{"id":4288,"date":"2022-08-09T03:05:48","date_gmt":"2022-08-09T03:05:48","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=4288"},"modified":"2022-08-09T03:06:16","modified_gmt":"2022-08-09T03:06:16","slug":"boolean-indexing","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-numpy\/boolean-indexing\/","title":{"rendered":"Boolean Indexing"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to access elements of a numpy array using boolean indexing.<\/p>\n\n\n\n

Introduction to numpy array boolean indexing #<\/a><\/h2>\n\n\n\n

Numpy allows you to use an array of boolean<\/a> values as an index of another array. Each element of the boolean array indicates whether or not to select the elements from the array.<\/p>\n\n\n\n

If the value is True<\/code>, the element of that index is selected. In case the value is False<\/code>, the element of that index is not selected. <\/p>\n\n\n\n

The following example uses boolean indexing to select elements of a numpy array using an array of boolean values:<\/p>\n\n\n

import<\/span> numpy as<\/span> np\n\na = np.array([1<\/span>, 2<\/span>, 3<\/span>])\nb = np.array([True<\/span>, True<\/span>, False<\/span>])\nc = a[b]\nprint(c)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[1<\/span> 2<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
\"Boolean<\/figure>\n\n\n\n
First, create a new numpy array<\/a> that includes three numbers from 1 to 3:<\/p>\n\n\n
a = np.array([1<\/span>, 2<\/span>, 3<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create another numpy array with three boolean values True<\/code>, True<\/code>, and False<\/code>:<\/p>\n\n\n
b = np.array([True<\/span>, True<\/span>, False<\/span>])<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, use the boolean array b<\/code> as the index of the array a<\/code> and assign the selected elements to the variable c<\/code>:<\/p>\n\n\n
c = a[b]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Because the first and second elements of the array b<\/code> are True<\/code>, the a[b]<\/code> returns a new array with the first and second elements of the array a<\/code>.<\/p>\n\n\n\n
Typically, you’ll use boolean indexing to filter an array. For example:<\/p>\n\n\n
import<\/span> numpy as<\/span> np\n\na = np.arange(1<\/span>, 10<\/span>)\nb = a > 5<\/span>\nprint(b)\n\nc = a[b]\nprint(c)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[False<\/span> False<\/span> False<\/span> False<\/span> False<\/span>  True<\/span>  True<\/span>  True<\/span>  True<\/span>]\n[6<\/span> 7<\/span> 8<\/span> 9<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
a = np.arange(1<\/span>, 10<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create a boolean array from the following expression:<\/p>\n\n\n
b = a > 5<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
This expression compares each element of the array a<\/code> with 5<\/code> and returns True<\/code> if it is greater than 5<\/code> or False<\/code> otherwise. The variable b<\/code> is an array of boolean values:<\/p>\n\n\n
[False<\/span> False<\/span> False<\/span> False<\/span> False<\/span>  True<\/span>  True<\/span>  True<\/span>  True<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, use the array b<\/code> as the index of array a<\/code> and assign the result to the variable c<\/code>:<\/p>\n\n\n
c = a[b]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The array c<\/code> contains only numbers from array a<\/code>, which are greater than 5<\/code>.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
  • Use boolean indexing to filter an array.<\/li><\/ul>\n
    \n\t
    \n\t\t
    Was this tutorial helpful ?<\/div>\n\t\t
    \n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t Yes <\/span>\n\t\t\t<\/button>\n\n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t
    \n\t\t
    \n\t\t\t
    <\/div>\n\t\t\t
First, create an array that has 9 numbers from 1 to 9 using the arange()<\/code><\/a><\/code> function:<\/p>\n\n\n