Learning Python 3 with the Linkbot/Defining Functions
To start off this chapter I am going to give you an example of what you could do but shouldn't (so don't type it in):
a = 23 b = -23 if a < 0: a = -a if b < 0: b = -b if a == b: print("The absolute values of", a, "and", b, "are equal.") else: print("The absolute values of", a, "and", b, "are different.")
with the output being:
The absolute values of 23 and 23 are equal.
The program seems a little repetitive. Programmers hate to repeat things -- that's what computers are for, after all! (Note also that finding the absolute value changed the value of the variable, which is why it is printing out 23, and not -23 in the output.) Fortunately Python allows you to create functions to remove duplication. Here is the rewritten example:
a = 23 b = -23 def absolute_value(n): if n < 0: n = -n return n if absolute_value(a) == absolute_value(b): print("The absolute values of", a, "and", b, "are equal.") else: print("The absolute values of", a, "and", b, "are different.")
with the output being:
The absolute values of 23 and -23 are equal.
The key feature of this program is the
(short for define) starts a function definition.
followed by the name of the function
absolute_value. Next comes a '(' followed by the parameter
n is passed from the program into the function when the function is called). The statements after the ':' are executed when the function is used. The statements continue until either the indented statements end or a
return is encountered. The
return statement returns a value back to the place where the function was called. We already have encountered a function in our very first program, the
Notice how the values of
b are not changed.
Functions can be used to repeat tasks that don't return
values. Here are some examples:
def hello(): print("Hello") def area(width, height): return width * height def print_welcome(name): print("Welcome", name) hello() hello() print_welcome("Fred") w = 4 h = 5 print("width =", w, " height =", h, " area =", area(w, h))
with output being:
Hello Hello Welcome Fred width = 4 height = 5 area = 20
That example shows some more stuff that you can do with functions. Notice that you can use no arguments or two or more. Notice also when a function doesn't need to send back a value, a return is optional.
Variables in functions
When eliminating repeated code, you often have variables in the repeated code. In Python, these are dealt with in a special way. So far all variables we have seen are global variables. Functions have a special type of variable called local variables. These variables only exist while the function is running. When a local variable has the same name as another variable (such as a global variable), the local variable hides the other. Sound confusing? Well, these next examples (which are a bit contrived) should help clear things up.
a = 4 def print_func(): a = 17 print("in print_func a = ", a) print_func() print("a = ", a)
When run, we will receive an output of:
in print_func a = 17 a = 4
Variable assignments inside a function do not override global variables, they exist only inside the function. Even though
a was assigned a new value inside the function, this newly assigned value was only relevant to
the function finishes running, and the
a's values is printed again, we see the originally assigned values.
Here is another more complex example.
a_var = 10 b_var = 15 e_var = 25 def a_func(a_var): print("in a_func a_var = ", a_var) b_var = 100 + a_var d_var = 2 * a_var print("in a_func b_var = ", b_var) print("in a_func d_var = ", d_var) print("in a_func e_var = ", e_var) return b_var + 10 c_var = a_func(b_var) print("a_var = ", a_var) print("b_var = ", b_var) print("c_var = ", c_var) print("d_var = ", d_var)
in a_func a_var = 15 in a_func b_var = 115 in a_func d_var = 30 in a_func e_var = 25 a_var = 10 b_var = 15 c_var = 125 d_var = Traceback (most recent call last): File "C:\def2.py", line 19, in <module> print("d_var = ", d_var) NameError: name 'd_var' is not defined
In this example the variables
d_var are all local variables when they are inside the function
a_func. After the statement
return b_var + 10 is run, they all cease to exist. The variable
a_var is automatically a local variable since it is a parameter name. The variables
d_var are local variables since they appear on the left of an equals sign in the function in the statements
b_var = 100 + a_var and
d_var = 2 * a_var .
Inside of the function
a_var has no value assigned to it. When the function is called with
c_var = a_func(b_var), 15 is assigned to
a_var since at that point in time
b_var is 15, making the call to the function
a_func(15). This ends up setting
a_var to 15 when it is inside of
As you can see, once the function finishes running, the local variables
b_var that had hidden the global variables of the same name are gone. Then the statement
print("a_var = ", a_var) prints the value
10 rather than the value
15 since the local variable
that hid the global variable is gone.
Another thing to notice is the
NameError that happens at the end. This appears since the variable
d_var no longer exists since
a_func finished. All the local variables are deleted when the function exits. If you want to get something from a function, then you will have to use
One last thing to notice is that the value of
e_var remains unchanged inside
a_func since it is not a parameter and it never appears on the left of an equals sign inside of the function
a_func. When a global variable is accessed inside a function it is the global variable from the outside.
Functions allow local variables that exist only inside the function and can hide other variables that are outside the function.
Linkbots in Functions : Making the Linkbot Move a Certain Distance
We've seen now that we can pass numbers and variables to a function in the function parameters. It turns out that you can pass just about anything into a function, including Linkbot objects. In the Chapter "Decisions", we wrote some code that could move a two-wheeled Linkbot around. In the code, we specified the angle to rotate the wheels, but it would be much cooler if we could tell the Linkbot to move some distance on the ground. When you're writing new functions, it's common to prototype how the function might be used before actually writing it, so lets try that now. We want our function to be used something like this:
driveDistance(myLinkbot, 10) # Drive a wheeled Linkbot 10 inches forward
Now that we're satisfied with how we want to use our function, now we have to worry about how to actually write it so that it does what we want it to do.
First, let us catalog what we know and what we have. So far we know about 2 functions that we can use to move motors on the Linkbot:
move(). Of the two, we have found that
move() is probably better for driving two-wheeled Linkbots. However, the
move() function takes angles as arguments. That leaves the question: How do we turn a distance into an angle?
It turns out there is an equation that you can use to figure out how many degrees a wheel has to turn to travel a certain distance. The equation is:
In this equation, "r" is the radius of the Linkbot's wheels specified in the same units as "distance" is specified.
If you would like to see a derivation where that equation comes from, click on the "derivation" link below.
To solve this question, we can consider how wheels roll on a surface. Let us consider a wheel rolling on a surface that doesn't slip on the surface because it will make our calculations easier. Let us consider the "circumference" of a wheel. If you took a string and wrapped it all the way around a circle, the length of that string is the "circumference". The circumference of a circle with radius "r" is
The following figure illustrates a blue wheel with a green string wrapped around it. As the wheel rolls, the string unrolls off of the wheel.
As we can see from the figure, if the wheel rolls one full revolution, it travels one circumference of distance. Knowing that one revolution is 360 degrees, we can write a ratio:
So if we want to travel a distance "distance", we can do
Using this equation, if we know the wheel radius and we know what distance we want to travel, we can calculate the degrees to turn the wheel! Now we can write our function.
For you math nerds out there, you might also know that the "arc-length" of a circle is
where is expressed in radians. Solving for gives
Converting to degrees, we get
which is exactly the same as our previous result!
Now, we can include that equation in our function. This allows us to re-use the function for any distance and we don't have to type in the equation over and over again.
# File: 01_driveDistance.py import linkbot import math # So that we can use math.pi myLinkbot = linkbot.CLinkbot('abcd') # Change abcd to your Linkbot's serial ID def driveDistance(linkbot, distance): r = 3.5 / 2 # If you have a wheel that's not 3.5 inches in diameter, change # "3.5" to the diameter of your wheel # Next: Calculate degrees we have to turn the Linkbot's wheels to go a # distance of "distance". degrees = (360) / (2 * math.pi * r) * distance # Finally, we turn the Linkbot's wheels that number of degrees. linkbot.move(degrees, 0, -degrees) driveDistance(myLinkbot, 10) # Drives the Linkbot 10 inches forward driveDistance(myLinkbot, -5) # Drives the Linkbot 5 inches backward
The program shown above uses the
driveDistance() function to drive a robot forward 10 inches and then backward 5 inches. You might wondering why we went through all the trouble of defining a function when we could accomplish the same task without functions.
- Consider if the task was much more complex than just two movements. If you have to drive the robot forwards and backwards more than 4 times, you are actually saving time and code by using a function.
- Writing repeated code via copy/paste can be very hard to debug. Imagine if you copy and pasted the equation 20 times for 20 robot movements, and then you found a bug in the copy-pasted code. You would have to correct the bug in each one of the 20 pasted code blocks. If you had written a function with a bug in it, you would only have to fix the equation inside the function.
- If you are writing code for someone else to use, it would make sense to encapsulate your code in a function. Imagine if you are working with a team of people and your job is to write a function that moves the robot forward and backward, another person's job is to write a function that turns the robot, and a third person has to write a function that changes the LED color. You could then take all three functions and put them into a single program and have a robot that accurately drives a certain distance, turns, and changes LED color.
#! /usr/bin/python #-*-coding: utf-8 -*- # converts temperature to Fahrenheit or Celsius def print_options(): print("Options:") print(" 'p' print options") print(" 'c' convert from Celsius") print(" 'f' convert from Fahrenheit") print(" 'q' quit the program") def celsius_to_fahrenheit(c_temp): return 9.0 / 5.0 * c_temp + 32 def fahrenheit_to_celsius(f_temp): return (f_temp - 32.0) * 5.0 / 9.0 choice = "p" while choice != "q": if choice == "c": c_temp = float(input("Celsius temperature: ")) print("Fahrenheit:", celsius_to_fahrenheit(c_temp)) choice = input("option: ") elif choice == "f": f_temp = float(input("Fahrenheit temperature: ")) print("Celsius:", fahrenheit_to_celsius(f_temp)) choice = input("option: ") elif choice == "p": #Alternatively choice != "q": so that print when anything unexpected inputed print_options() choice = input("option: ")
Options: 'p' print options 'c' convert from celsius 'f' convert from fahrenheit 'q' quit the program option: c Celsius temperature: 30 Fahrenheit: 86.0 option: f Fahrenheit temperature: 60 Celsius: 15.5555555556 option: q
#! /usr/bin/python #-*-coding: utf-8 -*- # calculates a given rectangle area def hello(): print('Hello!') def area(width, height): return width * height def print_welcome(name): print('Welcome,', name) def positive_input(prompt): number = float(input(prompt)) while number <= 0: print('Must be a positive number') number = float(input(prompt)) return number name = input('Your Name: ') hello() print_welcome(name) print() print('To find the area of a rectangle,') print('enter the width and height below.') print() w = positive_input('Width: ') h = positive_input('Height: ') print('Width =', w, ' Height =', h, ' so Area =', area(w, h))
Your Name: Josh Hello! Welcome, Josh To find the area of a rectangle, enter the width and height below. Width: -4 Must be a positive number Width: 4 Height: 3 Width = 4 Height = 3 so Area = 12
Rewrite the area2.py program from the Examples above to have a separate function for the area of a square, the area of a rectangle, and the area of a circle (
3.14 * radius**2). This program should include a menu interface.
def square(side): return side * side def rectangle(width , height): return width * height def circle(radius): return 3.14159 * radius ** 2 def options(): print() print("Options:") print("s = calculate the area of a square.") print("c = calculate the area of a circle.") print("r = calculate the area of a rectangle.") print("q = quit") print() print("This program will calculate the area of a square, circle or rectangle.") choice = "x" options() while choice != "q": choice = input("Please enter your choice: ") if choice == "s": side = float(input("Length of square side: ")) print("The area of this square is", square(side)) options() elif choice == "c": radius = float(input("Radius of the circle: ")) print("The area of the circle is", circle(radius)) options() elif choice == "r": width = float(input("Width of the rectangle: ")) height = float(input("Height of the rectangle: ")) print("The area of the rectangle is", rectangle(width, height)) options() elif choice == "q": print(" ",end="") else: print("Unrecognized option.") options()