Guess the Number

Beep is thinking of a secret number. Can you find it? You get a clue after every guess: “too high” or “too low”. It is one of the oldest and best games to program, and you will learn three big ideas on the way: random numbers, the while loop and comparing numbers.

What you need

• A computer with Python 3 and IDLE
• Your first Python lessons done (variables, print, if)
• About 40 minutes

Step 1: let the computer pick a secret

Python has a helper called random that can pick numbers for us. We have to say import random first. Then random.randint(1, 10) picks a whole number from 1 to 10, and it can pick 1 or 10 too.

Step 2: keep asking until the guess is right

A loop repeats code. You have used for loops that repeat a set number of times. A while loop is different: it keeps repeating while something is true, and stops when it becomes false. We do not know how many guesses you will need, so while is perfect.

Here is the whole game. Read it carefully before you run it.

import random

random.seed(7)
secret = random.randint(1, 10)
print("I am thinking of a number from 1 to 10.")
guess = 0
tries = 0
while guess != secret:
    guess = int(input("Your guess? "))
    tries = tries + 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
print("You got it in", tries, "tries!")

When you play, it looks like this (the numbers after the question marks are what you type):

I am thinking of a number from 1 to 10.
Your guess? 5
Too low!
Your guess? 8
Too high!
Your guess? 7
Too high!
Your guess? 6
You got it in 4 tries!

Here is what the important lines do:

• guess != secret is a comparison. The != sign means “is not equal to”. The loop keeps going while the guess is not the secret.
• int(input(...)) asks the question, then turns the typed text into a whole number so we can compare it.
• guess < secret asks “is the guess smaller than the secret?” and guess > secret asks “is it bigger?”.
• tries = tries + 1 adds one to the counter after each guess.
• elif means “otherwise, if”. It is a second question the computer only asks if the first answer was no.

What is a seed?

A computer cannot really roll dice in its head. It follows a clever recipe that makes numbers that look random. A seed is the starting point of that recipe. If you start from the same seed, you get the same numbers in the same order, every single time. With random.seed(7) the secret here is 6.

That is handy while you build and test a game, because you can predict what will happen. But it is no fun to play, because the secret never changes! Once your game works, delete the random.seed(7) line and the secret will be different each time. (On some computers the same seed can pick a different number. If your secret is not 6, your hints will look different, and that is fine.)

Step 3: a limit on guesses

A game gets more exciting when it can be lost. Let us give the player only five guesses. We use a variable called won that is False until the player gets it right.

import random

secret = random.randint(1, 10)
guesses_left = 5
won = False
print("I am thinking of a number from 1 to 10. You have 5 guesses.")
while guesses_left > 0 and not won:
    guess = int(input("Your guess? "))
    guesses_left = guesses_left - 1
    if guess == secret:
        won = True
        print("You got it!")
    elif guess < secret:
        print("Too low!")
    else:
        print("Too high!")
if not won:
    print("Out of guesses! My number was", secret)

The loop now has two reasons to stop, joined with and: the guesses run out, or the player wins. The loop only keeps going while both guesses_left > 0 and not won are true.

Your game might look like this if the secret happens to be 6:

I am thinking of a number from 1 to 10. You have 5 guesses.
Your guess? 5
Too low!
Your guess? 8
Too high!
Your guess? 6
You got it!

Challenges

import random

random.seed(7)
secret = random.randint(1, 10)
print("I am thinking of a number from 1 to 10.")
guess = 0
tries = 0
while guess != secret:
    guess = int(input("Your guess? "))
    tries = tries + 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
if tries == 1:
    print("WOW! First try! Are you a mind reader?")
else:
    print("You got it in", tries, "tries!")

What you learned

• random.randint(1, 10) picks a random whole number, and random.seed(...) makes it repeatable for testing.
• A while loop repeats for as long as something is true.
• <, >, == and != compare values.
• A game needs a way to be won and a way to be lost.