works to remove one instance of a value
to remove all instances of a value, run a while loop until the value is no longer in the list

pets.py

pets = [‘dog’, ‘cat’, ‘dog’, ‘goldfish’, ‘cat’, ‘rabbit’, ‘cat’]

print(pets)

while ‘cat’ in pets:

….pets.remove(‘cat’)

print(pets)

[‘dog’, ‘cat’, ‘dog’, ‘goldfish’, ‘cat’, ‘rabbit’, ‘cat’]

[‘dog’, ‘dog’, ‘goldfish’, ‘rabbit’]

Filling a dictionary with user input

can prompt for as much input as you need in each pass through a while loop

mountain_poll.py

response = {}

# Set a flag to indicate that polling is active.

polling_active = True

while polling_active:

….# Prompt for the person’s name and response.

….name = input(“\nWhat is your name? “)

….response = input (“Which mountain would you like to climb someday? “)

….# Store the response in the dictionary.

responses[name] = response

….# Find out if anyone else is going to take the poll.

repeat = input(“Would you like to let another person respond? (yes/ no) “)

….if repeat == ‘no’

……..polling_active = False

….# Polling is complete. Show the results.

….print(“\n— Pol Results —“)

….for name, response in responses.items():

……..print(f”{name} would like to climb {response}.”)

program defines an empty dictionary (responses) and sets a flag (polling_active)
Python will run the code in the while loop as long as polling_active is True

What is your name? Eric

Which mountain would you like to climb someday? Denali

Would you like to let another person respond? (yes/ no) yes

What is your name? Lynn

Which mountain would you like to climb someday? Devil’s Thumb

Would you like to let another person respond? (yes/ no) no

— Poll Results —

Eric would like to climb Denali.

Lynn would like to climb Devil’s Thumb.

Summary

learned how to use input() to allow users to provide their own information
learned to work with both text and numerical input
learned how to use while loops to allow programs to run as long as needed
saw several ways to control the flow of a while loop by setting an active flag, using the break statement, and using the continue statement
learned how to use a while loop to move items between lists and remove all instances of a value from a list
learned how while loops can be used with dictionaries

End of study session.

Learning Python: Day 18

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