Difference between revisions of "Python-3.4.3/C2/Accessing-parts-of-arrays/English"

Objectives

• Individual elements of single dimensional and multi-dimensional arrays.
• Rows and columns of arrays.
• Elements of an array, using slicing and striding.

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3 and
• IPython 5.1.0

Pre-requisite slide

• run basic Python commands on the IPython console.
• use arrays.

If not, see the relevant Python tutorials on this website.

Sample Arrays

Consider two arrays, A and C.

We will use these arrays throughout this tutorial.

Open the terminal, type ipython3 and press Enter

Open the terminal.

Type ipython3 and press Enter.

For this, we have to import numpy library.

Type import numpy as np and press Enter.

From here onwards, please remember to press the Enter key after typing every command on the terminal.

Highlight arange, reshape

We have already learnt about arange and reshape methods in an earlier tutorial.

Type A

A = array([1, 2, 3, 4, 5])

C = array([[1, 2, 3, 4, 5]

[6, 7, 8, 9, 10],

[11, 12, 13, 14, 15],

[16, 17, 18, 19, 20],

[21, 22, 23, 24, 25]])

A is a one dimensional array.

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.

<<PAUSE>>

Type A[2]

To access, the element 3 in array A, we say, A of 2.

Type A inside square brackets 2

In Python, arrays are zero-indexed.

This means, the position of the element starts with 0 instead of 1.

Type, C[2, 3]

14 is in the third row and the fourth column.

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

Type C inside square brackets 2 comma 3.

<<PAUSE>>

Type A[2] = -3

We shall now change 3 to -3 in A and 14 to -14 in C.

For this we simply assign the new value after accessing the element.

Type, A inside square brackets 2 is equal to minus 3

Type A

Highlight -3 and -14

You can see that the elements are changed now.

Likewise you can change any single element in an array.

<<PAUSE>>

Type, C[2]

First with rows and then with columns.

Let us access one row of C, say the third row.

Type C inside square brackets 2

We can see that the third row of the array is displayed now.

Negative Indexing

This means the index value of

• -1 gives the last element and
• -2 gives the second to last element of an array.

Type C inside square brackets 4

Highlight the outputs

Notice that both the outputs are same.

Type C inside square brackets minus 1 is equal to inside square brackets 0 comma 0 comma 0 comma 0 comma 0

Notice that zeros are displayed in the last row of the array C.

Highlight the last row

And check with the changes made.

<<PAUSE>>

[start:stop]

Slicing of an array is done to access parts of an array.

Slicing syntax is inside square brackets start colon stop.

syntax [start:stop:step]

Striding syntax is inside square brackets start colon stop colon step.

Highlight array([ 3, 8, 13])

0 and 3 corresponds to start and stop values for row slicing and 2 corresponds to column index.

We get the elements of rows indexed from 0 to 2 and column indexed by 2.

Hence we have sliced the array.

Type C[2,0:3]

Highlight 2 and 0:3

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.

Exercise 1

Try this exercise and then resume the video.

Obtain the following elements one by one from array C.

Highlight[7,8]

We get the elements 7 and 8.

Highlight [1, 6, 11, 16]

We get the elements 1, 6, 11 and 16

Highlight [6, 11, 16, 0]

We get the elements 6, 11, 16 and 2

Exercise 2

Try this exercise and then resume the video.

Obtain the elements [[8, 9], [13, -14]] from array C.

Highlight the output

We got the required elements.

<<PAUSE>>

We will try to access only the odd rows and columns i.e first, third and fifth.

Type, C inside square brackets 0 colon 5 colon 2 comma 0 colon 5 colon 2

Highlight the output

We can see that only the odd rows and columns are displayed.

Type C[1::2, ::2]

Highlight the output

This is called striding.

If no step is specified, a default value of 1 is assumed.

Type C inside square brackets 1 colon colon 2 comma colon colon 2

We get the elements as shown.

Exercise 3

Try this exercise and then resume the video.

Obtain the following elements from array C.

Summary slide

In this tutorial, we have learnt to,

1. Manipulate single and 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.

Evaluation

1. How do we access the element 18 from the given array A?
2. How do we obtain the elements [[21, 22], [31, 32]] from the given array B?

Solutions

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

1. To obtain the central four numbers in the array B, we say, B inside square brackets 1 colon 3 comma 1 colon 3

For more details, visit this website.

Thank You