Python: Simple Demo

You can follow along by starting the attached VM by clicking the Start Machine button shown above. Visual Studio Code should open automatically as shown in the screenshot below.

The program needs to start by picking a random secret number between 1 and 20. Then the user will begin making a guess after another till they get it right. This approach requires two variables: secret and guess. We can also track the number of tries (attempts) it took the user to find the number; for this, we will use the variable tries.

Python offers the random.randint() method, which returns a random integer within the specified bounds. For example, random.randint(1, 20) returns a random number between 1 and 20. Since the user has not made any attempts, we set tries to 0; moreover, we set guess to a value outside the valid range. To tell the user that the program picked a number, we use Python’s print() method to display an informational message on the screen. We show the current Python script below:

import random  # gives us tools for picking random numbers

secret = random.randint(1, 20)  # a <= secret <= b
tries = 0
guess = 0  # start with a value that cannot be the secret (since secret is 1..20)

print("I'm thinking of a number between 1 and 20")

This code snippet achieves the following:

1. Use the random.randint() method from the random library to pick a random number between 1 and 20 and save it to secret.
2. Creates two variables, tries and guess, and sets them to 0.
3. Display a message on the screen to inform the user that a number has been picked.

With our variables ready, we prompt the user to make a guess and set guess accordingly. We begin by saving the user’s input to a temporary text variable, then use Python’s int() to convert it to an integer. Furthermore, we need to increment tries each time the user makes a guess.

text = input("Take a guess: ")  # input() returns text (a string)
guess = int(text)  # convert the text to a number

tries = tries + 1  # add 1 try (written long-form for clarity)

So far, our program does the following:

1. Pick a random number between 1 and 20 and save it in secret.
2. Ask the user to take a guess, convert it to an integer, and save it in guess.
3. Increment tries by 1.

To check whether the user made a correct guess, we should be able to compare against various cases; this requires conditional statements and comparison operators.

The Incomplete Draft

In this task, we built a basic program that you can test for yourself and edit in the attached VM. Although this program picks a random number and reads the user’s input, it does not compare the two, which makes it practically useless as a game. In the next task, we upgrade this program so that it gives the user some feedback on how their guess compares to the secret number. The program below is saved as guess_v1.py in the /home/ubuntu/Python-Demo directory.

import random  # gives us tools for picking random numbers

secret = random.randint(1, 20)  # a <= secret <= b
tries = 0
guess = 0  # start with a value that cannot be the secret (since secret is 1..20)

print("I'm thinking of a number between 1 and 20")

text = input("Take a guess: ")  # input() returns text (a string)
guess = int(text)  # convert the text to a number

tries = tries + 1  # add 1 try

Before starting, please note that you can download the attached zip file. It includes all versions of the program that are present on the target VM. This archive allows you to review the project's evolution, compare implementations, or test each version independently on your local machine. Having all iterations in one package should make your work more efficient.

Next, we compare the user’s guess to the secret number and give a helpful hint: whether it is out of range, too low, too high, or if they got it right. It is like asking ourselves, “Is the number less than 1 or greater than 20?” If the answer is yes, we tell the user that the number is out of range. If the answer is no, we pose a second question: “Is the user’s guess less than the secret number?” If the answer is yes, we tell the user that the guess is too low. Let’s rephrase this in pseudo-code, i.e., English language that is closer to the programming language.

• If the guess is less than 1 or greater than 20, print “Out of range.” (If this is not the case, proceed to the next step.)
• Else if the guess is less than the secret number, print “Too low.” (If this is not the case, proceed to the next step.)
• Else if the guess is larger than the secret number, print “Too high.” (If this is not the case, proceed to the next step.)
• Else print “You got it.”

By reflecting on the above steps, it is only logical that if the guess is not less than or larger than the secret number, it would be equal to the secret number. In programming, whenever the condition after the “if” is not true (written as if CONDITION: in Python), the program checks the condition after the “else if” (written as elif CONDITION: in Python), and if it is also false, it checks the next “else if”. If there are no more “else if” statements left, the program executes the final “else” (written as else: in Python). Let’s convert the above pseudo-code to working Python code.

# Give a hint using if / elif / else.
if guess < 1 or guess > 20:
    print("That number is out of range. Try again.")
elif guess < secret:
    print("Too low, try again.")
elif guess > secret:
    print("Too high, try again.")
else:
    print("You got it in", tries, "tries!")

In this implementation, comparing the user’s guess to the chosen secret will lead to various cases:

• The user makes a guess outside the allowed bounds, i.e., 1 and 20
• The user’s guess is less than secret
• The user’s guess is greater than secret
• The user’s guess is correct

If the above looks confusing, consider the following numerical examples.

Numerical Example 1

We assume that the secret number is 10. If the user inputs 30, the first if condition will evaluate to true, and Python will execute the part after if guess < 1 or guess > 20: and print “That number is out of range. Try again.”

Numerical Example 2

We assume that the secret number is 10. Suppose the user inputs 5, the first if condition will evaluate to false because 5 is neither less than 1 nor greater than 20. Python will check the condition following the first elif and it will evaluate to true because 5 is indeed less than 10. Consequently, the program will print “Too low, try again.”

Numerical Example 3

We assume that the secret number is 10. Suppose the user inputs 15, the first if condition will evaluate to false because 15 is neither less than 1 nor greater than 20. The program will check the condition following the first elif and it will also evaluate to false because 15 is not less than 10. The program will check the condition following the second elif, and this one will evaluate to true, because 15 is larger than 10. As a result, the program will display, “Too high, try again.”

The First Draft

In this task, we built a working program that you can test for yourself and edit in the attached VM. The main limitation is that this program gives the user one chance. In the next task, we upgrade this program to allow the user an infinite number of tries to make the correct guess. The program below is saved as guess_v2.py in the /home/ubuntu/Python-Demo directory.

import random  # gives us tools for picking random numbers

secret = random.randint(1, 20)  # a <= secret <= b
tries = 0
guess = 0  # start with a value that cannot be the secret (since secret is 1..20)

print("I'm thinking of a number between 1 and 20")

text = input("Take a guess: ")  # input() returns text (a string)
guess = int(text)  # convert the text to a number

tries = tries + 1  # add 1 try

# Give a hint using if / elif / else.
if guess < 1 or guess > 20:
    print("That number is out of range. Try again.")
elif guess < secret:
    print("Too low, try again.")
elif guess > secret:
    print("Too high, try again.")
else:
    print("You got it in", tries, "tries!")

It is too hard to guess the number when given a single chance. To make this game more interesting, it should provide the user with at least a few chances. We will be highly tolerant and allow the user as many attempts as needed to find the secret number. To give the user more than one chance, we need to use our secret weapon: loops! In programming, loops, or more formally, iterations, allow us to execute the same lines of code multiple times. In more formal phrasing, we can iterate over the same code block as long as a specific condition holds.

Let’s look at a few examples of iteration in our daily lives. Consider the case where you want to buy a new t-shirt: you will see one shop after another until you find one that suits you. The condition here is finding a suitable t-shirt. Consider another example where you are searching for an empty parking space in a vast parking lot. You will check one row of cars after another until you finally find a space. The condition here is finding a suitable parking space.

Back to our game, we will keep repeating the code we wrote in Task 3 until the user’s guess matches the secret number. In Python, “does not equal” is written as !=. In other words, the condition “guess does not equal secret” is written as guess != secret. To write this as a while loop, we use the form while CONDITION:; in our case, it is while guess != secret:.

Let’s paste the code from Task 3 in the body of our while loop.

# Repeat until the user guesses the secret number.
while guess != secret:
    text = input("Take a guess: ")  # input() returns text (a string)
    guess = int(text)  # convert the text to a number
    
    tries = tries + 1  # add 1 try

    # Give a hint using if / elif / else.
    if guess < 1 or guess > 20:
        print("That number is out of range. Try again.")
    elif guess < secret:
        print("Too low, try again.")
    elif guess > secret:
        print("Too high, try again.")
    else:
        print("You got it in", tries, "tries!")

The code above will execute as follows. First, Python will check the condition after the while; if it is true, it will run all the indented lines. If the condition is false, it terminates.

Numerical Example 1

We assume that the secret number is 10. In the first run, the guess is 0. The while condition will evaluate to true because 10 is not equal to 0. The program will ask the user to “Take a guess,” and it will proceed from there.

Numerical Example 2

We assume that the secret number is 10. If the user entered 5 earlier, it means they did not guess the number in the previous attempt. The while condition will evaluate to true because 10 is not equal to 5. The program will give the user another chance and prompt them to “Take a guess.”

Numerical Example 3

We assume that the secret number is 10. If the user entered 10 earlier, it means they successfully guessed the number in the previous attempt. Let’s see if the program will prompt them for another guess. The while loop will evaluate to false because 10 is equal to 10; in other words, 10 != 10 is false. The indented code block within the while statement will no longer execute.

The Second Draft

In this task, we have completed coding our game. You can test it for yourself and edit in the attached VM. The program below is saved as guess_v3.py in the /home/ubuntu/Python-Demo directory.

import random  # gives us tools for picking random numbers

# ----------------------------
# Guess the Number (Beginner Demo)
# ----------------------------
# The computer picks a secret number.
# The player keeps guessing until they find it.

secret = random.randint(1, 20)  # a <= secret <= b
tries = 0
guess = 0  # start with a value that cannot be the secret (since secret is 1..20)

print("I'm thinking of a number between 1 and 20")

# Repeat until the user guesses the secret number.
while guess != secret:
    text = input("Take a guess: ")  # input() returns text (a string)
    guess = int(text)  # convert the text to a number
    
    tries = tries + 1  # add 1 try

    # Give a hint using if / elif / else.
    if guess < 1 or guess > 20:
        print("That number is out of range. Try again.")
    elif guess < secret:
        print("Too low, try again.")
    elif guess > secret:
        print("Too high, try again.")
    else:
        print("You got it in", tries, "tries!")

In this room, we covered three key pillars in imperative programming languages:

• Variables
• Conditionals with if and else
• Loops with while

If this is your first exposure to Python, don’t worry about being able to write a similar program yourself. Learning programming languages requires practice, and although you can now understand and explain this program, practice is indispensable to write something similar. We strongly encourage you to compare the code in this room with that of the JavaScript: Simple Demo room (coming soon); this will give you a good perspective on how different modern programming languages are designed and written.

In the next room, the JavaScript: Simple Demo room (coming soon), we will dive into an example program written using the JavaScript language. JavaScript runs in your web browser to make websites more interactive; moreover, it is powering an increasingly large number of web applications thanks to Node.js. You can explore it in the next room.