# Defining Functions

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# Creating Functions

To start off this chapter I am going to give you a 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 equalThe program seems a little repetitive. (Programmers hate to repeat things (That's what computers are for aren't they?)) Fortunately Python allows you to create functions to remove duplication. Here's the rewritten example:

a = 23 b = -23 def my_abs(num): if num < 0: num = -num return num if my_abs(a) == my_abs(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 equalThe 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

`my_abs`. Next comes a

`(`followed by the parameter

`num`(

`num`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 of course can be used to repeat tasks that don't return
values. Here's 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 = 20That example just 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

Of course, when eliminiating repeated code, you often have variables in the repeated code. These are dealt with in a special way in Python. Up till now, all variables we have see 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 variable. Sound confusing? Well, hopefully this next example (which is a bit contrived) will clear things up.

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 (innermost last): File "separate.py", line 20, in ? print "d_var = ",d_var NameError: d_var

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` is 15 since the function is called
with `a_func(b_var)`. Since at that point in time `b_var` is
15, the call to the function is `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 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 `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.

# Function walkthrough

*TODO* Move this section to a new chapter, Advanced Functions.

Now we will do a walk through for the following program:

def mult(a,b): if b == 0: return 0 rest = mult(a,b - 1) value = a + rest return value print "3*2 = ",mult(3,2)

Basically this program creates a positive integer multiplication function (that is far slower than the built in multiplication function) and then demonstrates this function with a use of the function.

**Question: **What is the first thing the program does?

**Answer: **The first thing done is the function mult is defined with the lines:

def mult(a,b): if b == 0: return 0 rest = mult(a,b - 1) value = a + rest return valueThis creates a function that takes two parameters and returns a value when it is done. Later this function can be run.

**Question: **What happens next?

**Answer: **The next line after the function,
`print "3*2 = ",mult(3,2)`

is run.

**Question: **And what does this do?

**Answer: **It prints `3*2 = `

and the return value of `mult(3,2)`

**Question: **And what does `mult(3,2)`

return?

**Answer: **We need to do a walkthrough of the `mult` function to find out.

**Question: **What happens next?

**Answer: **The variable `a` gets the value 3 assigned to it and the
variable `b` gets the value 2 assigned to it.

**Question: **And then?

**Answer: **The line `if b == 0:`

is run. Since `b` has the value 2
this is false so the line ` return 0`

is skipped.

**Question: **And what then?

**Answer: **The line `rest = mult(a,b - 1)`

is run. This line sets the
local variable `rest` to the value of `mult(a,b - 1)`

. The
value of `a` is 3 and the value of `b` is 2 so the function call
is `mult(3,1)`

**Question: **So what is the value of `mult(3,1)`

?

**Answer: **We will need to run the function `mult` with the parameters
3 and 1.

**Question: **So what happens next?

**Answer: **The local variables in the *new* run of the function are
set so that `a` has the value 3 and `b` has the value 1.
Since these are local values these do not affect the previous values
of `a` and `b`.

**Question: **And then?

**Answer: **Since `b` has the value 1 the if statement is false, so the
next line becomes `rest = mult(a,b - 1)`

.

**Question: **What does this line do?

**Answer: **This line will assign the value of `mult(3,0)` to rest.

**Question: **So what is that value?

**Answer: **We will have to run the function one more time to find that out.
This time `a` has the value 3 and `b` has the value 0.

**Question: **So what happens next?

**Answer: **The first line in the function to run is `if b == 0:`

. `b` has the value 0 so the next line to run is `return 0`

**Question: **And what does the line `return 0`

do?

**Answer: **This line returns the value 0 out of the function.

**Question: **So?

**Answer: **So now we know that `mult(3,0)` has the value 0. Now we
know what the line `rest = mult(a,b - 1)`

did since we have run
the function `mult` with the parameters 3 and 0. We have finished
running `mult(3,0)` and are now back to running `mult(3,1)`.
The variable `rest` gets assigned the value 0.

**Question: **What line is run next?

**Answer: **The line `value = a + rest`

is run next. In this run of the
function, `a=3`

and `rest=0`

so now `value=3`

.

**Question: **What happens next?

**Answer: **The line `return value`

is run. This returns 3 from the function.
This also exits from the run of the function `mult(3,1)`. After `return` is called, we go back to running `mult(3,2)`.

**Question: **Where were we in `mult(3,2)`?

**Answer: **We had the variables `a=3`

and `b=2`

and were examining
the line `rest = mult(a,b - 1)`

.

**Question: **So what happens now?

**Answer: **The variable `rest` get 3 assigned to it. The next line
`value = a + rest`

sets `value` to `3+3` or 6.

**Question: **So now what happens?

**Answer: **The next line runs, this returns 6 from the function. We are now
back to running the line `print "3*2 = ",mult(3,2)`

which can now
print out the 6.

**Question: **What is happening overall?

**Answer: **Basically we used two facts to calulate the multipule of the
two numbers. The first is that any number times 0 is 0 (`x * 0 =
0`). The second is that a number times another number is equal to the
first number plus the first number times one less than the second
number (`x * y = x + x * (y - 1)`). So what happens is `3*2` is
first converted into `3 + 3*1`. Then `3*1` is converted into
`3 + 3*0`. Then we know that any number times 0 is 0 so `3*0`
is 0. Then we can calculate that `3 + 3*0` is `3 + 0` which
is `3`. Now we know what `3*1` is so we can calculate that
`3 + 3*1` is `3 + 3` which is `6`.

This is how the whole thing works:

3*2 3 + 3*1 3 + 3 + 3*0 3 + 3 + 0 3 + 3 6

These last two sections were recently written. If you have any comments, found any errors or think I need more/clearer explanations please email. I have been known in the past to make simple things incomprehensible. If the rest of the tutorial has made sense, but this section didn't, it is probably my fault and I would like to know. Thanks.

# Examples

factorial.py

#defines a function that calculates the factorial def factorial(n): if n <= 1: return 1 return n*factorial(n-1) print "2! = ",factorial(2) print "3! = ",factorial(3) print "4! = ",factorial(4) print "5! = ",factorial(5)

Output:

2! = 2 3! = 6 4! = 24 5! = 120

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:

> python temperature2.py 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 area.py program done in 3.2 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.

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**Contents**Josh Cogliati jjc@honors.montana.edu Wikibooks Version: Wikibooks Non-programmers Python Tutorial Contents