Python-3.4.3/C3/Manipulating-lists/English-timed
From Script | Spoken-Tutorial
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| 00:01 | Welcome to the spoken tutorial on Manipulating Lists. |
| 00:06 | In this tutorial, we will learn about
slicing and striding of lists |
| 00:12 | Sort and reverse lists. |
| 00:15 | To record this tutorial, I am using Ubuntu Linux 16.04 operating system |
| 00:23 | Python 3.4.3
and IPython 5.1.0 |
| 00:30 | To practise this tutorial, you should know how to run basic Python commands on the ipython console and use lists |
| 00:41 | If not, see the relevant Python tutorials on this website. |
| 00:46 | We have already learnt about list and how to access individual elements in the list. |
| 00:53 | Now we will see about slicing of lists. |
| 00:57 | The syntax for slicing is p inside square brackets start colon stop. |
| 01:04 | It returns all the elements of p between start and stop values. |
| 01:11 | The element with the stop index value will not be included. |
| 01:16 | Let us start ipython.
Open the terminal. |
| 01:21 | Type ipython3 and press Enter. |
| 01:27 | From here onwards, remember to press the Enter key after typing every command on the terminal. |
| 01:34 | Let us understand the slicing with an example.
Type, primes is equal to then type as shown. |
| 01:45 | Now we will try to obtain all the primes between 10 and 20 from the above list of primes. |
| 01:52 | Type, primes inside square brackets 4 colon 8 |
| 01:59 | Recall that the start index value is 0. |
| 02:03 | Observe that the first element we want is 11 which has index 4 in the list.
So, start value is 4. |
| 02:12 | Also, 19 is the last element we require which has index 7 in the list. |
| 02:19 | The element with index equal to stop value will not be included.
So, end value is 8. |
| 02:26 | Pause the video.
Try this exercise and then resume the video. |
| 02:32 | Obtain the primes less than 10, from the list primes. |
| 02:37 | Switch back to terminal for the solution |
| 02:41 | Type, primes inside square brackets 0 colon 4 |
| 02:47 | Observe that in this case, our start value is 0. |
| 02:52 | 7 is the last element we require which has index 3 in the list.
So, the end value is 4. |
| 03:01 | Next we will learn to use step value in slicing. |
| 03:06 | Type 0 to 13 in a list and assign it to a variable num as shown. |
| 03:13 | Let us say, we want all the odd numbers less than 10 from the list num. |
| 03:19 | We will specify the step value in which the slice must be obtained. |
| 03:25 | Type num inside square brackets 1 colon 10 colon 2 |
| 03:31 | We have to start from element with index 1 upto index 10 in steps of 2. |
| 03:39 | It is called striding of list.
Press Enter to get the output. |
| 03:45 | We got the odd numbers less than 10. |
| 03:48 | When no step is specified, it is assumed to be 1. |
| 03:53 | Similarly, there are default values for start and stop as well. |
| 03:59 | Type, num inside square brackets colon 10 |
| 04:04 | It gives the first 10 elements of the list. |
| 04:08 | If we don't specify the start value, the first element of the list is taken as starting. |
| 04:15 | Type, num inside square brackets 10 colon |
| 04:21 | This gives us all the elements from 10th element to the end. |
| 04:26 | If we don't specify the stop value, the elements till the last index of the list will be returned. |
| 04:33 | Start or end value can be negative to indicate that they are counted from the end of the list. |
| 04:40 | Next let us get all the even numbers in the list "num" |
| 04:45 | Observe that all the even numbers are at even index locations. |
| 04:51 | So, type num inside square brackets colon colon 2 |
| 04:58 | We got all the even numbers.
This is called striding. |
| 05:04 | Pause the video.
Try this exercise and then resume the video. |
| 05:10 | Obtain all the multiples of three from the list num. |
| 05:15 | Switch back to terminal for the solution |
| 05:19 | Type, num inside square brackets colon colon 3 |
| 05:25 | It gives us every third element from the list (i.e) multiples of 3. |
| 05:31 | Next let us learn to sort a list. |
| 05:35 | Type, a is equal to inside square brackets 5, 1, 6, 7, 7, 10 |
| 05:45 | sort method is used to sort a list. |
| 05:49 | Type, a.sort open and close brackets |
| 05:55 | Type, a to get the output.
We can see that the contents of the list a is sorted now. |
| 06:04 | Python provides a built-in function called sorted. |
| 06:08 | sorted function sorts the list which is passed as an argument to it. |
| 06:14 | It returns a new sorted list. |
| 06:17 | Again we will assign the same value to a as shown. |
| 06:23 | Type, sorted inside brackets a |
| 06:28 | We can store this sorted list into another list variable sa. |
| 06:34 | Type, sa is equal to sorted inside brackets a |
| 06:41 | To see the sorted list, type sa |
| 06:46 | Python also provides the reverse method which reverses the list in place. |
| 06:52 | Type, r is equal to inside square brackets 1 comma 2 comma 3 comma 4 comma 5 |
| 07:01 | r dot reverse open and close brackets |
| 07:06 | To see the list r, type, r |
| 07:10 | We got the reverse of the list. |
| 07:13 | The original list r is changed now. |
| 07:17 | To reverse a list, we can also use striding with negative values. |
| 07:22 | Again we will assign the same value to r as shown. |
| 07:27 | Type, r inside square brackets colon colon minus 1 |
| 07:33 | We can also store this new reversed list in another variable. |
| 07:38 | Type, ra is equal to r inside square brackets colon colon minus 1 |
| 07:46 | To see the reversed list, type ra |
| 07:51 | Pause the video.
Try this exercise and then resume the video. |
| 07:57 | Given below is the list of marks of a student in an examination.
Obtain a list with marks in descending order. |
| 08:06 | Switch back to the terminal for the solution. |
| 08:10 | Type the list of marks as shown. |
| 08:14 | To get the marks in descending order type,
sorted inside brackets marks inside square brackets colon colon minus 1 |
| 08:26 | We can also get the same output by typing,
sorted inside brackets marks comma reverse equal to True |
| 08:38 | This brings us to the end of this tutorial.
Let us summarize. |
| 08:44 | In this tutorial, we have learnt to, Obtain the parts of lists using slicing and striding. |
| 08:52 | Sort lists using the sort method.
Use the method reverse to reverse the lists. |
| 09:00 | Here are some self assessment questions for you to solve |
| 09:05 | First. Given the list primes. How do you obtain the last 4 prime numbers? |
| 09:11 | Second. Given a list p, of unknown length. Obtain the first 3 characters of it. |
| 09:19 | And the answers,
First. The last four primes can be obtained from the given list as, primes inside square brackets minus 4 colon |
| 09:30 | Second. The first 3 characters can be obtained as,
p inside square brackets colon 3 |
| 09:37 | Please post your timed queries in this forum. |
| 09:41 | Please post your general queries on Python in this forum. |
| 09:46 | FOSSEE team coordinates the TBC project. |
| 09:50 | Spoken Tutorial Project is funded by NMEICT, MHRD, Govt. of India.
For more details, visit this website. |
| 10:01 | This is Priya from IIT Bombay signing off. Thanks for watching. |
