Class 4 : Let’s Write A Program Together

For this class we’re going to write a “Guess My Number” game program. In this game the program will pick a random number from 1 to 10 and the player will try to guess what the number is. The program will respond in different ways depending on whether the player guessed correctly or incorrectly. The player can also end the game whenever they want by telling the program “quit”. The interesting part of this program is you’re going to tell me how to write it instead of the other way around.

But before we get started, we need to learn a few more things about Python to help us build our game.

Getting Information From The Player

In order to play our game the player has to interact with it. We need a way to get guesses from the player so the game can compare its secret number to the players guess. To do this we use the raw_input() function. The raw_input() function let’s us ask the user for some information, and then wait for them to enter something using the keyboard. In the Python interactive mode it looks like this:

>>> guess = raw_input("Please enter a number: ")
Please enter a number:

At the point where the raw_input() function runs, the cursor is at the end of the “Please enter a number:” string, waiting for you to type something. You can type anything you want, when you hit the <ENTER> key whatever you typed will be assigned to the guess variable as a string. This is a very simple way to get input from the user using the keyboard.

Converting A String To A Number

We haven’t talked about this yet, but there is a difference between a string like “10” and the number 10. Try this in the interactive mode:

>>> 10 == 10
>>> "10" == 10

On the first line we are comparing the two number 10’s to each other to see if they are equal. Python knows they are, so it responds by printing True to the screen. But the next comparison, “10” == 10, why does Python respond with False? The simple answer is Python doesn’t think they’re equal. But why aren’t they equal? This can be confusing, “10” looks like the number ten. And 10 definitely looks like the number ten as well. For Python however, this isn’t true. The number 10 is exactly that, the numerical value 10. The string “10” is just a string, it has no numerical value, even though “10” looks like ten to us. The difference is in the representation. The “10” represents a string to Python, it doesn’t know that string represents ten to us. The 10 however does mean numerical ten to Python, ten things, ten cars, ten whatever.

What does this have to do with our game? A lot actually. When the game starts the program will randomly pick a number from 1 to 10, not a string, a number. However when the player types something into our guess = raw_input(“Please enter a number: ”) prompt, guess is a string variable. Even if the player enters a “1” and then a “0” and then hits enter, the guess variable will be a string. This is where a problem comes in. Let’s say we call the game’s variable for its number secret_number. If we write some Python code that compares them, like this:

if secret_number == guess:

it will fail because comparing a string to a number will always be False. We need to make Python compare two of the same kinds of things. For our game, both things need to be numbers. We need to convert the player’s guess variable to a number. Python can do this using the int() function. It looks like this:

guess_number = int(guess)

With this code we’re taking the player’s input, guess, which could be something like “8”, and converting it to the numerical value 8 and assigning it to the new variable guess_number. Now when we compare guess_number with secret_number, they are the same kind of thing (numbers) and will compare correctly when we write Python code like this:

if guess_number == secret_number:

Another Kind Of Loop

We’ve only used the for loop so far because it’s handy when you know ahead of time how many times you want to loop. For our game program we won’t know ahead of time how many guesses it will take our player to guess the secret_number. We also don’t know how many times they’ll want to play the game. This is a perfect use for the other loop Python supports, the while loop. The while loop is called a conditional loop because it will continue looping until some condition it is testing changes. Here’s an example of a while loop:

game_running = True
while game_running:
        # run some Python statements

What these program lines mean is that while the variable game_running is True, the while loop will keep looping. This also means that something in the while loop will have to change the value of game_running in order for the program to exit the loop. Forgetting to provide a way for the while loop to end creates what’s called an infinite loop. This is usually a bad thing and means in order to exit the program it has to be crashed or stopped in some other way.

More Things We Can Do With Lists

We’ve used Python lists before to hold things we want to deal with as one thing, like lists of turtles. We’ve created lists, we’ve appended things to lists, we’ve generated lists with the range() function. So far we’ve used the things in the list one at a time using the for loop. But how do we get to the individual things inside a list? For example, suppose I have this list in Python:

names = ["Andy", "George", "Sally", "Sharon", "Sam", "Chris"]

How can I get just the “Sally” name from the names list variable? We use something called list indexing to do that. Everything in a list has a position in the list, and all lists in Python start at position 0. The position is called an index, so to get “Sally” from the list, remembering all lists start at index 0, we do this:

name = names[2]

When we do this the variable name will be equal to “Sally” from our list. The “[2]” above is called the index into the list. We’ve told Python we want the thing inside the names list at index 2.

How Many Items Are In A List?

It’s often useful to be able to find out how many things are in a list. For instance, our names list above has six strings in it. But how could we find this out using Python? We use the len() function. It looks like this:

number_of_names_in_list = len(names)

This will set the variable number_of_names_in_list equal to six. Notice something about the number of items in the names list and the largest index, the name “Chris”. To get the name “Chris” from our names list we would do this:

name = names[5]

The last thing in the list is at index 5, but the number of things in the list is 6. This is because all lists start with index 0, which is included in the number of things in the list. So for the names list we have indexes 0, 1, 2, 3, 4 and 5, totallng 6 things.

How To Pick Random Things From A List?

Now we know how to pick individual things from a list, how to determine how long a list is and what the maximum index value in a list is. Can we use this information to choose a random thing from a list? For a minute let’s think about our turtle programs, we had a list something like this:

colors = ["black", "red", "organge", "yellow", "green", "blue"]

How could we pick a random color from this list to use when we were creating a turtle? We know the smallest index is 0, which would be the color “black”. We also know by looking at the list that our largest index is 5, the color blue. This is one less than the number of colors in the list. So we could do something like this:

colors = ["black", "red", "organge", "yellow", "green", "blue"]
turtle_color = colors[random.randint(0, 5)]

This Python statement would set the turtle_color variable to a random color from our colors list. But what if we added more colors to our list? Something like this:

colors = ["black", "red", "organge", "yellow", "green", "blue", "violet", "pink"]
turtle_color = colors[random.randint(0, 5)]

Unless we change the ‘5’ in the random.randint(5) function we’ll still be picking from the first six colors and ignoring the new ones we added. What if we’re picking random colors all over our program, we’d have to change all the lines that pick a color every time we changed the number of colors in our colors list. Can we get Python to handle this for us? Sure we can, we can use the len() function to help us out. We can change our code to look like this:

colors = ["black", "red", "organge", "yellow", "green", "blue", "violet", "pink"]
turtle_color = colors[random.randint(0, len(colors) - 1)]

What’s going on here? We still have our colors list variable, but now we’re using the len() function inside our random.randint() function. This is okay, the len() function returns a number and random.randint() expects a number as its second parameter. But now we’re telling random.randint() that the upper limit of the numbers we want to choose from is one less than the number of things in the colors list variable. And as we’ve seen, one less than the number of things in a list will always be the highest index in the list. By using the code above we can add or subtract as many items from the colors list as we want and our random selection will still work.

Our Completed Guess My Number Program

Here’s our Guess My Number program, complete with comments:

# guess my number

import random

# set our game ending flag to False
game_running = True

while game_running:
        # Greet the user to our game
        print "I'm thinking of a number between 1 and 10, can you guess it?"

        # have the program pick a random number between 1 and 10
        secret_number = random.randint(0, 10)

        # set the player's guess number to something outside the range
        guess_number = -1

        # loop until the player guesses our number
        while guess_number != secret_number:

                # get the player's guess from the player
                guess = raw_input("Please enter a number: ")

                # does the user want to quit playing?
                if guess == "quit":
                        game_running = False

                # otherwise, nope, player wants to keep going
                        # convert the players guess from a string to an integer
                        guess_number = int(guess)

                # did the player guess the program's number?
                if guess_number == secret_number:
                        print "Congratulations, you guessed my number!"

                # otherwise, whoops, nope, go around again
                        print "Oh, to bad, that's not my number..."

# say goodbye to the player
print "Thanks for playing!!"


We’ve completed our course and I hope you’ve had as much fun as I have! We’ve written some pretty amazing programs and learned quite a bit about programming and Python along the way. My wish is this interested you enough to keep learning about programming and to continue on to discover new things you can do with Python!