Python-3.4.3/C3/Accessing-parts-of-arrays/English-timed

Time	Narration
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.

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