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

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Visual Cue Narration
Show Slide 1

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

Hello friends and Welcome to the tutorial on 'Accessing pieces of arrays'.
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Learning objectives

At the end of this tutorial, you will be able to,
  1. Access and change individual elements of arrays, both one dimensional and multi-dimensional.
  2. Access and change rows and columns of arrays.
  3. Access and change other chunks from an array, using slicing and striding.
  4. Read images into arrays and perform processing on them, using simple array manipulations.


Show Slide 3

Pre-requisite slide

Before beginning this tutorial,we would suggest you to complete the tutorial on "Getting started with arrays".
Open the terminal
ipython -pylab
As usual, we start IPython, using
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Arrays, A and C

Let us begin with the help of an example. Let us have two arrays, A and C, as the sample arrays that we will use to work through this tutorial.
A = array([12, 23, 34, 45, 56])
C = array([[11, 12, 13, 14, 15],
           [21, 22, 23, 24, 25],
           [31, 32, 33, 34, 35],
           [41, 42, 43, 44, 45],
           [51, 52, 53, 54, 55]])
Pause the recording and type the arrays A and C. also make sure

that you have typed the arrays correctly. <Pause>

Let us begin with the most elementary thing, accessing individual elements. Also, let us first do it with the one-dimensional array A, and then do the same thing with the two-dimensional array.

A[2] To access, the element 34 in array A, we say, A of 2, note that we are using square brackets.
C[2, 3] Like lists, indexing starts from 0 in arrays, too. So, 34, the third element has the index 2.

Now, let us access the element 34 from C. To do this, we say, C of 2,3.

A[2] = -34
C[2, 3] = -34
34 is in the third row and the fourth column, and since indexing begins from zero, the row index is 2 and column index is 3.

Now, that we have accessed one element of the array, let us change it. We shall change 34 to -34 in both A and C. To do this, we simply assign the new value after accessing the element.

A[2]
C[2,3]
Let us check our operations,
C[2] Now that we have accessed and changed a single element, let us access and change more than one element at a time; first rows and then columns.

Let us access one row of C, say the third row. We do it by saying,

C[4] How do we access the last row of C? We could say,
C[-1] or as with lists,we could use negative indexing and say,
C[-1] = [0, 0, 0, 0, 0] Now, we could change the last row into all zeros, using either
C[-1] = 0 or, we can use,
C[:, 2] Now, how do we access one column of C? As with accessing individual elements, the column is the second parameter to be specified (after the comma). The first parameter, is replaced with a :. This specifies that we want all the elements of that dimension, instead of just one particular element. We access the third column by saying,
Pause the video here, try out the following exercise and resume the video.
Show Slide 5

Assignment 1

Change the last column of C to zeros.
Continue from paused state Switch to the terminal
C[:, -1] = 0
Switch to the terminal for solution. To change the entire last column of C to zeros, we simply say,
A[:] Since A is one dimensional, rows and columns of A don't make much sense. It has just one row and A of colon gives the whole of A.
Pause the video here, try out the following exercise and resume the video.
Show Slide 6

Assignment 2

Change A to [11, 12, 13, 14, 15].
Continue from paused state Switch to the terminal
A[:] = [11, 12, 13, 14, 15]
Switch to the terminal for solution. To change A, we say,
Switch to the browser and show the image

Switch back to the ipython terminal

Now, that we know how to access, rows and columns of an array, we shall learn how to access other pieces of an array. For this purpose, we will be using image arrays.

To read an image into an array, we use the imread command. We shall use the image squares.png present in /home/fossee. We first navigate to that path in the OS and see what the image contains.

I = imread('/home/fossee/squares.png') Let us now read the data in squares.png into the array I.
imshow(I) We can see the contents of the image, using the command imshow. We say, imshow(I) to see what has been read into I.
I We do not see white and black because, pylab has mapped white and black to different colors. This can be changed by using a different color map.

To see that I is really, just an array, we say, I, at the prompt

I.shape We see that an array is displayed.

To check the dimensions of any array, we can use .shape function.

As we can see,we got the dimensions of the image.The image,``squares.png`` has the dimensions of 300x300.
Show Slide 7

Squares.png Point at top-left quadrant of the image

Our goal for this part of the tutorial would be to get the top-left quadrant of the image. To do this, we need to access, a few of the rows and a few of the columns of the array.

To access, the third column of C, we said, C[:, 2]. Essentially, we are accessing all the rows in column three of C. Now, let us modify this to access only the first three rows, of column three of C.

We say,

C[0:3, 2] C[0:3, 2] gives, the elements of rows indexed from 0 to 3, 3 not included and column indexed 2. Note that, the index before the colon is included and the index after it is not included in the slice that we have obtained. This is very similar to the range function, where range returns a list, in which the upper limit or stop value is not included.
C[2, 0:3] Now, if we wish to access the elements of row with index 2, and in columns indexed 0 to 2 (included), we say,
Pause the video here, try out the following exercise and resume the video.
Show Slide 8

Assignment 3

First, obtain the elements [22, 23] from C. Then, obtain the elements [11, 21, 31, 41] from C. Finally, obtain the elements [21,31, 41, 0]. <Pause> Switch to the terminal for solution.
Continue from paused state Switch to the terminal
C[1, 1:3]
C[1, 1:3] gives the elements [22, 23]
C[0:4, 0] C[0:4, 0] gives the elements [11, 21, 31, 41]
C[1:5, 0] C[1:5, 0] gives the elements [21, 31, 41, 0]

Note that when specifying ranges, if you are starting from the beginning or going up-to the end, the corresponding element may be dropped. So, in the previous example to obtain [11, 21, 31, 41], we could have simply said,

C[:4, 0]
C[1:, 0]
We get the elements [21, 31, 41, 0]. If we skip both the indexes, we get the slice from end to end, as we already know.

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

Show Slide 9

Assignment 4

Obtain the elements [[23, 24], [33, -34]] from C.
Continue from paused state Switch to the terminal
C[1:3, 2:4]
Switch to the terminal for solution.
I[:150, :150] C[1:3, 2:4] will give us the required elements.

Now, we wish to obtain the top left quarter of the image. How do we go about doing it? Since, we know the shape of the image is 300, we know that we need to get the first 150 rows and the first 150 columns.

I[:150, :150] gives us the top-left corner of the image.
imshow(I[:150, :150]) We use the imshow command to see the slice we obtained in the form of an image and confirm.
Pause the video here, try out the following exercise and resume the video.
Show Slide 10

Assignment 5

Obtain the square in the center of the image.
Continue from paused state Switch to the terminal
imshow(I[75:225, 75:225])
Switch to the terminal for solution.
C[0:5:2, 0:5:2] Hence, we get the center of the image.

Our next goal is to compress the image, using a very simple technique, so as to reduce the space that the image takes on disk, while not compromising too heavily on the image quality. The idea is to drop alternate rows and columns of the image and save it. This way we will be reducing the data to one-fourth of the original data but losing only a little of visual information.

We shall first learn the idea of striding using the smaller array C. Suppose we wish to access only the odd rows and columns (first, third, fifth). We do this by,

C[::2, ::2] if we wish to be explicit, we say,
C[1::2, ::2] This is very similar to the step specified to the range function. It specifies, the jump or step in which to move, while accessing the elements. If no step is specified, a default value of 1 is assumed.
we get the elements, [[21, 23, 0], [41, 43, 0]] Pause the video here, try out the following exercise and resume the video.
Show Slide 11

Assignment 6

Obtain the following. [[12, 0], [42, 0]] [[12, 13, 14], [0, 0, 0]]
Continue from paused state

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Solution 6

The solution is on your screen.
I[::2, ::2] Now, that we know how to stride over an array, we can drop alternate rows and columns out of the image in I.
imshow(I[::2, ::2]) To see this image, we say,
imshow(I[::4, ::4]) This does not have much data to notice any real difference, but notice that the scale has reduced to show that we have dropped alternate rows and columns. If you notice carefully, you will be able to observe some blurring near the edges. To notice this effect more clearly, increase the step to 4.
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Summary slide

This brings us to the end of this tutorial. In this tutorial, we have learnt to,
  1. Manipulate single & multi dimensional arrays.
  2. Access and change individual elements by using their index numbers.
  3. Access and change rows and columns of arrays by specifying the row and column numbers.
  4. Slice and stride on arrays.
  5. Read images into arrays and manipulate them.


Show Slide 14

Self assessment questions slide

Here are some self assessment questions for you to solve
  1. Given the array, A = array([12, 15, 18, 21]), how do we access the element 18?
  2. Given the array,
B = array([[10, 11, 12, 13],
           [20, 21, 22, 23],
           [30, 31, 32, 33],
           [40, 41, 42, 43]])

Obtain the elements, [[21, 22], [31, 32]]

3. Given the array,

C = array([[10, 11, 12, 13],
           [20, 21, 22, 23]])

Change the array to

C = array([[10, 11, 10, 11],
           [20, 21, 20, 21]])
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Solution of self assessment questions on slide

And the answers,
  1. The element 18 in array A has index number 2.Hence, we access it as A of 2

Enumerated list ends without a blank line; unexpected unindent.

A[2]

2. To obtain the center four numbers in the array B, we say,B[1:3, 1:3]

B[1:3, 1:3]

3. We can change the elements of array C,by using slicing and striding

C[:2, 2:] = C[:2, :2]
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Acknowledgment slide

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

Contributors and Content Editors

Chandrika, Pravin1389