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Visual Cue Narration
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Containing title, name of the production team along with the logo of MHRD

Hello friends and Welcome to the tutorial on 'Manipulating Lists'.
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Learning objectives

At the end of this tutorial, you will be able to,
  1. Concatenate two lists
  2. Learn the details of slicing and striding of lists
  3. Sort and reverse lists.

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Pre-requisite slide

Before beginning this tutorial,we would suggest you to complete the tutorial on "Getting started with Lists".
Open the terminal and start ipython
let us start ipython on our terminal
primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29] We have already learnt about lists in Python, how to access individual elements in the list and some of the functions that can be run on the lists like max, min, sum, len and so on. Now let us learn some of the basic operations that can be performed on Lists.

We already know how to access individual elements in a List. But what if we have a scenario where we need to get a part of the entire list or what we call as a slice of the list? Python supports slicing on lists. Let us say I have the list,

primes[4:8] To obtain all the primes between 10 and 20 from the above list of primes we say
This gives us all the elements in the list starting from the element with the index 4, which is 11, upto the element with index 8 in the list but not including the eighth element. So we obtain a slice starting from 11 upto 19th. It is very important to remember that whenever we specify a range of elements in Python, the start index is included and end index is not included. So in the above case, 11, which was the element with the index 4, was included but 23 which was the element with index 8 was excluded.

Pause the video here, try out the following exercise and resume the video.

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Assignment 1

Obtain the primes less than 10, from the list primes.
Continue from paused state Switch to the terminal
Switch to the terminal for solution
It give us the primes below 10.
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Containing p[start:stop]

Generalizing, we can obtain a slice of the list "p" from the index "start" upto the index "end" but excluding "end" with the syntax p[start:stop]
Switch to terminal By default the slice fetches all the elements between start and stop including start but not stop. So as to say we obtain all the elements between start and stop in steps of one.
num = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] Python also provides us the functionality to specify the steps in which the slice must be obtained. Say we have
num[1:10:2] If we want to obtain all the odd numbers less than 10 from the list num we have to start from element with index 1 upto the index 10 in steps of 2
num[:10] When no step is specified, it is assumed to be 1. Similarly, there are default values for start and stop indices as well. If we don't specify the start index, it is implicitly taken as the first element of the list
num[10:] This gives us all the elements from the beginning upto the 10th element but not including the 10th element in the list. Similarly if the stop index is not specified, it is implicitly assumed to be the end of the list, including the last element of the list
num[::2] This gives all the elements starting from the 10th element in the list "num" upto the final element including that last element.

To get all the even numbers in the list "num", we do

Pause the video here, try out the following exercise and resume the video.
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Assignment 2

Obtain all the multiples of three from the list num.
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Solution 2

The solution is on your screen.

num[::3] gives us all the multiples of 3 from the list, since every

Block quote ends without a blank line; unexpected unindent.

third element in it, starting from 0, is divisible by 3.

a = [1, 2, 3, 4]
b = [4, 5, 6, 7]
a + b
The other basic operation that we can perform on lists is concatenation of two or more lists. We can combine two lists by using the "plus" operator. Say we have
c = a + b
When we concatenate lists using the "plus" operator we get a new list. We can store this list in a new variable,say c,
a = [5, 1, 6, 7, 7, 10]
It is important to observe that the "plus" operator always returns a new list without altering the lists being concatenated in any way.

We know that a list is a collection of data. Whenever we have a collection, we run into situations where we want to sort the collection. Lists support sort method which sorts the list in place

a Now the contents of the list a will be
a = [5, 1, 6, 7, 7, 10]
As the sort method sorts the elements of a list, the original list we had, is overwritten or replaced. We have no way to obtain the original list back. One way to avoid this is to keep a copy of the original list in another variable and run the sort method on the list. However Python also provides a built-in function called sorted which sorts the list which is passed as an argument to it and returns a new sorted list
sa = sorted(a) We can store this sorted list into another list variable
a = [1, 2, 3, 4, 5]
Python also provides the reverse method which reverses the list in place
a the reverse method reverses the list "a" and stores the reversed list in place i.e. in "a" itself. Lets see the list "a"
a[::-1] But again the original list is lost.

To reverse a list, we could use striding with negative indexing.

We can also store this new reversed list in another list variable.

Pause the video here, try out the following exercise and resume the video.

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Assignment 3

Given a list of marks of students in an examination, obtain a

list with marks in descending order.

marks = [99, 67, 47, 100, 50, 75, 62]

Continue from paused state Switch to the terminal
Switch to terminal for solution.
sorted(marks, reverse = True) OR
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Summary slide

This brings us to the end of this tutorial. In this tutorial, we have learnt to,
  1. Obtain parts of lists using slicing and striding.
  2. Concatenate lists using the plus operator.
  3. Sort lists using the sort method.
  4. Use the method reverse to reverse the lists.

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Self assessment questions slide

Here are some self assessment questions for you to solve
  1. Given the list primes, primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29], How do you obtain the last 4 primes?
  2. Given a list, p, of unknown length, obtain the first 3 (or all, if there are fewer) characters of it.
  3. reversed function reverses a list in place. True or False?

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Solution of self assessment questions on slide

And the answers,

1. The last four primes can be obtained from the given list as,


2. The first 3 characters can be obtained as,

  1. False. The function reverse will reverse a list in place.

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Acknowledgment slide

Hope you have enjoyed this tutorial and found it useful. Thank you!

Contributors and Content Editors