Difference between revisions of "Python-3.4.3/C2/Getting-started-with-arrays/English"

Learning objectives

• Create arrays using data
• Create arrays from lists
• Perform basic array operations
• Create an identity matrix and
• Use the method zeros

System Requirements

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

Pre-requisite slide

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

Overview of array

All elements in it must be of same data type.

Let us first open the terminal by pressing Ctrl+Alt+T keys simultaneously.

Highlight ipython prompt

We can see the ipython prompt.

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.

Type, a1 is equal to np dot array inside parentheses inside square brackets 1 comma 2 comma 3 comma 4

Notice that we have created a one dimensional array here.

Also notice that the object we passed to create an array is a list, i.e a1.

<<PAUSE>>

a2 = np.array([[1,2,3,4],[5,6,7,8]])

Two dimensional array is created by converting a list of lists to an array.

Type, a2 is equal to np dot array inside parentheses inside square brackets again inside square brackets 1 comma 2 comma 3 comma 4 comma inside square brackets 5 comma 6 comma 7 comma 8

This is our 2-dimensional array.

<<PAUSE>>

To arrange the elements in an array we use arange method.

The syntax is shown here.

Here, 1 is the start value and 9 is the stop value.

As you can see, we obtained a one dimensional array between 1 and 9 with 1 included and 9 excluded.

It will give the elements one less than the stop value.

<<PAUSE>>

Yes, we can do it.

object.reshape(rows, columns)

The syntax is:

object.reshape inside parentheses rows comma columns

Shape of the original array ar is not changed.

ar dot shape is equal to inside parentheses 2 comma 4.

We can see that the shape of the original array ar is changed now.

shape of an array

It returns a tuple of the shape of an array.

A tuple is nothing but an ordered list of elements.

a2.shape

Type a2 dot shape

a2.shape object is a tuple, and it returned a tuple (2, 4).

Assignment 1

Find out the shape of the arrays a1 and ar which we have created earlier in this tutorial.

Type, a1 dot shape

Since a1 is a single dimensional array, the column is empty.

ar is a two dimensional array.

Type a3 is equal to np dot array inside parentheses inside square brackets 1 comma 2 comma 3 comma inside single quotes a string

Here we are handling with different datatypes. So it should give us error.

Type a3

But we did not get any error.

Because all the elements get implicitly converted as strings.

Note that the output mentions dtype=’<U21'

A dtype is the datatype required to hold the objects in sequence.

The characters of dtype i.e. ’<U21' might differ with python version.

identity(n) method

It is a square matrix of order (n,n) with ones on the main diagonal and all other elements as zeros.

The syntax is identity inside parentheses n.

Type, np dot identity inside parentheses 2.

We can see all ones in the main diagonal as expected.

zeros(shape) method

It creates an m by n matrix with all elements as 0.

The Syntax is: zeros inside parentheses inside parentheses m, n

np.zeros((4,5))

Type, np dot zeros inside parentheses inside parentheses 4 comma 5.

Learning exercise

• zeros_like
• ones
• ones_like

First check the value of a1 which we assigned earlier.

Type a1

We see that a1 is a single dimensional array.

a1 * 2

It returned a new array with all the elements multiplied by 2.

Note that the value of a1 still remains the same.

Type, a1 plus 2

It returns a new array, with all the elements summed with two.

But again notice that the value of a1 has not been changed.

This will change the array a1 itself as we are assigning the new output to a1.

Next, we will see how to add two arrays.

Type, a1 is equal to np dot array inside parentheses inside square brackets 1, 2, 3, 4

This returns an array by adding element by element.

It returns an array with element by element multiplication.

Summary slide

In this tutorial, we have learnt to,

1. Create an array using the array() function.
2. Perform some basic operations on arrays like addition and multiplication.
3. Use methods like -

shapearange, reshapeidentity andzeros

Self assessment questions slide

x is equal to np.array inside parentheses inside square brackets 1, 2, 3, inside square brackets 5, 6, 7 is a valid statement?

• True
• False

Solution of self assessment questions on slide

The correct way is to assign the elements as a list of lists and then convert it to an array.

That is x is equal to np.array inside parentheses inside square brackets again inside square brackets 1, 2, 3, inside square brackets 5, 6, 7

Acknowledgment

For more details, visit this website.