manipulating-lists/script.rst
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.. Objectives
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.. ----------
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.. Clearly state the objectives of the LO (along with RBT level)
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.. Prerequisites
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.. -------------
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..   1. getting started with lists
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..   2. 
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..   3. 
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.. Author              : Madhu
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   Internal Reviewer   : 
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   External Reviewer   :
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   Checklist OK?       : <put date stamp here, if OK> [2010-10-05]
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Script
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------
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{{{ Show the slide containing the title }}}
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Hello friends. Welcome to this spoken tutorial on Manipulating Lists. 
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{{{ Show the slide containing the outline }}}
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We have already learnt a lot about Lists in Python. In this tutorial,
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we will learn more about advanced features of Lists in Python. We will
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see how to concatenate two lists, details of slicing and striding of
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lists, methods to sort and reverse lists.
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{{{ Shift to terminal and start ipython }}}
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To begin with let us start ipython, by typing::
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  ipython
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on the terminal
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We already know what Lists are in Python, how to access individual
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elements in the list and some of the functions that can be run on the
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lists like ``max, min, sum, len`` and so on. Now let us learn some of
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the basic operations that can be performed on Lists.
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We already know how to access individual elements in a List. But what
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if we have a scenario where we need to get a part of the entire list
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or what we call as a slice of the list? Python supports slicing on
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lists. Let us say I have the list::
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  primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
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To obtain the all the primes between 10 and 20 from the above list of
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primes we say::
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  primes[4:8]
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This gives us all the elements in the list starting from the element
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with the index 4 which is 11 in our list upto the element with index 8
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in the list but not including the eigth element. So we obtain a slice
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starting from 11 upto 19th. It is a very important to remember that
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when ever we specify a range of elements in Python the start index is
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included and end index is not included. So in the above case, 11 which
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was the element with the index 4 was included but 23 which was the
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element with index 8 was excluded.
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Following is an exercise you must do. 
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%% %% Obtain the primes less than 10, from the list ``primes``. 
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Please, pause the video here, do the exercise and then resume. 
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::
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  primes[0:4]
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will give us the primes below 10. 
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Generalizing, we can obtain a slice of the list "p" from the index
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"start" upto the index "end" but excluding "end" with the following
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syntax
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{{{ Show the slide containing p[start:stop] }}}
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By default the slice fetches all the elements between start and stop
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including start but not stop. So as to say we obtain all the elements
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between start and stop in steps of one. Python also provides us the
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functionality to specify the steps in which the slice must be
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obtained. Say we have::
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  num = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
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If we want to obtain all the odd numbers less than 10 from the list
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``num`` we have to start from element with index 1 upto the index 10 in
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steps of 2::
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  num[1:10:2]
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When no step is specified, it is assumed to be 1. Similarly, there are
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default values for start and stop indices as well. If we don't specify
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the start index it is implicitly taken as the first element of the
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list::
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  num[:10]
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This gives us all the elements from the beginning upto the 10th
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element but not including the 10th element in the list "num". Similary
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if the stop index is not specified it is implicitly assumed to be the
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end of the list, including the last element of the list::
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  num[10:]
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gives all the elements starting from the 10th element in the list
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"num" upto the final element including that last element. Now::
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  num[::2]
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gives us all the even numbers in the list "num".
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Following is an exercise that you must do. 
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%% %% Obtain all the multiples of three from the list ``num``.
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Please, pause the video here. Do the exercise and then continue. 
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::
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  num[::3]
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gives us all the multiples of 3 from the list, since every third
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element in it, starting from 0, is divisible by 3. 
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The other basic operation that we can perform on list is concatenation
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of two or more lists. We can combine two lists by using the "plus"
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operator. Say we have
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{{{ Read as you type }}}::
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  a = [1, 2, 3, 4]
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  b = [4, 5, 6, 7]
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  a + b
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When we concatenate lists using the "plus" operator we get a new
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list. We can store this list in a new variable::
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  c = a + b
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  c
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It is important to observe that the "plus" operator always returns a
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new list without altering the lists being concatenated in any way. 
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We know that a list is a collection of data. Whenever we have a
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collection we run into situations where we want to sort the
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collection. Lists support sort method which sorts the list inplace::
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  a = [5, 1, 6, 7, 7, 10]
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  a.sort()
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Now the contents of the list ``a`` will be::
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  a
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  [1, 5, 6, 7, 7, 10]
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Since the sort method sorts the list inplace the original list we had
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is overwritten or replaced. We have no way to obtain the original list
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back. One way to avoid this is to keep a copy of the original list in
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another variable and run the sort method on the list. However Python
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also provides a built-in function called sorted which sorts the list
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which is passed as an argument to it and returns a new sorted list::
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  a = [5, 1, 6, 7, 7, 10]
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  sorted(a)
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We can store this sorted list another list variable::
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  sa = sorted(a)
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Similarly to perform certain operations on the list we would like to
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reverse the list. Python provides reverse method which again reverses
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the list inplace::
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  a = [1, 2, 3, 4, 5]
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  a.reverse()
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reverses the list "a" and stores the reversed list inplace i.e. in "a"
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itself. Lets see the list "a"::
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  a
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  [5, 4, 3, 2, 1]
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But again the original list is lost. 
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.. #[punch: removed reversed, since it returns an iterator]
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To reverse a list, we could use striding with negative indexing.::
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   a[::-1]
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We can also store this new reversed list in another list variable.
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Following is an (are) exercise(s) that you must do. 
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%% %% Given a list of marks of students in an examination, obtain a
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      list with marks in descending order.
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      ::
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            marks = [99, 67, 47, 100, 50, 75, 62]
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Please, pause the video here. Do the exercise(s) and then continue. 
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::
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  sorted(marks)[::-1]
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OR
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::
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  sorted(marks, reverse = True)
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{{{ Show summary slide }}}
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This brings us to the end of another session. In this tutorial session
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we learnt
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  * Obtaining parts of lists using slicing and striding
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  * List concatenation
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  * Sorting lists 
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  * Reversing lists
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{{{ Show the "sponsored by FOSSEE" slide }}}
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This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India
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Hope you have enjoyed and found it useful.
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Thank you!
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