29 Mar Array Indexing
Array indexing is accessing array elements. In Numpy, access an array element using the index number. The 0th index is element 1 and the flow goes on as shown below:
- index 0 – element 1
- index 1 – element 2
- index 2 – element 3
- index 3 – element 4
In this lesson, we will cover the following topics to understand Array Indexing in NumPy:
- Access elements from a 1D Array
- Access elements from a 2D Array
- Access elements from a 3D Array
- Access elements from the last with Negative Indexing
Let us begin with accessing elements from a One-Dimensional i.e. 1D array:
Access elements from a One-Dimensional array
The following are some examples to access specific elements from a 1D array:
Example: Access the 1st element (index 0) from a One-Dimensional array
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import numpy as np n = np.array([10, 20, 30, 40, 50]) print(n[0]) |
Output
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1 2 3 |
10 |
Example: Access the 4th element (index 3) from a One-Dimensional array
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import numpy as np n = np.array([10, 20, 30, 40, 50]) print(n[3]) |
Output
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1 2 3 |
40 |
Access elements from a Two-Dimensional array
Accessing elements work as a matrix in a 2D Array i.e.
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a[0,0] = dimension 1 element 1st a[0,1] = dimension 1 element 2nd a[0,2] = dimension 1 element 3rd a[1,0] = dimension 2 element 1st a[1,1] = dimension 2 element 2nd a[1,2] = dimension 2 element 3rd a[2,0] = dimension 3 element 1st a[2,1] = dimension 3 element 2nd a[2,2] = dimension 3 element 3rd |
Following are some examples to access specific elements from a 2D array:
Example: Accessing 1st dimension elements from a 2D array
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import numpy as np n = np.array([[1,3,5],[4,8,12]]) print(n[0,0]) print(n[0,1]) print(n[0,2]) |
Output
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1 2 3 4 5 |
1 3 5 |
Example: Accessing 2nd dimension elements from a 2D array
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import numpy as np n = np.array([[1,3,5],[4,8,12]]) print(n[1,0]) print(n[1,1]) print(n[1,2]) |
Output
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1 2 3 4 5 |
4 8 12 |
Access elements from a Three-Dimensional Array
Following are some examples to access specific elements from a 3D array:
Example1
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import numpy as np n = np.array([[[5,10,15],[20,25,30]],[[35,40,45],[50,55,60]]]) print(n[0,0,0]) print(n[0,0,1]) print(n[0,0,2]) |
Output
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5 10 15 |
Example2
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import numpy as np n = np.array([[[5,10,15],[20,25,30]],[[35,40,45],[50,55,60]]]) print(n[1,0,0]) print(n[1,0,1]) print(n[1,0,2]) |
Output
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1 2 3 4 5 |
35 40 45 |
Access the array from the last with Negative Indexing
Arrays can be accessed with negative indexing. This gives the last element.
Example 1: Access the last element from a 1D array with negative indexing
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import numpy as np n = np.array([5, 10, 15]) print('Last element = ', n[-1]) |
Output
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Last element = 15 |
Example 2: Access the last element from a 2D array with negative indexing
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import numpy as np n = np.array([[1, 3, 5], [4, 8, 12]]) print('Last element = ', n[0, -1]) |
Output
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1 2 3 |
Last element = 5 |
Let us see another example:
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import numpy as np n = np.array([[1,3,5],[4,8,12]]) print('Last element = ', n[1,-1]) |
Output
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1 2 3 |
Last element = 12 |
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