In this tutorial, we will look at how to count zeros in a numpy array. We will also look at how to count zeros present in each row and each column of a 2d array.

## How to count zeros in a numpy array?

You can use `np.count_nonzero()`

or the `np.where()`

functions to count zeros in a numpy array. In fact, you can use these functions to count values satisfying any given condition (for example, whether they are zero or not, or whether they are greater than some value or not, etc).

Note that using `np.count_nonzero()`

is simpler of the two methods. The following is the syntax to count zeros using this function –

# arr is a numpy array # count of zeros in arr n_zeros = np.count_nonzero(arr==0)

Let’s look at some examples of how to use the above functions. First, we will create a couple of numpy arrays that we will be using throughout this tutorial.

import numpy as np # one-dimensional array arr_1d = np.array([3, 0, 5, 2, 1, 0, 8, 6]) print(arr_1d) # two-dimensional array arr_2d = np.array([[4, 3, 0], [0, 0, 2], [2, 5, 6]]) print(arr_2d)

Output:

[3 0 5 2 1 0 8 6] [[4 3 0] [0 0 2] [2 5 6]]

Now we have a one-dimensional array and a two-dimensional array for which we will be counting the zeros.

## Count all zeros in the array

To count all the zeros in an array, simply use the `np.count_nonzero()`

function checking for zeros. It returns the count of elements inside the array satisfying the condition (in this case, if it’s zero or not).

Let’s use this function to count the zeros in `arr_1d`

created above:

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# count zeros in 1d array n_zeros = np.count_nonzero(arr_1d==0) # display the count of zeros print(n_zeros)

Output:

2

We get 2 as the output since there are two zero elements in the 1d array `arr_1d`

.

You can also use the same syntax to count zeros in higher dimensional arrays. Let’s count the number of zeros in `arr_2d`

using `np.count_nonzero()`

# count zeros in 2d array n_zeros = np.count_nonzero(arr_2d==0) # display the count of zeros print(n_zeros)

Output:

3

We get 3 as the output since there are three zero value elements in the array `arr_2d`

.

## Count of zeros in each row

To count zeros in each row, pass `axis=1`

to the `np.count_nonzero()`

function. Let’s count zeros in each row of `arr_2d`

# count zeros in each row n_zeros = np.count_nonzero(arr_2d==0, axis=1) # display the count of zeros print(n_zeros)

Output:

[1 2 0]

It returns a numpy array of the count of zeros in each row. You can see that we have one zero-element in the first row, two in the second row, and no such elements in the third row.

## Count of zeros in each column

To count zeros in each column, pass `axis=0`

to the `np.count_nonzero()`

function. Let’s count the zeros in each column of `arr_2d`

# count zeros in each column n_zeros = np.count_nonzero(arr_2d==0, axis=0) # display the count of zeros print(n_zeros)

Output:

[1 1 1]

We have one zero-element in each column of the array `arr_2d`

.

For more on the `np.count_nonzero()`

function, refer to its documentation.

## Using `np.where()`

to count zeros in an array

Alternatively, you can use `np.where()`

to count the zeros in an array. `np.where()`

is generally used to find indexes of elements satisfying a condition in a numpy array.

You can use this function to find indexes of zero-valued elements in the array and then count them to get the count of zeros in the array. Let’s count the zeros in the array `arr_1d`

using this method:

# count zeros with np.where result = np.where(arr_1d==0) # show the result of np.where print(result) # count of zeros n_zeros = result[0].size # display the count of zeros print(n_zeros)

Output:

(array([1, 5], dtype=int64),) 2

You can see that `np.where()`

results in a tuple of numpy arrays showing the indexes satisfying the condition. We see that zeros are present at index 1 and 5 in the array `arr_1`

. To get the count, we use the `.size`

attribute of this index array.

You can also use `np.where()`

to count zeros in higher-dimensional arrays as well. For example, let’s use it to count zeros in `arr_2d`

# count zeros with np.where result = np.where(arr_2==0) # show the result of np.where print(result) # count of zeros n_zeros = result[0].size # display the count of zeros print(n_zeros)

Output:

(array([0, 1, 1], dtype=int64), array([2, 0, 1], dtype=int64)) 3

The returned value from `np.where()`

is a tuple of two arrays, the first one shows the row indexes of elements matching the condition (element equal to zero) and the second array gives the column indexes for those elements. Counting the indexes in any of these arrays gives the count of zeros in the array.

With this, we come to the end of this tutorial. The code examples and results presented in this tutorial have been implemented in a Jupyter Notebook with a python (version 3.8.3) kernel having numpy version 1.18.5

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- Trim zeros from a numpy array in Python
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