Learning Python 3 with the Linkbot/Decisions

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If statement

As always I believe I should start each chapter with a warm-up typing exercise, so here is a short program to compute the absolute value of an integer:

n = int(input("Number? "))
if n < 0:
   print("The absolute value of", n, "is", -n)
else:
   print("The absolute value of", n, "is", n)

Here is the output from the two times that I ran this program:

Number? -34
The absolute value of -34 is 34
Number? 1
The absolute value of 1 is 1

So what does the computer do when it sees this piece of code? First it prompts the user for a number with the statement "n = int(input("Number? "))". Next it reads the line "if n < 0:". If n is less than zero Python runs the line "print("The absolute value of", n, "is", -n)". Otherwise it runs the line "print("The absolute value of", n, "is", n)".

More formally Python looks at whether the expression n < 0 is true or false. An if statement is followed by an indented block of statements that are run when the expression is true. Optionally after the if statement is an else statement and another indented block of statements. This second block of statements is run if the expression is false.

There are a number of different tests that an expression can have. Here is a table of all of them:

operator function
< less than
<= less than or equal to
> greater than
>= greater than or equal to
== equal
!= not equal

Another feature of the if command is the elif statement. It stands for else if and means if the original if statement is false but the elif part is true, then do the elif part. And if neither the if or elif expressions are true, then do what's in the else block. Here's an example:

"Notice the difference between lines marked 1 and 2. On the line marked 1, the single equals sign (assignment operator) is being used to assign the value of a + 1 to the variable a. On the line marked 2, the double equals sign (equality operator) is being used to check if a is equal to 5. "
a = 0
while a < 10:
    a = a + 1                 #1
 
    if a > 5:
        print(a, ">", 5)
    elif a == 5:              #2
        print(a, "==", 5)
    elif a <= 3:
        print(a, "<=", 3)
    else:
        print("Neither test was true")

and the output:

1 <= 3
2 <= 3
3 <= 3
Neither test was true
5 == 5
6 > 5
7 > 5
8 > 5
9 > 5
10 > 5

Notice how the elif a <= 3 is only tested when the if statement fails to be true. There can be more than one elif expression, allowing multiple tests to be done in a single if statement.

Moving a Linkbot's Motors

In the previous chapter, Who Goes There?, we saw that you can move a linkbot's motor with the moveJoint() function. It is typically used like:

myLinkbot.moveJoint(1, 180)

where the first argument is the motor you want to move and the second argument is the angle in degrees you want to move it. The above code will move motor "1" in the positive direction by 180 degrees.

What if you want to move more than one motor at a time? The linkbot also has a move() function that does just that. It looks something like this:

myLinkbot.move(90, 180, 270)

The move() function expects three numbers inside the parentheses. The first number is the angle to move motor 1, the second number is the angle to move motor 2, and the third number is the angle to move motor 3. If your Linkbot only has two motors, the value for the face that does not have a motor is ignored. For example, if I have a Linkbot-I and I ran the above sample code, the "180" is ignored because motor 2 is not a movable motor. Instead, the Linkbot will move motor 1 forward 90 degrees and motor 3 forward 270 degrees.

When the move() function is executed, the Linkbot will move any/all of its motors simultaneously.

01_command_linkbot.py

In this example, we'd like to create a program that we can use to move our Linkbot around. This demo will use the python break command. In the previous chapter, we saw that Python can get stuck in "infinite loops". The break command can be used to exit loops even if the loop condition is still true.

Lets start with just moving the Linkbot forward and backward first. To run this demo, you'll need a Linkbot-I and two wheels. A caster is also recommended but optional.

# File: 01_command_linkbot.py
import linkbot
 
myLinkbot = linkbot.CLinkbot('ABCD') # Replace ABCD with your Linkbot's serial ID
 
while True: # This is an infinite loop. Get out by using the "break" statement
    command = input('Please enter a robot command or "quit" to exit the program: ')
    if command == 'forward':
        # Rotate the wheels 180 degrees to move the robot forward
        myLinkbot.move(180, 0, -180)    # Motor 3 has to spin in the negative 
                                        # direction to move the robot forward
    elif command == 'backward':
        myLinkbot.move(-180, 0, 180)
    elif command == 'quit':
        break
    else:
        print('I\'m sorry. I don\'t understand that command.')
 
print('Goodbye!')

When you run the above program, you will be shown a command prompt that looks like:

Please enter a robot command or "quit" to exit the program:

You can try typing the commands "forward" or "backward" and see what the robot does!

A Note on Strings

You might have noticed that towards the end of the program, there is a line that reads print('I\'m sorry. I don\'t understand that command.') with funny backslashes in the code. When you run the program though, it doesn't print those backslashes. These backslashes actually serve an important purpose.

When Python encounters either a single quote ' or double quote " , it knows that what follows will be a string that is terminated by the same type of quotation mark that started it. But, what if we want to include some quotation marks as part of the string? Consider the following piece of code:

mystring = "Hello there. My dog says "woof!" when he's bored."

What we are trying to do here, is store the string:

Hello there. My dog says "woof!" when he's bored.

in the variable "mystring", including the quotation marks and all. When Python sees the first Quotation mark at "Hello th..., it says "Ok, this is going to be a string and as soon as I see another quotation mark, that will be the end of the string". Soon, Python sees another quotation mark at ...says "woof! and thinks "I've found the end of the string!", but this is not the end of our string! We wanted Python to go all the way to the end at ...bored." . How do we tell Python that the quotation marks around "woof!" are special, and that our string doesn't actually end there?

The answer is backslashes. If there is a backslash preceding a quotation mark, that is a special indicator to Python to include that quotation mark as part of the string, rather than terminating the string. Therefore, the fixed code should be:

mystring = "Hello there. My dog says \"woof!\" when he's bored."

In this example, the single quote in he's is automatically included as part of the string because the string started with a double-quotation mark, and thus Python is looking for a double-quotation mark to terminate the string.

Sample runs

Sample runs:

Please enter a robot command or 'quit' to exit the program: forward
Please enter a robot command or 'quit' to exit the program: backward
Please enter a robot command or 'quit' to exit the program: aoeu
I'm sorry. I don't understand that command.
Please enter a robot command or 'quit' to exit the program: quit
Goodbye!


Examples

equality.py

# This Program Demonstrates the use of the == operator
# using numbers
print(5 == 6)
# Using variables
x = 5
y = 8
print(x == y)

And the output

False
False

high_low.py

# Plays the guessing game higher or lower 
 
# This should actually be something that is semi random like the
# last digits of the time or something else, but that will have to
# wait till a later chapter.  (Extra Credit, modify it to be random
# after the Modules chapter)
number = 7
guess = -1
 
print("Guess the number!")
while guess != number:
    guess = int(input("Is it... "))
 
    if guess == number:
        print("Hooray! You guessed it right!")
    elif guess < number:
        print("It's bigger...")
    elif guess > number:
        print("It's not so big.")

Sample run:

Guess the number!
Is it... 2
It's bigger...
Is it... 5
It's bigger...
Is it... 10
It's not so big.
Is it... 7
Hooray! You guessed it right!

even.py

# Asks for a number.
# Prints if it is even or odd
 
number = float(input("Tell me a number: "))
if number % 2 == 0:
    print(int(number), "is even.")
elif number % 2 == 1:
    print(int(number), "is odd.")
else:
    print(number, "is very strange.")

Sample runs:

Tell me a number: 3
3 is odd.
Tell me a number: 2
2 is even.
Tell me a number: 3.4895
3.4895 is very strange.

average1.py

# keeps asking for numbers until 0 is entered.
# Prints the average value.
 
count = 0
sum = 0.0
number = 1 # set to something that will not exit the while loop immediately.
 
print("Enter 0 to exit the loop")
 
while number != 0:
    number = float(input("Enter a number: "))
    if number != 0:
        count = count + 1
        sum = sum + number
    if number == 0:
        print("The average was:", sum / count)
Sample runs

Sample runs:

Enter 0 to exit the loop
Enter a number: 3
Enter a number: 5
Enter a number: 0
The average was: 4.0
Enter 0 to exit the loop
Enter a number: 1
Enter a number: 4
Enter a number: 3
Enter a number: 0
The average was: 2.66666666667

average2.py

# keeps asking for numbers until count numbers have been entered.
# Prints the average value.
 
#Notice that we use an integer to keep track of how many numbers, 
# but floating point numbers for the input of each number
sum = 0.0
 
print("This program will take several numbers then average them")
count = int(input("How many numbers would you like to average: "))
current_count = 0
 
while current_count < count:
    current_count = current_count + 1
    print("Number", current_count)
    number = float(input("Enter a number: "))
    sum = sum + number
 
print("The average was:", sum / count)

Sample runs:

This program will take several numbers then average them
How many numbers would you like to average: 2
Number 1
Enter a number: 3
Number 2
Enter a number: 5
The average was: 4.0
This program will take several numbers then average them
How many numbers would you like to average: 3
Number 1
Enter a number: 1
Number 2
Enter a number: 4
Number 3
Enter a number: 3
The average was: 2.66666666667

Exercises

Write a program that asks the user their name, if they enter your name say "That is a nice name", if they enter "John Cleese" or "Michael Palin", tell them how you feel about them ;), otherwise tell them "You have a nice name."

Solution
name = input('Your name: ')
if name == 'Ada':
    print('That is a nice name.')
elif name == 'John Cleese':
    print('... some funny text.')
elif name == 'Michael Palin':
    print('... some funny text.')
else:
    print('You have a nice name.')


Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated." at the end, otherwise print "Good job!"

Solution
number = 7
guess = -1
count = 0
 
print("Guess the number!")
while guess != number:
    guess = int(input("Is it... "))
    count = count + 1
    if guess == number:
        print("Hooray! You guessed it right!")
    elif guess < number:
        print("It's bigger...")
    elif guess > number:
        print("It's not so big.")
 
if count > 3:
    print("That must have been complicated.")
else:
    print("Good job!")


Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."

Solution
number1 = float(input('1st number: '))
number2 = float(input('2nd number: '))
if number1 + number2 > 100:
    print('That is a big number.')


Modify the Linkbot program presented earlier to include the commands "turnright", "turnleft", and "beep". Make the "turnright" command turn the robot to the right, "turnleft" turn the robot to the left, and "beep" beep the robot buzzer for 1 second.

Solution
import linkbot
import time # So we can use time.sleep() later
 
myLinkbot = linkbot.CLinkbot('abcd') # Replace abcd with your Linkbot's serial ID
 
while True: # This is an infinite loop. Get out by using the "break" statement
    command = input('Please enter a robot command or "quit" to exit the program: ')
    if command == 'forward':
        # Rotate the wheels 180 degrees to move the robot forward
        myLinkbot.move(180, 0, -180) # Motor 3 has to spin in the negative direction to move the robot forward
    elif command == 'backward':
        myLinkbot.move(-180, 0, 180)
    elif command == 'turnright':
        myLinkbot.move(180, 0, 180)
    elif command == 'turnleft':
        myLinkbot.move(-180, 0, -180)
    elif command == 'beep':
        myLinkbot.setBuzzerFrequency(440)
        time.sleep(1)
        myLinkbot.setBuzzerFrequency(0)
    elif command == 'quit':
        break
    else:
        print('I\'m sorry. I don\'t understand that command.')
 
print('Goodbye!')


Learning Python 3 with the Linkbot
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