Python-3.4.3/C3/Accessing-parts-of-arrays/English-timed
From Script | Spoken-Tutorial
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| 00:01 | Welcome to the spoken tutorial on "Accessing parts of arrays". |
| 00:07 | In this tutorial, we will learn to access and change: Individual elements of single dimensional and multi-dimensional arrays. |
| 00:17 | Rows and columns of arrays. |
| 00:20 | Elements of an array, using slicing and striding. |
| 00:25 | To record this tutorial, I am using
Ubuntu Linux 16.04 operating system |
| 00:33 | Python 3.4.3 and IPython 5.1.0 |
| 00:40 | To practise this tutorial, you should know how to run basic Python commands on the IPython console. |
| 00:49 | use arrays. |
| 00:51 | If not, see the relevant Python tutorials on this website. |
| 00:57 | Let us begin with the help of an example. |
| 01:01 | Consider two arrays, A and C.
We will use these arrays throughout this tutorial. |
| 01:09 | Let us start ipython.
Open the terminal. |
| 01:15 | Type ipython3 and press Enter. |
| 01:21 | Let us create the two arrays in terminal.
For this, we have to import numpy library. |
| 01:29 | Type import numpy as np and press Enter. |
| 01:36 | From here onwards, please remember to press the Enter key after typing every command on the terminal. |
| 01:43 | Type A is equal to np dot arange inside parentheses 1 comma 6 |
| 01:51 | Type, C is equal to np dot arange inside parentheses 1 comma 26 dot reshape inside parentheses 5 comma 5 |
| 02:03 | We have already learnt about arange and reshape methods in an earlier tutorial. |
| 02:09 | Now, let us see the contents of A and C.
Type A |
| 02:16 | Type C |
| 02:19 | In A, we have only one row with elements from 1 to 6.
A is a one dimensional array. |
| 02:28 | In C, we have 1 to 26 elements in the form of matrix of 5 rows and 5 columns.
So C is a two dimensional array. |
| 02:40 | Next, let us see about accessing individual elements in an array. |
| 02:45 | To access, the element 3 in array A, we say, A of 2. |
| 02:51 | Type A inside square brackets 2 |
| 02:56 | In Python, arrays are zero-indexed.
This means, the position of the element starts with 0 instead of 1. |
| 03:06 | Now, let us access the element 14 from array C. |
| 03:11 | 14 is in the third row and the fourth column.
To do this, we say, C of 2,3. |
| 03:19 | Type C inside square brackets 2 comma 3. |
| 03:24 | Next we will learn how to change the value of an array. |
| 03:29 | We shall now change 3 to -3 in A and 14 to -14 in C. |
| 03:37 | For this we simply assign the new value after accessing the element. |
| 03:43 | Type, A inside square brackets 2 is equal to minus 3 |
| 03:50 | Type, C inside square brackets 2 comma 3 is equal to minus 14 |
| 03:57 | Let us check our operations.
Type A |
| 04:03 | Type C
You can see that the elements are changed now. |
| 04:10 | Likewise you can change any single element in an array. |
| 04:15 | Next let us learn to change more than one elements at a time.
First with rows and then with columns. |
| 04:24 | Let us access one row of C, say the third row. |
| 04:29 | Type C inside square brackets 2 |
| 04:34 | We can see that the third row of the array is displayed now. |
| 04:40 | Python programming supports negative indexing of arrays. |
| 04:45 | This means the index value of -1 gives the last element and -2 gives the second to last element of an array. |
| 04:56 | We can access the last row of C in 2 ways. |
| 05:01 | Type C inside square brackets 4 |
| 05:06 | Or with negative indexing as C inside square brackets minus 1
Notice that both the outputs are same. |
| 05:17 | Now, we will learn to change the last row into all zeros. |
| 05:22 | Type C inside square brackets minus 1 is equal to inside square brackets 0 comma 0 comma 0 comma 0 comma 0 |
| 05:34 | Type C
Notice that zeros are displayed in the last row of the array C. |
| 05:42 | We can also type, C inside square brackets minus 1 is equal to 2. |
| 05:50 | Type C
And check with the changes made. |
| 05:57 | Now let us learn to slice an array. |
| 06:00 | Slicing of an array is done to access parts of an array. |
| 06:05 | Slicing syntax is inside square brackets start colon stop. |
| 06:11 | Striding uses the ‘step’ value to jump between the elements in an array. |
| 06:17 | Striding syntax is inside square brackets start colon stop colon step. |
| 06:25 | Switch back to the terminal. |
| 06:28 | Type, C inside square brackets 0 colon 3 comma 2 |
| 06:34 |
0 and 3 corresponds to start and stop values for row slicing and 2 corresponds to column index. |
| 06:44 | We get the elements of rows indexed from 0 to 2 and column indexed by 2.
Hence we have sliced the array. |
| 06:54 | Now we will access the elements of row with index 2, and first 2 columns. |
| 07:01 | Type C inside square brackets 2 comma 0 colon 3
2 corresponds to row index and 0 and 3 corresponds to start and stop values for column slicing. |
| 07:17 | Pause the video. Try this exercise and then resume the video. |
| 07:23 | Obtain the following elements one by one from array C. |
| 07:28 | Switch to the terminal for the solution. |
| 07:32 | Type C inside square brackets 1 comma 1 colon 3 |
| 07:39 | We get the elements 7 and 8. |
| 07:43 | Type C inside square brackets 0 colon 4 comma 0 |
| 07:50 | We get the elements 1, 6, 11 and 16 |
| 07:55 | Type C inside square brackets 1 colon 5 comma 0 |
| 08:02 | We get the elements 6, 11, 16 and 2 |
| 08:07 | We can also get the same elements by typing C inside square brackets 1 colon comma 0 |
| 08:16 | Pause the video. Try this exercise and then resume the video. |
| 08:22 | Obtain the elements [[8, 9], [13, -14]] from array C. |
| 08:28 | Switch to the terminal for the solution. |
| 08:31 | Type, C inside square brackets 1 colon 3 comma 2 colon 4 |
| 08:39 | We got the required elements. |
| 08:42 | Next we will learn the idea of striding using the smaller array C. |
| 08:47 | We will try to access only the odd rows and columns i.e first, third and fifth. |
| 08:54 | Type, C inside square brackets 0 colon 5 colon 2 comma 0 colon 5 colon 2 |
| 09:04 | We can also type C inside square brackets colon colon 2 comma colon colon 2 |
| 09:13 | We can see that only the odd rows and columns are displayed. |
| 09:18 | The step 2 specifies the jump between the elements.
This is called striding. |
| 09:26 | If no step is specified, a default value of 1 is assumed. |
| 09:32 | Type C inside square brackets 1 colon colon 2 comma colon colon 2 |
| 09:41 | We get the elements as shown. |
| 09:44 | Pause the video. Try this exercise and then resume the video. |
| 09:50 | Obtain the following elements from array C. |
| 09:54 | The solution is on your screen. |
| 09:57 | This brings us to the end of this tutorial.
In this tutorial, we have learnt to, Manipulate single and multi dimensional arrays. |
| 10:08 | Access and change individual elements by using their index numbers. |
| 10:14 | Access and change rows and columns of arrays by specifying the row and column numbers. |
| 10:21 | Slice and stride on arrays. |
| 10:24 | Here are some self assessment questions for you to solve. |
| 10:28 | How do we access the element 18 from the given array A? |
| 10:33 | How do we obtain the elements [[21, 22], [31, 32]] from the given array B? |
| 10:40 | And the answers,
First. The element 18 in array A has index number 2. Hence, we can access it as A of 2 |
| 10:50 | Second. To obtain the central four numbers in the array B, we say, B inside square brackets 1 colon 3 comma 1 colon 3 |
| 11:01 | Please post your timed queries in this forum. |
| 11:05 | Please post your general queries on Python in this forum. |
| 11:10 | FOSSEE team coordinates the TBC project. |
| 11:14 | Spoken-tutorial is funded by NMEICT, MHRD, Govt. of India.
For more details, visit this website. |
| 11:24 | This is Priya from IIT Bombay signing off. Thanks for watching. |
