.. Objectives

.. ----------

.. 8.1 LO: getting started with functions (3)

.. At the end of this tutorial, you will be able to 

.. 1. define function

.. #. define functions with arguments

.. #. learn about docstrings

.. #. learn about return values

..     can return multiple values

.. #. read code

.. Prerequisites

.. -------------

..   1. should have ``ipython`` installed. 

..   #. getting started with ``ipython``.

.. Author              : Anoop Jacob Thomas <anoop@fossee.in>

   Internal Reviewer   : 

   External Reviewer   :

   Checklist OK?       : <put date stamp here, if OK> [2010-10-05]

==============================

Getting started with functions

==============================

{{{ show welcome slide }}}

Hello and welcome to the tutorial getting started with functions.

{{{ switch to next slide, outline slide }}}

In this tutorial we will learn about functions in python, how to

define functions, arguments to functions, docstrings, and function

return value.

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While writing code, we always want to reduce the number of lines of

code and functions is a way of reusing the code. Thus the same lines

of code can be used again and again. A function is a portion of code

within a larger program that performs a specific task and is

relatively independent of the remaining code. Now let us get more

familiar with functions,

{{{ switch to next slide, f(x) a mathematical function }}}

Consider a mathematical function f(x) = x square. Here x is a variable

and with different values of x the value of function will change. When

x is one f(1) will return the value 1 and f(2) will return us the

value 4. Let us now see how to define the function f(x) in python.

{{{ switch to next slide, define f(x) in Python }}}

In your Ipython interpreter type the following,

::

    def f(x):

    	return x*x

Well that defined the function, so before learning what we did let us

see if it returns the expected values, try,

::

    f(1)

    f(2)

Yes, it returned 1 and 2 respectively. And now let us see what we did,

we wrote two lines. The first line ``def f(x)`` is used to define the

name and the parameters to the function. ``def`` is a keyword and

``f`` is the name of the function and ``x`` the parameter of the

function.

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%% 1 %% Write a python function named cube which computes the cube of

   a given number n.

Pause here and try to solve the problem yourself.

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The problem can be solved as,

::

    def cube(n):

    	return n**3

And now let us see how to write functions without arguments.

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let us define a new function called ``greet`` which will print ``Hello

World``.

::

    def greet():

    	print "Hello World!"

now try calling the function,

::

    greet()

Well that is a function which takes no arguments. Also note that it is

not mandatory for a function to return values. The function ``greet``

neither takes any argument nor returns any value.

Now let us see how to write functions with more than one argument.

{{{ switch to next slide, exercise 2 }}}

%% 2 %% Write a python function named ``avg`` which computes the

   average of ``a`` and ``b``.

Pause here and try to solve the problem yourself.

{{{ switch to next slide, solution 2 }}}

The problem can be solved as,

::

    def avg(a,b):

    	return (a + b)/2

Thus if we want a function to accept more arguments, we just list them

separated with a comma between the parenthesis after the function name

in the ``def`` line.

{{{ switch to next slide, docstring }}}

It is always a good practice to document the code that we write, and

for a function we define we should write an abstract of what the

function does, and that is called a docstring. Let us modify the

function ``avg`` and add docstring to it. Do the following,

::

    def avg(a,b):

        """ avg takes two numbers as input (a & b), and

	returns the average of a and b"""

	return (a+b)/2

Note that docstrings are entered in the immediate line after the

function definition and put as a triple quoted string. And here as far

as the code functionality is concerned, we didn't do anything. We just

added an abstract of what the function does.

Now try this in the ipython interpreter.

::

    avg?

It displays the docstring as we gave it. Thus docstring is a good way

of documenting the function we write.

Try to do this,

::

    f?

It doesn't have a docstring associated with it. Also we cannot infer

anything from the function name, and thus we are forced to read the

code to understand anything about the function.

{{{ switch to next slide, exercise 3 }}}

%% 3 %% Add docstring to the function f.

Pause here and try to do it yourself.

{{{ switch to next slide, solution }}}

We need to define the function again to add docstring to the function

``f`` and we do it as,

::

    def f(x):

    	"""Accepts a number x as argument and,

	returns the square of the number x."""

	return x*x

{{{ switch to next slide, exercise 4 }}}

%% 4 %% Write a python function named ``circle`` which returns the

   area and perimeter of a circle given radius ``r``.

Pause here and try to solve the problem yourself.

{{{ switch to next slide, solution 4 }}}

The problem requires us to return two values instead of one which we

were doing till now. We can solve the problem as,

::

    def circle(r):

    	"""returns area and perimeter of a circle given radius r"""

	pi = 3.14

	area = pi * r * r

	perimeter = 2 * pi * r

	return area, perimeter

A python function can return any number of values. There is no

restriction for it.

Let us call the function ``circle`` as,

::

    a, p = circle(6)

    print a

    print p

Now we have done enough coding, let us do some code reading exercise,

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What does the function ``what`` do?

.. def what( n ):

..     if n < 0: n = -n

..     while n > 0:

..         if n % 2 == 1:

..             return False

..         n /= 10

..     return True

Pause here and try to figure out what the function ``what`` does.

{{{ switch to next slide, even_digits }}}

.. def even_digits( n ):

..    """returns True if all the digits of number n is even

..    returns False if all the digits of number n is not even"""

..     if n < 0: n = -n

..     while n > 0:

..         if n % 2 == 1:

..             return False

..         n /= 10

..     return True

The function returns ``True`` if all the digits of the number ``n``

are even, otherwise it returns ``False``.

Now one more code reading exercise,

{{{ switch to next slide, what }}}

What does the function ``what`` do?

.. def what( n ):

..     i = 1

..     while i * i < n:

..         i += 1

..     return i * i == n, i

Pause here and try to figure out what the function ``what`` does.

{{{ switch to next slide, is_perfect_square }}}

.. def is_perfect_square( n ):

..     """returns True and square root of n, if n is a perfect square,

..     otherwise returns False and the square root of the 

..     next perfect square"""

..     i = 1

..     while i * i < n:

..         i += 1

..     return i * i == n, i

The function returns ``True`` and the square root of ``n`` if n is a

perfect square, otherwise it returns ``False`` and the square root of

the next perfect square.

This brings us to the end of this tutorial, in this tutorial we covered

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- Functions in Python

- Passing parameters to a function

- Returning values from a function

We also did few code reading exercises.

{{{ switch to next slide, Thank you }}}

Thank you!