Python and NumPy: List vs NumPy Arrays

Let’s look at basic numeric operations of Scalars, Vectors and Matrices using Python. In order to do this we need to use the Numeric Python NumPy:

Python

Scalars

In Python scalars can be created using numbers:

Python
3

They can also be made using square brackets:

Python
3

Addition of numbers can be carried out using:

Python
8

However if square brackets are enclosed around the scalars, instead of adding the sum of the two scalars, they are instead concatenated together into a single array:

Python
[3,5]

Vectors

This may be useful in some cases. However supposing we want to add two 1 row by 4 columns vectors together:

Python
[1, 2, 3, 4, 5, 6, 7, 8]

We find that once again we concatenate them instead of adding them together. For numeric operations, it is hence recommended to use a NumPy array opposed to a Python list:

Python
array([ 6,  8, 10, 12])
Python

These show up in the Variable Explorer, note the types are list and int32 respectively:

When a is examine din the variable explorer, each element in the list has its own Type (in this case all integers), each has a size, in this case each individual element has a size of 1 and these can be customised:

In contrast the NumPy array is totally numerical, and each cell can only contain one value.

So for instance, the following elements in the list can be updated:

Python

Trying the same for the NumPy array gives an error:

Python
Traceback (most recent call last):

  File "<ipython-input-5-f1938797665c>", line 1, in <module>
    b[0]='g'

ValueError: invalid literal for int() with base 10: 'g'
Python
Traceback (most recent call last):

  File "<ipython-input-6-316b8a7bcf3b>", line 1, in <module>
    b[1]=[1,2,3]

ValueError: setting an array element with a sequence.

In essence we are only allowed a scalar numeric value for each cell in the NumPy Array whereas a list can be made more complicated.

Matrix

Instead of a vector, we can create a matrix. It is again possible to do both as a list and as NumPy array:

Python

Note the matrix is input using the form:

Python

The inner square brackets surrounding Row0 corresponds to the single dimension of a Row vector, i.e. Row0 by itself is a Row Vector. The outside square brackets however correspond to the fact that, Row0, alongside Row1, together make up a Matrix.

In other words if on the outside there is 1 square bracket we have a 1D object and if there are two square brackets on the outside we have a 2D object.

Once again we can see the difference between the list and the NumPy Array.

3D Stack

To make a 3D array or stack of matrices we can use:

Python

Note this 3D Stack is of the form:

Python

Once again we can see the difference between the List and the NumPy Array:

The NumPy Array is listed as a Stack and has dimensions of 2 pages, 2 rows and 4 columns. By default axis 0 is selected an one may go through each slice (showing all elements of axis 0):

It is worth looking at the shape of the NumPy Arrays b, d and f using:

Python
np.shape(1)
Out[8]: ()

np.shape(b)
Out[8]: (4,)

np.shape(d)
Out[9]: (2, 4)

np.shape(f)
Out[10]: (2, 2, 4)

Here we see the Scalar gives no dimensions. This is because there is only 1 Row and 1 Column and thus there is no need to index any values in it as the entire scalar is just a single value. The Row Vector has 4 Columns. It also has 1 Row but because it is a single Row there is once again no need to index it. The Matrix has 2 Rows by 4 Columns and the Stack has 2 Pages, 2 Rows and 4 Columns.

This is perhaps seen more clearly with g, which has a different size in all the dimensions:

Python

This Stack has 2 Pages, 3 Rows and 4 Columns. The value 8 is on the 0th Page, 1st row and 3rd column. It can be indexed using:

Python
8

Or alternatively:

Python
8

Check your understanding: Index into g to yield the numbers 19, 17 and 2 respectively.

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