Project: the guessing game

What you will be able to do

After this lesson you can

• plan a loop that has more than one reason to stop,
• use a flag variable, which remembers whether something happened,
• test each way in which a game can end, and
• extend the game with your own ideas.

The rules

The computer has a secret number. The player has 6 tries to guess it. After each wrong guess, the program says Too low or Too high. If the player guesses the number, the player wins. If the six tries are used up, the player loses, and the program shows the secret.

Step 1: the reasons to stop

A loop needs a condition. Here the game ends for two different reasons: the player wins, or the tries are all used. The loop must continue while neither has happened.

• It continues while tries < max_tries and the player has not won.
• To remember whether the player has won, we use a flag: a variable that is True or False. We call it won, and it starts as False.

The condition is then while tries < max_tries and not won:. Read it aloud: "while there are tries left and the player has not won".

Step 2: the game

The last point is important. The loop does not know why it stopped. The flag tells the program.

Step 3: test every way of ending

A game can end in three ways, and each needs a test.

Check the edge as well: the winning guess on the last try, which is the sixth. Type 1, 2, 3, 4, 5, 42. The program should say that the player wins.

This is a strategy for the player, not for the program. Try it in the game: guess the middle each time.

Step 4: extend it

Pick one, and do it without looking at a solution. Test again after each change.

1. A random secret. Use import random and secret = random.randint(1, 100). Now the game is different each time.
2. A smarter hint. Say Very close when the guess is within 3 of the secret.
3. Play again. Wrap the whole game in another loop that asks Play again? (y/n).
4. Count the best score. Remember the smallest number of tries over several games.
5. A range check. Refuse a guess that is outside 1 to 100, without using up a try.

Common mistakes

• Using only one reason to stop. Without the limit, the game can run for ever.
• Forgetting the flag, and then not knowing in the end whether the player won.
• Counting the tries in the wrong place, so the message says Guess 0 of 6.
• Not testing the last try. An off-by-one fault shows there.
• Not changing the text from input() into a number.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Project: the guessing game. The checker gives your game a list of guesses, and tests the three endings.
2. Build. Do Step 4 for one extension.
3. Stretch. Open the problem The Collatz puzzle and write a program that counts the steps for the start number that you choose.

Remember these

• A loop can end for more than one reason. Put all of them in the condition, with and.
• A flag variable remembers whether something happened.
• After the loop, use the flag to say why it ended.
• Test each way in which the program can end, and the edge: the win on the last try.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why do we need the flag won?

2. What does the condition while tries < max_tries and not won: mean in words?

3. How many tests does the game need at least?