.. 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 }}}

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

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

{{{ switch to next slide, Function }}}

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

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

In your Ipython interpreter type the following,

::

    def f(x):

    	return x*x

::

    f(1)

    f(2)

{{{ switch to next slide, problem statement 1 }}}

%% 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.

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

The problem can be solved as,

::

    def cube(n):

    	return n**3

{{{ switch to next slide, greet function }}}

World``.

::

    def greet():

    	print "Hello World!"

::

    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 }}}

::

    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

::

    def avg(a,b):

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

	returns the average of a and b"""

	return (a+b)/2

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,

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

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

{{{ switch to next slide, summary }}}

- Functions in Python

- Passing parameters to a function

- Returning values from a function

We also did few code reading exercises.

Thank you!