========

 Script

========

{{{ show the welcome slide }}}

Welcome this 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 the ``break``, ``continue`` and ``pass`` keywords

and how to use them. 

{{{ 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. 

E%% %% Pause the video here and write a ``while`` loop to print the

squares of all the even numbers below 10. Then, return to the video.

::

  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

E%% %% Pause the video here and write a ``for`` loop to print the

squares of all the even numbers below 10. Then, return to the video. 

::

  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. 

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

E%% %%Pause the video here and using the ``continue`` keyword modify

the ``for`` loop to print the squares of even numbers below 10, to

print the squares of only multiples of 4. (Do not modify the range

function call.) Then, resume the video. 

::

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

      if n%4:

          continue      

      print n*n

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``. 

Thank You!