03. Iterations

8/28/26About 13 minAbout 1878 words

03. Iterations

While & for-loops

Concept: More random numbers

For random floating-point numbers: Python's random module has random.random(), which returns a random float between 0.0 (inclusive) and 1.0 (exclusive). You can scale that yourself, e.g. random.random() * 10 gives a float from 0 up to (but not including) 10. randint(a, b) gives a random integer including both endpoints.

Exercise 1 — Countdown

Ask the user for a starting number.

Use a while loop to count down to zero.

When the countdown reaches zero, print:

GO!

Example:

Starting number: 5

5
4
3
2
1
GO!

Use time.sleep() so that the numbers don't appear immediately one after another, but leave one second between them.

Exercise 2 — Number guessing game

Create a random number between 1 and 100 using randint().

Ask the user to guess the number repeatedly.

After every guess, tell them:

Too high
Too low
Correct!

Keep asking until they guess correctly.

At the end, print how many attempts they needed.

Example:

Guess the number (1-100): 50
Too high!

Guess the number (1-100): 25
Too low!

Guess the number (1-100): 37
Correct!

You needed 3 attempts.

Use a while loop and randint().

Exercise 3 — Password attempts

Create a program with a secret password.

The user gets three attempts to enter the correct password.

After each incorrect attempt, print how many attempts remain.

If the password is correct:

Login successful!

If all three attempts are wrong:

Too many incorrect attempts.

Then use:

sys.exit(1)

to terminate the program.

If the login succeeds, terminate using:

sys.exit(0)

Use while, if/else, ==, sys.exit() and input().

Exercise 4 — Dice rolling simulator

Ask the user how many dice they want to roll.

For each die:

generate a random number between 1 and 6
print the result

Afterwards, print the total.

For example:

How many dice? 5

Die 1: 4
Die 2: 6
Die 3: 2
Die 4: 5
Die 5: 1

Total: 18

Use a for loop and range().

Then extend the program:

Ask whether the user wants to roll the dice again.

If they enter "yes", repeat the whole process.

This means you'll need a while loop containing a for loop.

Exercise 5 — ATM simulator

Write a small ATM program.

Start with:

balance = 1000

Ask the user for their name and PIN.

The correct PIN is:

Give the user a maximum of three attempts to enter the PIN.

If the PIN is incorrect three times, print an error message to stderr:

Too many incorrect PIN attempts.

and exit with:

sys.exit(1)

After successfully logging in, repeatedly show a menu:

====================
ATM
====================
1. Show balance
2. Deposit money
3. Withdraw money
4. Exit
====================

The user chooses an option.

Option 1: Print the current balance.
Option 2: Ask how much money they want to deposit and add it to the balance.
Option 3: Ask how much they want to withdraw. Don't allow them to withdraw more than the current balance. If they try, print the error to the standard error output (sys.stderr).
Option 4: Print a goodbye message and exit gracefully.

For an invalid menu option, print an error to stderr but keep the ATM running.

Exercise 6 — Lottery simulator

Create a lottery simulator.

Ask the user how many lottery tickets they want to play.

For every ticket, generate six random numbers between 1 and 45 and display them.

After all tickets have been generated, display the total number of tickets and the total number of numbers generated.

Then ask the user whether they want to generate another set of tickets.

If they answer "yes", start again.

If they enter anything else, print a goodbye message and end the program successfully.

Challenge: Make sure that each ticket contains six independently generated numbers.

Exercise 7 — Temperature monitoring

Write a program that simulates a temperature sensor.

Ask the user for:

the minimum allowed temperature
the maximum allowed temperature
the number of measurements to simulate

Generate that number of random temperature measurements between 10 and 40 degrees. Use floating-point values rather than integers.

For every measurement, display:

measurement number
measured temperature
whether the temperature is normal, too low, or too high

Keep track of how many measurements were:

too low
normal
too high

At the end, display a summary such as:

Temperature report
------------------
Measurements: 10
Too low: 2
Normal: 6
Too high: 2

If more than half of the measurements are outside the allowed range, print a warning message to the error output and terminate with an error status.

Otherwise, print that the sensor is operating normally.

Exercise 8 — Quiz game

Create a small quiz game.

The program should ask the user several multiple-choice questions.

For every question:

display the question
display four possible answers
ask the user to choose an answer
check whether the answer is correct
keep track of the score

After every question, briefly wait before showing the next question.

At the end, calculate the percentage of correct answers.

Give the player a result:

90% or higher → Excellent
75–89% → Very good
50–74% → Passed
below 50% → Failed

Ask the player whether they want to play again.

If they choose "yes", start a new game with the score reset to zero.

If they enter anything else, end the program.

Exercise 9 — Guess the number: difficulty levels

Create a more advanced version of the number guessing game.

First, ask the user to choose a difficulty:

Easy
Medium
Hard

The difficulty determines:

Difficulty	Range	Maximum attempts
Easy	1–20	10
Medium	1–100	7
Hard	1–500	5

Generate a random number within the selected range.

The player repeatedly enters guesses.

After every guess, tell them whether their guess is:

too high
too low
correct

Keep track of the number of attempts.

If they guess correctly, display a success message including the number of attempts.

If they use all their attempts without guessing correctly, display the correct number and an error message.

Ask whether they want to play again.

Extra challenge: After every incorrect guess, also tell the player whether their guess was very close to the secret number. Decide yourself what "very close" means.

Exercise 10 — Mini command-line game

Create a small text-based adventure game.

The player starts with:

a name
100 health points
0 gold

At the beginning, ask the player for their name.

Then repeatedly present a menu:

========================
Adventure Game
========================
1. Explore
2. Rest
3. Show status
4. Quit
========================

Explore

When the player explores, generate a random event.

Possible events include:

finding gold
encountering an enemy
finding nothing
finding a healing potion

Finding gold increases the player's gold.

An enemy causes random damage.

A healing potion restores some health, but health may never exceed 100.

Display what happened.

Rest

Resting restores some health.

However, resting should take a few seconds, with a short pause before the player is told that they have recovered.

Show status

Display:

player name
health
gold

Use a ternary expression to indicate whether the player is currently "Healthy" or "Injured".

Quit

Display a goodbye message and terminate successfully.

Game over

If the player's health reaches zero, print an appropriate message to the error output and terminate with an error status.

The game should continue running until the player quits or dies.

Break and continue

Exercise 1 — Input validator

Create a program that repeatedly asks the user to enter a positive integer.

The program should:

accept the number if it is valid
reject invalid input
keep asking until the user enters a valid number
stop immediately when a valid number has been entered

Afterwards, print the number and whether it is even or odd.

Then ask the user whether they want to enter another number.

Use continue to skip the rest of the current iteration when the input is invalid, and break when the program should leave the loop.

Also handle a special input where the user enters "quit" to leave the program immediately.

Exercise 2 — Shopping list calculator

Create a small shopping calculator.

Repeatedly ask the user for the price of an item.

The user can enter:

a positive number → add it to the total
0 → stop entering products
a negative number → reject it and ask again
"quit" → immediately stop the program

Keep track of:

number of valid products
total price
highest price
lowest price

Invalid negative prices should not affect any of these values.

At the end, print a receipt containing the number of products, total price, average price, highest price and lowest price.

If no products were entered, print an appropriate message instead of trying to calculate an average.

Exercise 3 — Number analysis

Ask the user how many numbers they want to enter.

Then repeatedly ask for that many numbers.

For every number:

if it is negative, ignore it and continue with the next number
if it is zero, stop processing numbers immediately
otherwise, process it normally

For the numbers that were actually processed, calculate:

how many numbers were entered
how many were even
how many were odd
the total
the average
the largest number
the smallest number

At the end, display a report.

For example:

Numbers processed: 7
Even numbers: 4
Odd numbers: 3
Total: 126
Average: 18.0
Smallest: 4
Largest: 42

The zero should end the input process, while negative numbers should simply be skipped.

Exercise 4 — Random number challenge

Create a game in which the computer generates random numbers between 1 and 100.

The player has to guess whether the next number will be higher or lower than the current number.

For example:

Current number: 47

Will the next number be higher or lower? higher

Next number: 82
Correct!

Current number: 82

Will the next number be higher or lower? lower

Next number: 91
Wrong!

Keep playing until:

the player gives a wrong answer
two consecutive random numbers are equal
the player chooses to quit

Keep track of the player's score.

After every correct prediction, increase the score and continue the game.

If the player enters an invalid answer, don't generate a new number. Ask the question again.

At the end, print the final score.

Exercise 5 — Simple download manager

Simulate downloading a number of files.

Ask the user how many files they want to download.

For every file:

ask for the filename
generate a random download time between 1 and 5 seconds
display that the download has started
wait the generated number of seconds
randomly decide whether the download succeeds or fails
display the result

If a download fails, skip the remaining processing for that file and continue with the next file.

The user should also be able to enter "cancel" instead of a filename. In that case, stop all remaining downloads immediately.

At the end, display:

number of files successfully downloaded
number of failed downloads
number of canceled downloads
total number of files processed

Use continue when a download fails and break when the user cancels the entire download process.

Extra challenge: Before starting the downloads, ask the user whether they want to see the simulated waiting times. If they answer "no", still perform the waits, but don't display the individual waiting times.