Python Turtle - Lesson 5

Part 1: Branching

Video link

Branching control structure

The branching control structure allows the program’s flow it take alternative paths. Let’s use a practical example to better understand this.

We’re going to use the lesson_4_pt_2.py file. You can either save it as lesson_5_pt_1a.py or download and use the lesson_5_pt_1a.py file.

 1import turtle
 2
 3
 4def draw_poly(length, sides):
 5    for i in range(sides):
 6        my_ttl.forward(length)
 7        my_ttl.right(360 / sides)
 8
 9
10# setup window
11screen = 500
12window = turtle.Screen()
13window.setup(screen, screen)
14
15# create instance of turtle
16my_ttl = turtle.Turtle()
17my_ttl.shape("turtle")
18
19num_sides = int(input("How many sides?> "))
20size = int(input("Length of sides?> "))
21
22draw_poly(size, num_sides)

Run the program, and at the prompt, instead of providing a number, provide a word, for example dog.

This will raise the following error:

1Traceback (most recent call last):
2  File "<string>", line 19, in <module>
3ValueError: invalid literal for int() with base 10: 'dog'

This error occurs because in line 19 we are trying to convert the literal (string) dog into an integer. Since dog is not a whole number, it causes an error.

What we need to do is check that user has entered a whole number before converting it into an integer.

Create a new file in, enter the code below then save it as lesson_5_pt_1b.py.

1user_value = input("Enter a number: ")
2
3print(user_value.isdigit())

PRIMM:

  • Predict what you think will happen when you run the code twice:

    • first time enter the value 10

    • second time enter the value dog

  • Run the code. Did it follow your predictions?

  • Let’s investigate that code.

Remember that Python inputs are strings. Strings have special operations called methods. One of those is the isdigit method. isdigit returns the Boolean value of True if all the characters in a string are digits.

String Methods

Python has many useful string methods. If you want to explore them W3Schools’ Python String Methods is a good place to start.

We can tell if the user’s input is a number or not. Now we need to tell the computer how to respond to this information.

The if statement

Adjusts your lesson_5_pt_1b.py code so it is the same as the code below.

1user_value = input("Enter a number: ")
2
3if user_value.isdigit():
4    print("That's a number")

PRIMM

  • Predict what you think will happen when you run the code twice:

    • first time enter the value 10

    • second time enter the value dog

  • Run the code. Did it follow your prediction?

  • Let’s investigate that code.

Flowcharts

Flowcharts are great at demonstrating how selection works. We have already used the condition symbol (diamond) in our for loops. They are also used for the conditions in if statements.

Code flowchart:

flowchart lesson 5 1

Code breakdown:

  • Line 3: if user_value.isdigit():

    • This defines the if statement.

    • The if tells Python that this is an if statement.

    • The next part is called a conditional.

      • Conditionals are operations that return a Boolean value (True or False).

      • This specific conditional is user_value.isdigit()

      • We already know the results from our previous work:

        • 10 → True

        • dog → False

    • Ends with :

      • This has the same use as for loops and functions. It indicates that an indented code block follows.

    • The indented code block, will only run if the condition returns True. In our example:

      • 10 → user_value.isdigit() returns True → run indented code block

      • dog → user_value.isdigit() returns False → don’t run indented code block

  • Line 4: print("That's a number")

    • This is the indented code block that will run if user_value.isdigit() is True

We can now respond to a digit being entered. But what if we want to provide a different response when user_value.isdigit() is False?

The if … else statement

Adjust your lesson_5_pt_1b.py code by adding lines 5 and 6 in the code below.

1user_value = input("Enter a number: ")
2
3if user_value.isdigit():
4    print("That's a number")
5else:
6    print("That's not a number")

PRIMM

  • Predict what you think will happen when you run the code twice:

    • first time enter the value 10

    • second time enter the value dog

  • Run the code. Did it follow your prediction?

  • Let’s investigate that code.

Code flowchart:

flowchart lesson 5 2

Code breakdown:

  • Lines 3 and 4 operate the same as the previous code.

  • Line 5 - else:

    • The else statement uses the if statement’s condition.

    • In this case, it says: if user_value.isdigit() is False then run the following indented code block.

    • The : tells Python that an indented code block follows.

  • Line 6 - print("That's not a number")

    • This is the indented code block that will run if user_value.isdigit() is False

To check out what is happening in detail stepping through the code with the debugger. Use the inputs of 10 and dog.

Using if … else to capture errors

Go back to lesson_5_pt_1a.py and adjust the code by replacing line 19 with the following code:

1# get user input
2num_sides = input("How many sides?> ")
3if num_sides.isdigit():
4    num_sides = int(num_sides)
5else:
6    print("Invalid input")
7    quit()

Your code should look like the code below:

 1import turtle
 2
 3
 4def draw_poly(length, sides):
 5    for i in range(sides):
 6        my_ttl.forward(length)
 7        my_ttl.right(360 / sides)
 8
 9
10# setup window
11screen = 500
12window = turtle.Screen()
13window.setup(screen, screen)
14
15# create instance of turtle
16my_ttl = turtle.Turtle()
17my_ttl.shape("turtle")
18
19# get user input
20num_sides = input("How many sides?> ")
21if num_sides.isdigit():
22    num_sides = int(num_sides)
23else:
24    print("Invalid input")
25    quit()
26
27size = input("Length of sides?> ")
28
29draw_poly(size, num_sides)

Then replace line 27 with this code:

1size = input("Length of sides?> ")
2if size.isdigit():
3    size = int(size)
4else:
5    print("Invalid input")
6    quit()

Your code should look like the code below:

 1import turtle
 2
 3
 4def draw_poly(length, sides):
 5    for i in range(sides):
 6        my_ttl.forward(length)
 7        my_ttl.right(360 / sides)
 8
 9
10# setup window
11screen = 500
12window = turtle.Screen()
13window.setup(screen, screen)
14
15# create instance of turtle
16my_ttl = turtle.Turtle()
17my_ttl.shape("turtle")
18
19# get user input
20num_sides = input("How many sides?> ")
21if num_sides.isdigit():
22    num_sides = int(num_sides)
23else:
24    print("Invalid input")
25    quit()
26
27size = input("Length of sides?> ")
28if size.isdigit():
29    size = int(size)
30else:
31    print("Invalid input")
32    quit()
33
34draw_poly(size, num_sides)

Let’s test this code to see if it works.

PRIMM

  • Predict what you think will happen when you run the code in the following scenarios:

    • valid sides value and valid size value

    • valid sides value and invalid size value

    • invalid sides value and valid size value

    • invalid sides value and invalid size value

  • Run the code. Did it follow your prediction?

  • Let’s investigate that code.

More testing tips

  • When testing branching code you need to test all possible paths.

  • Test if statements for both True conditions and False conditions.

  • This code had four possible branches so we needed to test all four of them

Code flowchart:

flowchart lesson 5 3

Code breakdown:

  • Line 19: # get user input → a comment used to structure the code

  • Line 20: num_sides = input("How many sides?> ") → accepts user input and assigns it to num_sides

  • Line 21: if num_sides.isdigit(): → tests if num_sides only contains numbers

    • if num_sides.isdigit() is True then run the code block from line 20

  • Line 22: num_sides = int(size) takes the value assigned to num_sides converts it to an integer, then reassigns it to num_sides

  • Line 23: else: → if num_sides is not all numbers execute following code block (lines 22 to 23)

  • Line 24: print("Invalid input") → informs the user of their mistake

  • Line 25: quit() → exits the program

  • Line 27: size = input("Length of sides?> ") → accepts user input and assigns it to size

  • Line 28: if size.isdigit(): → tests if size only contains numbers

    • If size.isdigit() is True then run the code block from line 27

  • Line 29: size = int(size) takes the value assigned to size converts it to an integer, then reassigns it to size

  • Line 30: else: → if size is not all numbers execute following code block (lines 29 to 30)

  • Line 31: print("Invalid input") → informs the user of their mistake

  • Line 32: quit() → exits the program

Refactor Code - DRY

Looking at our code, does it pass the DRY test?

The # get user input section from line 17 to 30 definitely has repetition in it. Twice the code:

  1. asks the user for input

  2. checks if that input is all numbers

  3. either converts or quits the program depending on the if statement.

During all this, the only parts of the code that differs are:

  • Line 20 and 27 the input prompt is different:

    • Line 20 → "How many sides?> "

    • Line 27 → "Length of sides?> "

  • in their respective sections different variable names are used:

    • Lines 20 to 25 → num_sides

    • Lines 27 to 32 → size

This looks like a prefect opportunity to refactor the code using a function.

What is refactoring?

Refactoring is changing your code without changing the way it works. This is normally done to make code more efficient or more maintainable.

  • Efficient code uses less computing resources (processing power, storage, internet bandwidth etc.).

  • Maintainable code is easier for programmers to understand, fix, update and enhance.

To refactor our code we need to add the following function at line 10 of your code:

def get_number(prompt):
    num = input(prompt)
    if num.isdigit():
        return int(num)
    else:
        print("Invalid input")
        quit()

Then remove the code under # get user input from lines 19 to 32, and replace it with two calls to the function:

# get user input
num_sides = get_number("How many sides?> ")
size = get_number("Length of sides?> ")

In the end your code should look like the code below:

 1import turtle
 2
 3
 4def draw_poly(length, sides):
 5    for i in range(sides):
 6        my_ttl.forward(length)
 7        my_ttl.right(360 / sides)
 8
 9
10def get_number(prompt):
11    num = input(prompt)
12    if num.isdigit():
13        return int(num)
14    else:
15        print("Invalid input")
16        quit()
17
18
19# setup window
20screen = 500
21window = turtle.Screen()
22window.setup(screen, screen)
23
24# create instance of turtle
25my_ttl = turtle.Turtle()
26my_ttl.shape("turtle")
27
28# get user input
29num_sides = get_number("How many sides?> ")
30size = get_number("Length of sides?> ")
31
32draw_poly(size, num_sides)

When you refactor code, it is important to ensure the code still works the same. So run the code to ensure that it still works the same way.

Remember to test all 4 possible branches:

  • valid sides value and valid size value

  • valid sides value and invalid size value

  • invalid sides value and valid size value

  • invalid sides value and invalid size value

If your code still works the same, let’s investigate the code we added.

Code flowchart:

flowchart lesson 5 4

Code breakdown:

  • The get_number function:

    • def get_number(prompt): → defines our new function with one argument prompt:

      • we observed that the prompt was one of the differences between our two blocks of similar code

      • using this argument means we can provide a different prompt each time we call the function

    • num = input(prompt) → uses the prompt argument and assigns the user input to num

    • if num.isdigit(): → checks if num only contains numbers

    • return int(num) → converts the value assigned to num then sends it to the main program:

      • return is new

      • return sends a value back to the main program and then ends the function.

    • else: → if num does not contain only numbers, run the following code block

    • print("Invalid input") → informs the user their input is not correct

    • quit() → exits the program

  • num_sides = get_number("How many sides?> ") → calls the get_number function

    • get_number() → calls the function

    • "How many sides?> " → provides the prompt string to the function

    • num_sides = takes the value returned by the function and assigns it to num_sides

  • size = get_number("Length of sides?> ") → calls the get_number function

    • get_number() → calls the function

    • "Length of sides?> " → provides the prompt string to the function

    • size = takes the value returned by the function and assigns it to size

Playing with colour

Let’s keep adding features to our program. Turtle allows you to also change the colour of your shapes and lines using the method color:

color accepts two arguments:

  • first argument → line colour

  • second argument → fill colour

Spelling colour / color

Like most programming languages, Python uses US spelling. Using Australian spelling (eg. colour) will generate an error.

It up to the programmer to decide what spelling to follow in their naming of variables and functions. I choose to use the US spelling. The consistent spelling reduces the likelihood of errors.

Now let’s change the colour of our shape.

Make the changes in code the code below to:

  • Line 5

  • Line 6

  • Line 35

 1import turtle
 2
 3
 4def draw_poly(length, sides, color):
 5    my_ttl.color("black", color)
 6    my_ttl.begin_fill()
 7    for i in range(sides):
 8        my_ttl.forward(length)
 9        my_ttl.right(360 / sides)
10    my_ttl.end_fill()
11
12
13def get_number(prompt):
14    num = input(prompt)
15    if num.isdigit():
16        return int(num)
17    else:
18        print("Invalid input")
19        quit()
20
21
22# setup window
23screen = 500
24window = turtle.Screen()
25window.setup(screen, screen)
26
27# create instance of turtle
28my_ttl = turtle.Turtle()
29my_ttl.shape("turtle")
30
31# get user input
32num_sides = get_number("How many sides?> ")
33size = get_number("Length of sides?> ")
34
35draw_poly(size, num_sides, "red")

PRIMM

  • Predict what you think will happen when you run the code:

  • Run the code. Did it follow your prediction?

  • Let’s investigate that code.

Code breakdown:

  • def draw_poly(length, sides, color): → accepts a third argument color

  • my_ttl.color("black",color) → sets the turtle colour

    • line colour → "black"

    • fill colour → the value in the color argument

Turtle colours

Turtle allows the use of named colours. It also allows RBG and Hexadecimal colours, but named colours are enough for our needs.

Here is a list of all the named colours.

Now that we can change colour, can we let the user choose between red, blue and green for the fill colour?

We will need to capture the error when the user enters anything other than "red", "blue" or "green". That means using an if statement, but the if … else statement only allows two branches. We need to have four.

To choose between three or more branches we need to learn about the last part of the if statement: elif.

The if … elif … else statement

The elif statement is effectively a else + if statement. It allows branching between multiple blocks of code. The best way to explore this is by using it in our code.

Create a function so the user can choose between red, blue and green for the fill colour.

Adjust your code so it is the same as the code below.

Changes are in:

  • Lines 22 to 32

  • Line 47

  • Line 49

 1import turtle
 2
 3
 4def draw_poly(length, sides, color):
 5    my_ttl.color("black", color)
 6    my_ttl.begin_fill()
 7    for i in range(sides):
 8        my_ttl.forward(length)
 9        my_ttl.right(360 / sides)
10    my_ttl.end_fill()
11
12
13def get_number(prompt):
14    num = input(prompt)
15    if num.isdigit():
16        return int(num)
17    else:
18        print("Invalid input")
19        quit()
20
21
22def get_color():
23    color = input("Fill colour (red, blue, green)?> ").lower()
24    if color == "red":
25        return color
26    elif color == "blue":
27        return color
28    elif color == "green":
29        return color
30    else:
31        print("Invalid input")
32        quit()
33
34
35# setup window
36screen = 500
37window = turtle.Screen()
38window.setup(screen, screen)
39
40# create instance of turtle
41my_ttl = turtle.Turtle()
42my_ttl.shape("turtle")
43
44# get user input
45num_sides = get_number("How many sides?> ")
46size = get_number("Length of sides?> ")
47fill = get_color()
48
49draw_poly(size, num_sides, fill)

PRIMM

  • Predict what you think will happen when you run the code:

  • Run the code. Did it follow your prediction?

  • Let’s investigate that code.

There are a few new concepts for us to breakdown:

  • Line 23: color = input("Fill colour (red, blue, green)?> ").lower() → lower() is new

    • lower() is another string method

    • it converts all the letters in a string to their lowercase version

  • Line 24: if color == "red": → tests if the user inputted "red"

  • Line 25: return color

    • sends the value of color (in this case "red" back to the main program)

    • ends the function

  • Line 26: elif color == "blue":

    • is only executed when the condition in line 21 is False

    • checks if the value of color is "blue"

  • Line 27: return color

    • sends the value of color (in this case "blue" back to the main program)

    • ends the function

  • Line 28: elif color == "green":

    • is only executed if the conditions in line 21 and line 23 are both False

    • checks if the value of color is "green"

  • Line 29: return color

    • sends the value of color (in this case "green" back to the main program)

    • ends the function

  • Line 30: else:

    • is only executed if the conditions in line 21, line 23 and line 24 are all False

  • Line 31 and line 32 are the same as the get_number function

Code flowchart:

flowchart lesson5 5

The if … elif … else statement is very useful and flexible. You will use it in various configurations, so let look at it’s rules.

if…elif…else structure

The structure of a full if … elif … else statement is:

  • the if component

    • always at the beginning of an if … elif … else statement

    • the only compulsory component

    • there can only be one if per if … elif … else statement

  • the elif component

    • must come after the if statement and before the else statement

    • is optional

    • there can be as many elif components as needed

    • it is only used when all the conditions before it are False

  • the else component

    • must be at the end of an an if … elif … else statement

    • it is optional

    • there can only be one per if … elif … else statement

    • it is only used when all the conditions before it are False

Part 1 Exercises

In this course, the exercises are the make component of the PRIMM model. So work through the following exercises and make your own code.

Exercise 1

Download lesson_5_ex_1.py file and save it to your lesson folder. Below is its code.

1# Amy's security guard program
2
3######################################################
4## Write a program that asks for a person's name    ##
5## if that person is Amy they are granted entry     ##
6## other wise they are politely told to go away     ##
7######################################################

Follow the instructions in the comments and use your Python knowledge to create a password checker. Remember to apply the DRY principle

Exercise 2

Download lesson_5_ex_2.py file and save it to your lesson folder. Below is its code.

 1# Amy's security guard program
 2
 3friends = "Bruce"
 4
 5######################################################
 6## Write a program that asks for a person's name    ##
 7## and then grants entry of that person is Amy      ##
 8## or a friend of Amy.                              ##
 9## Everyone else is told, politely, to go away      ##
10######################################################

Follow the instructions in the comments and use your Python knowledge to create an enhanced password checker. Remember to apply the DRY principle

Exercise 3

Download lesson_5_ex_3.py file and save it to your lesson folder. Below is its code.

 1import turtle
 2
 3#####################################################
 4## Adjust the code below to allow the user to      ##
 5## choose the coordinates where the shape is drawn ##
 6#####################################################
 7
 8
 9def draw_poly(length, sides, color):
10    my_ttl.color("black", color)
11    my_ttl.begin_fill()
12    for i in range(sides):
13        my_ttl.forward(length)
14        my_ttl.right(360 / sides)
15    my_ttl.end_fill()
16
17
18def get_number(prompt):
19    num = input(prompt)
20    if num.lstrip("-").isdigit():
21        return int(num)
22    else:
23        print("Invalid input")
24        quit()
25
26
27def get_color():
28    color = input("Fill colour (red, blue, green)?> ").lower()
29    if color == "red":
30        return color
31    elif color == "blue":
32        return color
33    elif color == "green":
34        return color
35    else:
36        print("Invalid input")
37        quit()
38
39
40def move_pen():
41    # replace 'pass' with your code to get coordinates from user #
42    pass
43
44# setup window
45screen = 500
46window = turtle.Screen()
47window.setup(screen, screen)
48
49# create instance of turtle
50my_ttl = turtle.Turtle()
51my_ttl.shape("turtle")
52
53# get user input
54num_sides = get_number("How many sides?> ")
55size = get_number("Length of sides?> ")
56fill = get_color()
57
58draw_poly(size, num_sides, fill)

Follow the instructions in the comments (check line 41) and use your Python knowledge to enhance our shape drawing code. Remember to apply the DRY principle.

Part 2: While Loop

Video link

In Python we have two forms of iteration. We have already looked at the for loop. In the section we will look at the other iteration control structure, the while loop.

The two types of loops map to two different types of iteration:

  • definite iteration

    • Is used when you do know how many times the loop will need to run.

    • definite iteration uses for loops since they loop for a set number of times.

  • indefinite iteration

    • Is used when you don’t know how many times the loop will need to run.

    • indefinite iteration uses while loops since they will loop as long as the condition is True

Card dealing is a good analogy of the definite and indefinite loops distinction:

  • Dealing for Uno:

    • How many cards does each player get?

    • The rules say seven.

    • So, we need to deal around the players seven times.

    • This is definite iteration as you know how many times you have to go around the group.

  • Dealing for Snap:

    • How many cards does each player get?

    • Depends on how many players, you need to keep going until you have dealt the whole deck.

    • This indefinite iteration as you will need to go around the group While there are still cards left in the deck.

In summary:

  • for loop is count controlled → we know how many times to run it.

  • while loop is condition controlled → we don’t know how many times to run it.

To understand while loops, let’s look at a number guessing game.

Number guessing game

Download the lesson_5_pt_2.py file and save it to your lesson folder. Below is its code.

 1import random
 2
 3
 4def get_number(prompt):
 5    num = input(prompt)
 6    if num.isdigit():
 7        return int(num)
 8    else:
 9        print("Invalid input")
10        quit()
11
12
13number = random.randint(1, 100)
14
15guess = get_number("Guess a number between 1 and 100> ")
16
17if guess == number:
18    print("Correct!")
19else:
20    print("Incorrect. The number was", number)

PRIMM

  • Predict what you think will happen when you run the code:

  • Run the code. Did it follow your prediction?

  • Let’s investigate that code.

What is the random module?

The random module gives us access to a range of functions that produce random results.

To see all the commands, you can go the W3Schools Python Random Module page.

Code breakdown:

  • Line 1: import random:

    • We will be using the random function called randint, so we need to import random.

  • Lines 4 to 10 are the same get_number function we have used previously.

  • Line 13: number = random.randint(1,100)

    • random.randint(1,100)

      • use the randint function from the random module

      • generate a random integer between 1 and 100 (inclusive)

    • number = → assign the returned integer to the variable number

  • Line 15: guess = get_number("Guess a number between 1 and 100> ")

    • get_number("Guess a number between 1 and 100> ") → calls the get_number function to ask the user for a number

    • guess = → assigns the returned integer to the variable guess

  • Line 17: if guess == number:

    • checks if the user’s guess and the random number are the same

    • the == symbol is a comparison operator (see below). It checks if two values are the same

    • if the two values are the same → run the code block on line 16

  • Line 19: else: → if the user’s guess and the random number are not the same → run the code block on line 18.

Comparison operators

A comparison operator compares two values and returns either True or False.

Python’s condition testing uses many comparison operators:

Operator

Meaning

==

checks if two values are the same (equal to)

!=

checks if two values are not the same (not equal to)

>

checks if the left value is greater than the right value

<

checks if the left value is less than the right value

>=

checks if the left value is greater than or equal to the right value

<=

checks if the left value is less than or equal to the right value

So, we’ve made a simple game, but it is not a good one. A one-in-one-hundred chance of guessing a number is not going to keep the user entertained for too long. How about we adjust the code to allow the user to have ten guesses?

Now that sounds like iteration, but what kind? Since we know how many times this will need to loop (10), it’s definite iteration. Definite iteration requires a for loop.

Change your code so it looks like the code below. Specifically:

  • line 15 → provide user instructions

  • lines 17 to 23 → place the guessing process within a for loop

  • line 23 → make sure you remove the number reveal

  • line 25 → reveal the number after all 10 guesses have finished

 1import random
 2
 3
 4def get_number(prompt):
 5    num = input(prompt)
 6    if num.isdigit():
 7        return int(num)
 8    else:
 9        print("Invalid input")
10        quit()
11
12
13number = random.randint(1, 100)
14
15print("You have 10 turns to guess a number between 1 and 100")
16
17for turn in range(10):
18    guess = get_number("Guess a number between 1 and 100> ")
19
20    if guess == number:
21        print("Correct!")
22    else:
23        print("Incorrect. Try again")
24
25print("The number was", number)

PRIMM

  • Predict what you think will happen when you run the code:

  • Run the code. Did it follow your prediction?

  • We won’t worry about investigating as there is nothing new in this code.

This is better, but still isn’t great. There is a one-in-ten chance of getting the right number. Each guess is a stab in the dark with no knowledge gained from the previous guesses. How about we give the user hints and let them know that their guess is too high or too low?

Change the if … else statement into the if … elif … else statement on lines 20 to 25 in the code below:

 1import random
 2
 3
 4def get_number(prompt):
 5    num = input(prompt)
 6    if num.isdigit():
 7        return int(num)
 8    else:
 9        print("Invalid input")
10        quit()
11
12
13number = random.randint(1, 100)
14
15print("You have 10 turns to guess a number between 1 and 100")
16
17for turn in range(10):
18    guess = get_number("Guess a number between 1 and 100> ")
19
20    if guess > number:
21        print("Guess is too high")
22    elif guess < number:
23        print("Guess is too low")
24    else:
25        print("Correct!")
26
27print("The number was", number)

We’ve done a fair bit of coding without any serious testing. So this time lets keep running our code until we cover all four branches:

  1. guess is too high

  2. guess is too low

  3. guess is correct

  4. all 10 guess used up without guessing the number

This might be easier to do if we know the random number. Feel free to add a line that prints the random number, but make sure you comment it out after testing.

PRIMM

  • Predict what you think will happen when you run the code:

  • Run the code. Did it follow your predictions?

  • We won’t worry about investigating as there is nothing new in this code.

Did you identify a problem when the user guesses the number before using all ten guesses? The game prints Correct! but then continues to ask them to guess numbers. This is because we created a definite iteration using for which is set to always give ten guesses.

What we want is an indefinite iteration that loops until the user guesses the number. To do this we will use a while loop.

Using a while loop

Change your code so it is the same as the code below. Specifically:

  • line 15 → add guess = 0

  • line 17 → change the for statement to while guess != number:

 1import random
 2
 3
 4def get_number(prompt):
 5    num = input(prompt)
 6    if num.isdigit():
 7        return int(num)
 8    else:
 9        print("Invalid input")
10        quit()
11
12
13number = random.randint(1, 100)
14
15guess = 0
16
17while guess != number:
18    guess = get_number("Guess a number between 1 and 100> ")
19
20    if guess > number:
21        print("Guess is too high")
22    elif guess < number:
23        print("Guess is too low")
24    else:
25        print("Correct!")
26
27print("The number was", number)

Again you want to run this code enough time that you have covered all four possible branches:

  1. guess is too high

  2. guess is too low

  3. guess is correct

  4. all 10 guess used up without guessing the number

PRIMM

  • Predict what you think will happen when you run the code:

  • Run the code. Did it follow your predictions?

  • Let’s investigating the new code to see how a while loop works.

Code breakdown:

  • Line 17: while guess != number:

    • guess != number → this is the loop condition

    • It tests if guess and number are the same:

      • It will return True when guess and number are not the same.

      • It will return False when guess and number are the same.

    • while tells Python to loop the following code block if the loop condition returns True

  • Line 15: guess = 0

    • In our while statement we use the variable guess before getting an input from the user → this will raise an error.

    • We need to assign value to guess before the while statement

      • The problem is the value we assign to guess cannot be the same as the value assigned to number. If it is, the while loop will not run at all, and the user will not provide input.

      • To solve this, we assign 0 because it is outside the range of random.randint (1-100).

      • This way, guess != number will always return True the first time the condition is tested.

Code flowchart:

flowchart lesson 5 6

Using while to enhance our error capture

We now have a somewhat fun game where the user has a good chance of guessing the number. We can also use while to improve our error capture in the get_number function.

At the moment, if the user provides an input which isn’t an integer, the game ends. This is a bit harsh, especially if they have already made three or four guesses.

Adjust your get_number function so that it is the same as in the code below.

 1import random
 2
 3
 4def get_number(prompt):
 5    while True:
 6        num = input(prompt)
 7        if num.isdigit():
 8            return int(num)
 9        else:
10            print("Invalid input")
11
12
13number = random.randint(1, 100)
14
15guess = 0
16
17while guess != number:
18    guess = get_number("Guess a number between 1 and 100> ")
19
20    if guess > number:
21        print("Guess is too high")
22    elif guess < number:
23        print("Guess is too low")
24    else:
25        print("Correct!")
26
27print("The number was", number)

Again you want to run this code enough time that you have covered all four possible branches:

  1. guess is too high

  2. guess is too low

  3. guess is correct

  4. all 10 guess used up without guessing the number

PRIMM

  • Predict what you think will happen when you run the code:

  • Run the code. Did it follow your predictions?

  • Let’s investigating the new code to see how this use of a while loop works

Code breakdown:

  • Line 5: while True:

    • This is called an infinite loop, since the condition will always be True, the loop will always run.

    • Infinite loops are frequently cause by errors, although not in this case.

    • Infinite loops can be ‘broken out’ of by using the break statement.

    • If the Infinite loop is in a function, it can also be ‘broken out’ of by using the return statement.

  • Lines 6 to 10 are the same as before, except they form a code block inside of the while loop.

  • It is worth noting the importance of line 8.

    • Since the while loop is infinite, the program will keep asking for input until it executes line 8.

    • In line 8 the value assigned to num is converted into an integer and then returned to the main program

    • In this situation the return statement ends the function. In ending the function, it also exits the while loop.

Code flowchart:

flowchart lesson 5 7

The end effect of these changes is the program that will ask the user for a number until the user enters an integer.

Part 2 Exercise

In this course, the exercises are the make component of the PRIMM model. So work through the following exercises and make your own code.

Exercise 4

Download lesson_5_ex_4.py file and save it to your lesson folder. Below is its code.

 1import turtle
 2
 3
 4def draw_poly(length, sides, color):
 5    my_ttl.color("black", color)
 6    my_ttl.begin_fill()
 7    for i in range(sides):
 8        my_ttl.forward(length)
 9        my_ttl.right(360 / sides)
10    my_ttl.end_fill()
11
12
13############################################
14## adjust the get_number code so it loops ##
15## until the user provides a valid input  ##
16############################################
17
18
19def get_number(prompt):
20    num = input(prompt)
21    if num.lstrip("-").isdigit():
22        return int(num)
23    else:
24        print("Invalid input")
25        quit()
26
27
28###########################################
29## adjust the get_color code so it loops ##
30## until the user provides a valid input ##
31###########################################
32
33
34def get_color():
35    color = input("Fill colour (red, blue, green)?> ").lower()
36    if color == "red":
37        return color
38    elif color == "blue":
39        return color
40    elif color == "green":
41        return color
42    else:
43        print("Invalid input")
44        quit()
45
46
47def move_pen():
48    x_val = get_number("x axis position?> ")
49    y_val = get_number("y axis position?> ")
50    my_ttl.penup()
51    my_ttl.goto(x_val, y_val)
52    my_ttl.pendown()
53
54
55# setup window
56screen = 500
57window = turtle.Screen()
58window.setup(screen, screen)
59
60# create instance of turtle
61my_ttl = turtle.Turtle()
62my_ttl.shape("turtle")
63
64# get user input
65num_sides = get_number("How many sides?> ")
66size = get_number("Length of sides?> ")
67fill = get_color()
68
69move_pen()
70draw_poly(size, num_sides, fill)

Follow the instructions in comments and make changes to the get_number and get_colour functions so they capture user input errors.