.. Objectives

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

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

.. 1. use the ``for`` loop 

.. #. use the ``while`` loop

.. #. Use ``break``, ``continue`` and ``pass`` statements to play around

..    with loops.

.. Prerequisites

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

.. 1. getting started with ipython

.. #. getting started with for

.. #. conditionals

.. Author              : Puneeth

   Internal Reviewer   : Anoop Jacob Thomas<anoop@fossee.in>

   External Reviewer   :

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

Script

------

{{{ Show the slide containing title }}}

Hello Friends. Welcome to the tutorial on loops in Python. 

{{{ Show the outline slide }}}

In this tutorial, we shall look at ``while`` and ``for`` loops. We

shall then look at how to break out of them, or skip some iterations

in loops.

.. #[[Anoop: for loop is a pre-requisite and has been already covered,

   so i think our emphasize can be on while loops]]

.. #[[punch: I think, we should have both of them. It gives a better

.. context and comparison.]

{{{ switch to the ipython terminal }}}

We have an ``ipython`` terminal, that we shall use through out this

tutorial. 

We shall first begin with the ``while`` loop. The ``while`` loop is

used for repeated execution as long as a condition is ``True``. 

Let us print the squares of all the odd numbers less than 10, using

the ``while`` loop.

::

  i = 1

  while i<10:

      print i*i

      i += 2

This loop prints the squares of the odd numbers below 10. 

The ``while`` loop, repeatedly checks if the condition is true and

executes the block of code within the loop, if it is. As with any

other block in Python, the code within the ``while`` block is indented

to the right by 4 spaces. 

{{{ switch to next slide }}}

Following is an exercise that you must do. 

%%1%% Write a ``while`` loop to print the squares of all the even

numbers below 10. 

Please, pause the video here. Do the exercise and then continue. 

{{{ switch to next slide after a seconds break}}}

::

  i = 2

  while i<10:

      print i*i

      i += 2

Let us now solve the same problem of printing the squares of all odd

numbers less than 10, using the ``for`` loop. As we know, the ``for``

loop iterates over a list or any other sequential data type. So, we

use the ``range`` function to get a list of odd numbers below 10, and

then iterate over it and print the required stuff. 

::

  for n in range(1, 10, 2):

      print n*n

Following is an exercise that you must do. 

{{{ switch to next slide }}}

%%2%% Write a ``for`` loop to print the squares of all the even

numbers below 10. 

Please, pause the video here. Do the exercise and then continue. 

{{{ switch to next slide after a seconds break }}}

::

  for n in range(2, 10, 2):

      print n*n

Let us now look at how to use the keywords, ``pass``, ``break`` and

``continue``.

As we already know, ``pass`` is just a syntactic filler. It is used

for the sake of completion of blocks, that do not have any code within

them. 

::

  for n in range(2, 10, 2):

      pass

``break`` is used to break out of the innermost loop. The ``while``

loop to print the squares of all the odd numbers below 10, can be

modified using the ``break`` statement, as follows

::

  i = 1

  while True:

      print i*i

      i += 2

      if i<10:

          break

``continue`` is used to skip execution of the rest of the loop on this

iteration and continue to the end of this iteration. 

.. #[[Anoop: should add slides for break, continue, pass]]

Say, we wish to print the squares of all the odd numbers below 10,

which are not multiples of 3, we would modify the ``for`` loop as

follows.  ::

  for n in range(1, 10, 2):

      if n%3 == 0:

          continue      

      print n*n

Following is an exercise that you must do. 

{{{ switch to next slide }}}

%%3%%Using the ``continue`` keyword modify the ``for`` loop, with the

``range(2, 10, 2)``, to print the squares of even numbers below 10, to

print the squares of only multiples of 4.

Please, pause the video here. Do the exercise and then continue. 

{{{ switch to next slide after a seconds break}}}

::

  for n in range(2, 10, 2):

      if n%4:

          continue      

      print n*n

{{{ Show summary slide }}}

That brings us to the end of this tutorial. In this tutorial, we have

learnt about looping structures in Python and the use of the keywords

``pass``, ``break`` and ``continue``. 

{{{ Show the "sponsored by FOSSEE" slide }}}

This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

Hope you have enjoyed and found it useful.

Thank you!