PoojaMoolya: Created page with "{| border=1 | '''Time''' | '''Narration''' |- | 00:01 | Welcome to the spoken tutorial on "'''Accessing parts of arrays'''". |- | 00:07 |..."

2019-05-31T11:03:02Z

Created page with "{| border=1 | <center>'''Time'''</center> | <center>'''Narration'''</center> |- | 00:01 | Welcome to the spoken tutorial on "'''Accessing parts of arrays'''". |- | 00:07 |..."

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{| border=1
| <center>'''Time'''</center>
| <center>'''Narration'''</center>

|-
| 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 '''

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| 02:16
| Type '''C '''

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| 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.

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| 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'''.

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| 03:11
|14 is in the third '''row''' and the fourth '''column'''.

To do this, we say, C of 2,3.

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| 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.

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| 03:37
|For this we simply assign the new value after accessing the '''element'''.

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| 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'''

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| 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'''.

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| 04:29
|Type '''C '''''inside square brackets '''''2'''

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| 04:34
|We can see that the third row of the '''array''' is displayed now.

|-
| 04:40
| '''Python programming''' supports '''negative indexing''' of '''arrays'''.

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| 04:45
|This means the index value of -1 gives the last element and -2 gives the second to last element of an array.

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| 04:56
| We can access the last '''row''' of C in 2 ways.

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| 05:01
|Type '''C '''''inside square brackets '''''4'''

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| 05:06
| Or with '''negative indexing''' as''' C '''''inside square brackets minus '''''1'''

Notice that both the outputs are same.

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| 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.

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| 05:42
| We can also type,''' C '''''inside square brackets minus '''''1 '''''is equal to''''' 2.'''

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| 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.

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| 07:32
| Type '''C '''''inside square brackets '''''1 '''comma''' 1 '''colon '''3'''

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| 07:39
|We get the '''elements''' 7 and 8.

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| 07:43
| Type '''C '''''inside square brackets '''''0 '''colon '''4 '''comma''' 0 '''

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| 07:50
|We get the '''elements''' 1, 6, 11 and 16

|-
|07:55
| Type '''C '''''inside square brackets '''''1 '''colon '''5 '''comma''' 0'''

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| 08:02
|We get the '''elements''' 6, 11, 16 and 2

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| 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 '''

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| 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.

|}

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

PoojaMoolya: Created page with "{| border=1 | '''Time''' | '''Narration''' |- | 00:01 | Welcome to the spoken tutorial on "'''Accessing parts of arrays'''". |- | 00:07 |..."