While loops#

Normally Python reads one statement at a time, one at a time. But loops give us the ability to repeat a set of statements. In this lesson we will be learning about while loops.

Introduction #

A while loop is a compound statement. Like an if statement while loops include a conditional expression and a suite of statements that will be repeated until the expression evaluates to falsy.

Each time that the suite of statements is repeated in a loop it is called an iteration. Likewise, the process of repeating a loop is called iterating.

Syntax #

The syntax for a while loop is:

while CONDITION:
    BODY


`CONDITION`	an expression evaluated in a boolean context that determines if the loop keeps going
`BODY`	statements to execute each iteration

Here’s a simple example that will keep asking the user for their name until they answer.

Listing 57 while loop example#

name = ""

while not name:
  name = input("What is your name? ")

print(f"Hello there {name}!")

Exercises #

Exercise 16 (Heads or Tails)

Write a while loop that asks the user "Heads or tails?" until they answer with either "heads" or "tails". Hint: Use the not in operator.

Solution to Exercise 16 (Heads or Tails)

Listing 58 Heads or Tails Exercise#

answer = None
while answer not in ["heads", "tails"]:
  answer = input("Heads or tails? ")

Exercise 17 (Random numbers less than 50)

Write a while loop that prints a random number between 1 and 100 as long as the number is less than 50.

Solution to Exercise 17 (Random numbers less than 50)

Listing 59 Random numbers less than 50 Exercise#

import random

num = 0
while num < 50:
  num = random.randint(1, 100)
  print(num)

Quitting a loop #

The break statement can be used to stop loop iteration.

This example intentionally creates an infinite loop then in each iteration asks the user if they would like to keep going and uses the break statement if they reply with "n".

Listing 60 stopping a loop with break#

import random

while True:
    num = random.randint(1, 10)
    print("The number is:", num)
    reply = input("Keep going? ")
    if reply == "n":
        break

Exercise#

Exercise 18 (Random number not divisible by ten)

Write a while loop that prints a random number between 1 and 1000 as long as the number is less than 500. If the number is divisible by 10, quit the loop. Hint: Use the % operator.

Solution to Exercise 18 (Random number not divisible by ten)

import random
num = -1
while num < 500:
    num = random.randint(1, 1000)
    print(num)
    if num % 10 == 0:
        print("num is divisible by 10, quitting")
        break

Skipping part of an iteration #

You can skip the rest of the statements in a loop iteration by using the continue statement.

This example uses the continue statement to skip the rest of the iteration if the user does not enter a number.

Listing 61 using the continue statement#

balance = 100
while balance > 0:
    reply = input("amount: ")
    if not reply.isnumeric():
        print("Numbers only.")
        continue
    balance = balance - int(reply)
    print("Your balance is:", balance)

Exercise 19 (Dice)

Assign chances to 0
Assign rolls to an empty list
Write a loop that repeats 10 times. In each iteration:
1. Assign random number between 1 and 6 to the num variable
2. Print "You rolled: num"
3. Add 1 to chances
4. Ask the user if they want to keep the roll
5. If they reply with "n", use the continue statement to skip the rest of the iteration
6. Append num to a list of rolls
7. Print the rolls list

Solution to Exercise 19 (Dice)

chances, rolls = 0, []

while chances < 10:
  num = random.randint(1, 6)
  print(f"You rolled: {num}")

  chances += 1

  keep = input("Keep? ")
  if keep.lower() == "n":
    continue

  rolls.append(num)
  print(f"Your rolls so far: {rolls}")

Loop patterns #

In this section we’re going to go over some of the ways that loops are commonly used.

Incrementing and decrementing #

Often in programming we want to repeat a suite of statements a specific number of times.

One way to do this is to keep track of which iteration the loop is currently executing by incrementing or decrementing a number.

Listing 62 incrementing the i variable#

i = 0

while i < 10:
  print("Iteration:", i+1)
  i = i + 1

Exercise#

Exercise 20 (Countdown)

Write a loop that counts down from 3 to 1. In each iteration print out the current number then use the time.sleep function to pause for one second.

Solution to Exercise 20 (Countdown)

import time

num = 3
while num >= 1:
  print(f"{num}...")
  num = num - 1
  time.sleep(1)

Infinite loops #

When a program is written in such a way that the loop condition is always met the result is called an infinite loop. These are easy to cause by mistake.

The simplest infinite loop is while True with no break:

Listing 63 while True#

import time

while True:
  print("Are we there yet?")
  time.sleep(1)

A common mistake is to forget to increment your counter, as demonstrated below.

Listing 64 forgetting to increment i#

import time

i = 0

while i < 10:
  print("Iteration:", i)
  time.sleep(1)

Infinite loops are not always a bad thing though. In the example below an infinite loop is used to always return the user to a main menu.

Listing 65 infinite loop used to keep a program running#

CHOICES = {
  "draw": "draw a card",
  "play": "play a card",
  "pass": "pass this turn",
}

def menu():
  """Print user interface, and act according to user choice"""
  print("menu:", ", ".join(CHOICES))
  selection = input("> ").lower().strip()
  if selection not in CHOICES:
    print(f'"{selection}" is not a valid selection, try again.')
    return

  text = CHOICES[selection]
  print(f"{text}...")

def main():
  """."""
  while True:
    menu()
    print()

main()

You can use a while loop to iterate over the items in a list. To do this we’ll increment an i variable just like we did above but we’ll use the len() function to determine the length of the list and therefore how many times the loop should repeat.

Then we can use the i variable to access each item in the list.

Listing 66 iterating over a list#

i = 0
colors = ["red", "green", "blue"]

while i < len(colors):
  color = colors[i]
  print("Color", i, "is:", colors[i])
  i += 1

Exercise 21 (Lunch menu)

Print a lunch menu. Make a list of lunch choices for a menu. Use a while loop to print out each item in the list with the number next to it.

Solution to Exercise 21 (Lunch menu)

Listing 67 Lunch Menu Exercise#

choices = [
  "Pepperoni pizza",
  "Ham sandwich",
  "Fish tacos",
  "Bean burrito",
]

i = 0

print("Lunch menu")
print("----------", "\n")

while i < len(choices):
  item = choices[i]
  print(f"{i+1}. {item}")
  i += 1

You can also use this to iterate over list-like values. For example, a string can be used like a list of characters. So we can use the same pattern to iterate over all of the characters in a string.

Listing 68 iterating over the characters in a string#

word = input("Enter a word: ")

i = 0
while i < len(word):
    print("Letter", i, "is:", word[i])
    i += 1

Exercise 22 (Vowels)

Print the vowels in a word. Ask the user for a word then iterate over each letter. If the letter is a vowel print the letter. (Hint: make a list of vowels the use the in operator)

Solution to Exercise 22 (Vowels)

Listing 69 Vowels Exercise#

VOWELS = ["a", "e", "i", "o", "u", "y"]
word = input("Enter a word: ")

i = 0
while i < len(word):
  letter = word[i]
  if letter in VOWELS:
    print(letter, end="")
  i += 1
print()

Nested loops #

Just like you can have an if statement inside of another if statement, you can also have a while loop inside of another while loop.

This example prints out a score card for three rounds of three players each.

rounds, players = 3, 3
r = 0
while r < rounds:
    print("Round:", r+1, "\n")
    p = 0
    while p < players:
        print("Player", p+1, "score:", "______________")
        p += 1
    print()
    r += 1

Round: 1 

Player 1 score: ______________
Player 2 score: ______________
Player 3 score: ______________

Round: 2 

Player 1 score: ______________
Player 2 score: ______________
Player 3 score: ______________

Round: 3 

Player 1 score: ______________
Player 2 score: ______________
Player 3 score: ______________

A few things to pay attention to with nested loops:

The child p counter needs to be set to 0 inside the parent loop. If you forget this, the score line for each player will only be printed in the first round.
The child p counter must be incremented inside of the child loop.
In this case it doesn’t matter, but you generally want to increment the r counter at the end of the parent loop, just in case you need to use the current value of r inside of the child loop.

Here’s another example that prints out a grid of x, y coordinates.

rows, cols = 5, 5
r = 0
while r < rows:
    c = 0
    while c < cols:
        output = str(r) + "," + str(c) + "   "
        print(output, end="")
        c += 1
    print("\n")
    r += 1

0,0   0,1   0,2   0,3   0,4   

1,0   1,1   1,2   1,3   1,4   

2,0   2,1   2,2   2,3   2,4   

3,0   3,1   3,2   3,3   3,4   

4,0   4,1   4,2   4,3   4,4

Exercises#

Exercise 23 (multiplication-table)

Print a multiplication table grid with 9 rows and 9 columns.

Solution to Exercise 23 (multiplication-table)

Listing 70 Multiplication Table Exercise#

SIZE = 9

print()

x = 1
while x <= SIZE:
    print("  ", end="")
    y = 1
    while y <= SIZE:
        val = x * y
        print(str(val).rjust(4), end="  ")
        y += 1
    x+=1
    print("\n")

main()

Exercises #

Exercise 24 (Name Cheer)

Print a cheer for your name. For each letter print "Gimme a letter!". Then end it with, "What does it spell? name!"

Solution to Exercise 24 (Name Cheer)

Listing 71 Name Cheer Exercise#

import time

NAME = "Alissa"

i = 0
while i < len(NAME):
    print("Gimme a", NAME[i].upper() + "!")
    i += 1
    time.sleep(0.5)

print("\nWhat does it spell?")
print(NAME.upper() + "!")

Exercise 25 (Check Password)

Assign a secret variable to a string of your choice for the correct password.
Assign a chances variable to 3
Assign a password variable to None
Assign a locked variable to True
Write a while loop that continues as long as password is not equal to secret.
In the loop:
1. Ask the user for the password using the input() function, and assign the result to password.
2. If password is equal to secret, set locked to False
3. Quit the loop if there are no more chances
4. subtract 1 from chances
After the loop: if locked is False
1. Print "Welcome!"

Solution to Exercise 25 (Check Password)

Listing 72 Check Password Exercise#

secret = "00000"
password, locked, chances = None, True, 3

while password != secret:
  password = input("Enter the password: ")
  if password == secret:
    locked = False

  if not chances:
    print("Too many attempts, try again later.")
    break

  chances = chances - 1

if not locked:
  print("Welcome.")

Exercise 26 (Jolly Good Fellow)

Print the lyrics to the jolly good fellow song. "For he's a jolly good fellow..." three times, then “Which nobody can deny!” once.

Solution to Exercise 26 (Jolly Good Fellow)

Listing 73 Jolly Good Fellow Exercise#

i = 0
while i < 3:
    print("For he's a jolly good fellow...")
    i += 1

print("Which nobody can deny!")

Exercise 27 (12 Days of Christmas)

Print the 12 Days of Christmas song.

Create a nested list, where each item is a list containing two items: the ordinal word for the day (ie “second”) and the gift for that day.
Iterate over each day to print the parent day lyric "On the ordinal day of Christmas my true love gave to me gift."
Use a nested while loop to iterate over each of the days lower than parent day in descending order.
For each child day, print the lyric "day gift,"

Solution to Exercise 27 (12 Days of Christmas)

Listing 74 12 Days of Christmas Exercise#

import time

DAYS = [
    ['first', "a partridge in a pear tree"],
    ['second', "turtle doves"],
    ['third', "french hens"],
    ['forth', "calling birds"],
    ['fifth', "golden rings"],
    ['sixth', "geese a laying"],
    ['seventh', "swans a swimming"],
    ['eighth', "maids a milking"],
    ['ninth', "ladies dancing"],
    ['tenth', "lords a leaping"],
    ['eleventh', "pipers piping"],
    ['twelfth', "drummers drumming"],
]

i = 0
while i < len(DAYS):
    day, gift = DAYS[i]
    print("On the", day, "day of Christmas my true love gave to me")

    x = i
    while x >= 0:
        gift = DAYS[x][1]
        time.sleep(0.5)

        # indent the line
        line = "  "

        # add the "and" in "and a partridge in a pear tree"
        if i and x == 0:
            line += "and "

        # add the number of gifts
        if x:
            line += str(x+1) + " "

        # add the gift given
        line += gift

        # add the "," or "."
        if x:
            line += ","
        else:
            line += "."

        # print the line
        print(line)

        x -= 1

    i += 1

    print()

Exercise 28 (Hangman)

Write a hangman game.

Choose a short word, give the player 6 chances to guess letters
Each turn:
1. Print their chances with x to show used chances, and _ to show remaining
  example: chances: xx____
2. Print the word, but replace _ for unguessed letters
  example: 5 letters: _e___
3. Ask the player to guess a letter
4. Bonus: make sure the user enters exactly one character
Tell the user at the end if they won or lost

Solution to Exercise 28 (Hangman)

Listing 75 Hangman Exercise#

import random

WORDS = [ "hello", "goodbye", "secret", "satisfy", "apple", "bear" ]

def info(turn, chances, guess):
    """print the used vs unused chances and guessed vs unguessed letters
       example:
           chances: xx____ 7 letters: _e_____
    """
    print(" "*40, "chances:", ("x"*turn)+("_"*(chances-turn)), len(guess), "letters:", guess)

def main():
    """."""
    turn, chances, word = 1, 6, random.choice(WORDS)
    guess = "_" * len(word)

    info(0, chances, guess)

    while guess != word and turn <= chances:
        char = input("Guess a letter: ").lower()
        if not char:
            continue
        elif len(char) > 1:
            print(" Just one letter!")
            continue
        if char in word:
            letters = list(guess)
            for i,c in enumerate(word):
                if c == char:
                    letters[i] = c
            guess = "".join(letters)
        print(" "*40, "chances:", ("x"*turn)+("_"*(chances-turn)), len(word), "letters:", guess)
        turn += 1
    if guess == word:
        print("You win!!")
    else:
        print("Too bad. The word was:", word)

main()

Reference #

Glossary #

While Loops#

while loop#: A compound statement that repeats a suite of statements as long as a condition evaluates to truthy.
infinite loop#: When a program is written in such a way that the loop continues forever.
increment#
incrementing#: Adding to a number, usually 1.
decrement#
decrementing#: Subtracting from a number, usually 1.

Python Class

While loops

Contents

While loops#

Introduction #

Syntax #

Exercises #

Quitting a loop #

Exercise#

Skipping part of an iteration #

Loop patterns #

Incrementing and decrementing #

Exercise#

Infinite loops #

Iterating over a list #

Nested loops #

Exercises#

Exercises #

Reference #

Glossary #

While Loops#

See also #