Learning Python 3 with the Linkbot/Decisions

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:

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 " ". Next it reads the line " ". If  is less than zero Python runs the line " ". Otherwise it runs the line " ".

More formally Python looks at whether the expression  is true or false. An  statement is followed by an indented block of statements that are run when the expression is true. Optionally after the  statement is an   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:

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

and the output:

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

Notice how the  is only tested when the   statement fails to be true. There can be more than one  expression, allowing multiple tests to be done in a single   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  function. It is typically used like: 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  function that does just that. It looks something like this: The  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  function is executed, the Linkbot will move any/all of its motors simultaneously.

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  command. In the previous chapter, we saw that Python can get stuck in "infinite loops". The  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.

A Note on Strings

You might have noticed that towards the end of the program, there is a line that reads  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: When Python sees the first Quotation mark at , 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   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   . How do we tell Python that the quotation marks around   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: In this example, the single quote in  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.

Examples
equality.py

And the output

False False

high_low.py

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

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

average2.py

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."

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!"

Write a program that asks for two numbers. If the sum of the numbers is greater than 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.