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Count Values Less than a Given Value in Numpy Array

In this tutorial, we will look at how to count the values in a Numpy array that are less than a given value (let’s say k) with the help of some examples.

Steps to get the count of all the values less than k in Numpy

count of values less than a given value in numpy array

In general, to find the count of values in a Numpy array that satisfy the given condition, you can –

  1. Use boolean indexing to filter the array for only the values that satisfy the condition.
  2. Calculate the length of the filtered array from step 1.

Thus, first, filter the Numpy array to contain only the values that are less than the given value and then find its length to get the required count.

Let’s now look at a step-by-step example.

Step 1 – Create a Numpy array

First, we will create a Numpy array that we will be using throughout this tutorial.

import numpy as np

# create a numpy array
ar = np.array([1, 2, 3, 4, 5, 6, 7])
# display the array
print(ar)

Output:

[1 2 3 4 5 6 7]

Here, we used the numpy.array() function to create a one-dimensional Numpy array containing some numbers.

Step 2 – Filter the array using a boolean expression

To get all the values from a Numpy array less than a given value, filter the array using boolean indexing.

First, we will specify our boolean expression, ar < k and then use the boolean array resulting from this expression to filter our original array.

For example, let’s get all the values in the above array that are less than 4 (k = 4).

# values in array less than 4
ar_filtered = ar[ar < 4]
print(ar_filtered)

Output:

[1 2 3]

We get all the values in the array ar that are less than 4.

Step 3 – Get the length of the filtered array

To get the count of values that satisfy the given condition (whether it’s less than k or not) find the length of the resulting filtered array from step 2 using the Python built-in len() function.

# length of the filtered array
print(len(ar_filtered))

Output:

3

We get the count of values in the array ar that are less than the given value (4).

We can combine the code from the last two steps into a single line of code.

# count of values in array less than 4
print(len(ar[ar < 4]))

Output:

3

We get the same result as above and we removed the extra variable ar_filtered.

In this tutorial, we looked at how to count values in a Numpy array that are less than a given value k. Note that in this method we’re not counting unique elements that are less than k, rather we’re counting all values in an array that are less than k (which may include duplicates depending on the array).

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Author

  • Piyush

    Piyush is a data scientist passionate about using data to understand things better and make informed decisions. In the past, he's worked as a Data Scientist for ZS and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects.