Check If Two Numpy Arrays are Equal

The Numpy library in Python comes with a number of useful built-in functions to work with arrays. In this tutorial, we will look at how to check if two Numpy arrays are equal or not with the help of some examples.

How to check for equality of two Numpy arrays?

We say two Numpy arrays are equal if the corresponding elements in both arrays are equal.

You can use the Numpy built-in array_equal() function to check whether two arrays are equal or not. The following is the syntax –

import numpy as np
# compare numpy arrays a1 and a2 for equality
np.array_equal(a1, a2) 

It returns True if both arrays are equal and False if the arrays are not equal.

There are other methods are well that you can use to check for equality of two Numpy arrays (see the examples below).

Examples

Let’s now look at examples of some of the methods that we can use to check the equality of two Numpy arrays.

First, let’s create some Numpy arrays that we will be using throughout this tutorial.

import numpy as np

# create numpy arrays
a = np.array([1, 2, 3])
b = np.array([1, 2, 3])
c = np.array([1, 2, 4])

# display the arrays
print(f"a = {a}")
print(f"b = {b}")
print(f"c = {c}")

Output:

a = [1 2 3]
b = [1 2 3]
c = [1 2 4]

Here, we used the np.array() function to create three Numpy arrays – a, b, and c. You can see that arrays a and b are equal.

Example 1 – Using Numpy array_equal() function

To compare two arrays for equality using the Numpy array_equal() function, pass both the arrays as arguments to the function.

Let’s check if the arrays a and b are equal or not.

# check if arrays a and b are equal
print(np.array_equal(a, b))

Output:

True

We get True as the output which indicates that both arrays are equal.

Let’s check if arrays a and c are equal.

# check if arrays a and c are equal
print(np.array_equal(a, c))

Output:

False

We get False as the output since the arrays a and c are not equal.

For more on the Numpy array_equal() function, refer to its documentation.

Example 2 – Using the == operator and the all() function

You can also use a combination of the == operator and the Numpy all() to check if the two arrays are equal or not.

First, let’s see what we get if we only use the equality operator, == to compare two Numpy arrays.

# compare arrays a and b with ==
print(a==b)

Output:

[ True  True  True]

We get a boolean array with each value representing whether the corresponding elements at that index are equal or not. Now, if all the values in this boolean array are True we can say that both arrays are equal.

You can use the Numpy array all() function to check whether all the values in a given array are True or not. Let’s apply() the all() function on the result of the == operator.

# check if arrays a and b are equal
print((a==b).all())

Output:

True

We get True as the output. This means that the Numpy arrays a and b are equal.

Let’s now use this method to check whether arrays a and c are equal or not.

# check if arrays a and c are equal
print((a==c).all())

Output:

False

We get False as the output indicating the arrays a and c are not equal.

Note that using this method to compare two arrays of different lengths for equality will result in an error.

# create numpy arrays of different lengths
a = np.array([1, 2, 3])
b = np.array([1, 2])
# check if arrays a and b are equal
print((a==b).all())

Output:

/var/folders/s7/bqr5_87n7vs5c76jfk340cqw0000gn/T/ipykernel_82233/3831394073.py:5: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
  print((a==b).all())
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Input In [38], in <module>
      3 b = np.array([1, 2])
      4 # check if arrays a and b are equal
----> 5 print((a==b).all())

AttributeError: 'bool' object has no attribute 'all'

Here, we’re comparing two arrays of different lengths using the == operator which gives us a warning that “elementwise comparison failed” and results in False. Then we’re applying the Numpy all() function (which only works on Numpy arrays) to this boolean value which results in the above error.

Summary – Compare two Numpy arrays for equality

In this tutorial, we looked at how to check if two Numpy arrays are equal or not. The following are the key takeaways from this tutorial.

• Two Numpy arrays are considered equal if their corresponding elements are equal.
• Use the Numpy array_equal() function to check if two Numpy arrays are equal or not.
• Alternatively, you can also use a combination of the == operator and the all() function to compare two arrays from equality. Note that this method gives an error when comparing arrays of different lengths.

You might also be interested in –

• Different ways to Create NumPy Arrays
• Numpy – Remove Duplicates From Array
• Numpy – Print Array With Commas