Part 4: While loops — Python Värmland

Python Värmland

Part 4: While loops

The short version - for experienced programmers

While loops work as you'd expect in Python.

number = 1

while number != 10:
    print(number)
    number = number + 1

Note the importance of the indentation - it denotes the loop block. Again, whitespace is significant in Python. Always use four spaces to indent. No tabs, no two spaces. Use four spaces.

The full version - for beginner programmers

Answer to the exercise in Part 3

Here's one possible answer to the exercise from Part 3.

user_age = input("How old are you? ")
dog_age = input("How old is the dog? ")

user_age = int(user_age)
dog_age = int(dog_age)

# Convert the dog's age to "human years".
dog_age = dog_age * 7

print(f"The dog's age in human years is {dog_age}.")

if dog_age < user_age:
    print("The dog is younger than you.")
elif dog_age == user_age:
    print("The dog is the same age as you.")
else:
    print("The dog is older than you.")

You might be wondering why we didn't use the "greater than" >= operator to make sure the dog is older than the user at the end. That's because we don't have to - we have already made sure that the dog isn't younger than the user, and not the same age either. The only possibility that remains is that it is older than the user.

while loops

Loops are an essential part of all kinds of programming. It allows you to tell the computer to do the same thing over and over, a number of times. Let's have a look at a very simple example.

number = 0

while number < 10:
    print(number)
    number = number + 1

Previously, we've printed variabled by writing print(f"{number}"), but now we're just typing print(number). How come? That's because now we only want to print the number, and nothing else. If, however, we want to print the number as part of a sentence (technically, as part of a "string"), we have to use the print(f"The number is {number}.") syntax.

Anyway, when we run the program above, we get:

0
1
2
3
4
5
6
7
8
9

We only have the one print statement, but we still get 10 numbers printed. Why? That's because we're printing inside a loop - something that repeats several times.

If we translate the above program into english, it would be something like:

Set the variable "number" to 0

While the variable "number" is less than 10, do this:
    Print the variable "number"
    Increase the variable "number" by 1

In other words, when we run through the loop the first time, the variable number is set to 0, and subsequently, 0 is printed. Then, number is increased by 1 by the line number = number + 1. That might seem confusing, but think of it as meaning "set number to what it already is (0), plus 1", which means that it now becomes 1. Then, we start back from the while line again. Since number, which is now 1, is still less than 10, the program will run through the loop again. This time, number will be increased from 1 to 2. After another few loops, number will be increased to from 9 to 10. But then when we get back to the while line, the loop will stop, because number is no longer less than 10. Therefore, the program ends after the 9 has been printed.

Let's make a simple game

Now we're going to make a simple number guessing game. The computer will randomly decide on a number, that the user will have to guess. After each guess, the computer will tell us if the number is correct, too low, or too high. That's where the while loop comes in. "While the guess is not correct", we will continue the loop. When we eventually get a correct guess, the loop will exit and we can print a "well done" message.

Modules

But first, we need to take a brief moment to talk about Python "modules". We've used a few functions in the past, such as print, input, and int. Those functions are available to us from the get-go - we can just use them any time we want. Some functions, however, reside in "modules" which we must "import" in order to be able to use them. Python comes with a standard library consisting of a large number of very useful functions. One of these is the random function, which (among other things) contains the randint function which gives us a random integer.

This simple program shows us how this works.

import random

number = random.randint(1, 10)

As you can see, we don't just type number = randint(1, 10). That's because we need to tell Python which randint function we want. In theory, there could be another function with the same name in a different module, therefore we use the syntax "modulename.functionname" to call functions from modules.

The 1 and 10 inside randint's parentesis are called "arguments", They tell the function that we want the random integer to be no lower than 1, and no higher than 10.

Now for the game itself

Now that we understand the basics of modules, it's time to get to the game itself. Open IDLE, go to File and then New file and type in the following (copy & paste is cheating; you learn more efficiently if you type it in yourself):

# Make Python's "random" module available to us
import random

# In the random module, there's a function called
# randint. We can use it to give us any integer from
# 1 to 100.
number = random.randint(1, 100)

print("I'm thinking of a number between 1 and 100.")
print("Try to guess which one it is.")

# Let's give the guess variable a starting number
# that we know will never be correct.
guess = 0

# Until guess is equal to the number, repeat this
# loop:
while guess != number:
    guess = input("What's your guess? ")
    guess = int(guess)  # Convert guess to integer

    # Check if it is too low or too high
    if guess < number:
        print("Your guess is too low.")
    elif guess > number:
        print("Your guess is too high.")

# We have now exited the loop, which means that
# guess must be equal to number, IE, correct.
print("Correct! Well done!")

Now, save it (Control-s) giving it the name number_guess.py, then run it (F5). If somehow it doesn't work and you get stuck, you can always exit the program by pressing Control-c.

Exercise

As we've mentioned before, the key to really learning how to program is to experiment, and to alter code to see what happens. So in that spirit, make the following three changes to the number guessing game:

  1. When the game ends, ask the user if they would like to play again by answering "y" or "n". Hint: You will need another outer while loop for this.
  2. When the game starts, ask the user for what the highest possible number should be, instead of always using 100. Remember that you will need to convert the answer like we've done before, once you get it from the user.
  3. Change the program so that it keeps track of how many guesses it takes to get it right, and then print that number when the game ends.

Previous: Part 3: If statements | Next: Part 5: Lists and for loops | Index

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