Functions - python (part 6)

Defining Functions

Creating 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:
def absolute_value(n):
    if n < 0:
        n = -n
    return n

a = 23
b = -23

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 def statement. def (short for define) starts a function definition. def is followed by the name of the function absolute_value. Next comes a '(' followed by the parameter n (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.
Notice how the values of a and 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(w, h):
    return w * h

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 print_func, when the function finishes running, and the a's values is printed again, we see the originally assigned values.
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
The output is:
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:\Python24\def2", line 19, in -toplevel-
     print "d_var = ", d_var

NameError: name 'd_var' is not defined
In this example the variables a_var, b_var, and 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 b_var and 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_varis 15, making the call to the function a_func(15). This ends up setting a_var to 15 when it is inside of a_func.
As you can see, once the function finishes running, the local variables a_var and b_var that had hidden the global variables of the same name are gone. Then the statementprint "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 return something.
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.
Examples
temperature2.py
# 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":
        temp = input("Celsius temperature: ")
        print "Fahrenheit:", celsius_to_fahrenheit(temp)
    elif choice == "f":
        temp = input("Fahrenheit temperature: ")
        print "Celsius:", fahrenheit_to_celsius(temp)
    elif choice != "q":
        print_options()
    choice = raw_input("option: ")
Sample Run:
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
area2.py
# By Amos Satterlee
print
def hello():
    print 'Hello!'

def area(width, height):
    return width * height

def print_welcome(name):
    print 'Welcome,', name

name = raw_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 = input('Width: ')
while w <= 0:
    print 'Must be a positive number'
    w = input('Width: ')

h = input('Height: ')
while h <= 0:
    print 'Must be a positive number'
    h = input('Height: ')

print 'Width =', w, 'Height =', h, 'so Area =', area(w, h)
Sample Run:
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
Exercises
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.