Numpy arrays are very versatile when it comes to manipulating and extracting values. In this tutorial, we will look at how to get the last column of a two-dimensional Numpy array with the help of some examples.
Steps to get the last column of a Numpy Array
Let’s look at a step-by-step example of how to extract the last column from a two-dimensional Numpy array.
Step 1 – Create a 2D Numpy array
First, we will create a 2D Numpy array that we will use throughout this tutorial.
import numpy as np
# create 2D Numpy array
ar = np.array([
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
])
# display the array
print(ar)
Output:
[[1 2 3] [4 5 6] [7 8 9]]
Here, we used the numpy.array() function to create a two-dimensional Numpy array having three rows and three columns.
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You can see that the last column of the above array contains the values 3, 6, and 9.
Step 2 – Slice the array to get the last column
You can use slicing to extract the last column of a Numpy array. The idea is to slice the original array for all the rows and just the last column. Using a negative index can be useful here (the column index of the last column is -1).
For example, to get the last column of the array ar use the syntax ar[:, -1].
Let’s get the last column of the array created above.
# get the last column ar[:, -1]
Output:
array([3, 6, 9])
We get a Numpy array of the values in the last column of our original array, ar. Note that the resulting array, here, is one-dimensional.
ar[:, -1].shape
Output:
(3,)
If you instead want the last column as a Numpy array with shape (3, 1) (like a column vector) use the following syntax –
# get the last column ar[:, [-1]]
Output:
array([[3],
[6],
[9]])
We get the last column from the original array as a separate column (sort of like a column vector). Let’s see its shape.
ar[:, [-1]].shape
Output:
(3, 1)
You can see that the resulting array is of shape (3, 1)
Summary
In this tutorial, we looked at how to extract the last column of a two-dimensional Numpy array. The following is a short summary of the steps mentioned in this tutorial.
- Create a 2D Numpy array (skip this step if you already have an array to operate on).
- Use slicing to get the last column of the array –
- If you want the values in the last column as a simple one-dimensional array, use the syntax
ar[:, -1]. - If you want the resulting values in a column vector (for example, with shape
(n, 1)where n is the total number of rows), use the syntaxar[: [-1]].
- If you want the values in the last column as a simple one-dimensional array, use the syntax
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