Array slicing is similar to list slicing in Python. Array indexing also begins from 0. However, since arrays can be multidimensional, we have to specify the slice for each dimension.
As we are mainly working with 2 dimensional arrays in this guide, we need to specify the row and column like what we do in a matrix. We can select a single element from an array or even a subarray containing multiple rows and columns of the original array.
To do array slicing, we use square brackets
Syntax
nameofarray[start_row:end_row, start_col:end_col]
Example - This retrieves the element in row 1, column 2
myArray[0:1, 1:2]
Note: If the start is not specified, it will be set to 0. If the end is not specified, it will run the length of the array in the dimension.
Let's create a simple array with shape (3,3) to work with. With a (3,3) shaped array, we know that its rows and columns can be referred to using their indices: 0, 1 and 2.
To retrieve all the elements from the first row, specify the row parameter as 0, and leave the column parameter empty.
Syntax
myArray[0,:]
You can also retrieve elements in a different row by replacing 0 with the proper index as long as they are within range.
In this example, we are retrieving elements from the 2nd column. Thus we would need to specify the column parameter and leave the row parameter empty.
Syntax
myArray[:,1]
Suppose we want to retrieve a subarray consisting of the first 2 rows and all 3 columns. We can specify the parameters as below.
Syntax
myArray[:2,:]
Slicing can also be used to reverse the order of elements in arrays.
Let's first create an array to work with using np.arange and then use the syntax below.
Syntax
myArray[::-1]
Take note that slicing only allows you to view elements while still being linked to the original array.
This means that if we edit myArray_new then myArray will change as well.
1. Use this array for the following practice:
myArray = np.array([[11,12,13], [14,15,16], [17,18,19]])
a. Get a subarray of the first row and first 2 columns. Your results should look like this:
[11 12]
b. Change all elements in 1st and second row to 0. Your results should look like this:
[[ 0 0 0] [ 0 0 0] [17 18 19]]
[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] and reverse the order.
1a. Get a subarray of the first row and first 2 columns. Your results should look like this:
1b. change all elements in 1st and second row to 0
2. Create an array that contains [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] and reverse the order.
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