Home Backend Development Python Tutorial Detailed explanation of the usage of where() function in Python

Detailed explanation of the usage of where() function in Python

Mar 29, 2018 pm 01:09 PM
python where Detailed explanation

This article mainly introduces you to the detailed usage of the where() function in Python. The editor thinks it is quite good. Now I will share it with you and give you a reference. I hope it can help you.

Usage of where()

First of all, let me emphasize that the where() function only returns different values ​​for different inputs.

1 When the array is a one-dimensional array, the returned value is a one-dimensional index, so there is only one set of index arrays

2 When the array is a two-dimensional array, the array value that meets the conditions What is returned is the position index of the value, so there will be two sets of index arrays to represent the position of the value

For example,


>>>b=np.arange(10)
>>>b
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
>>>np.where(b>5)
 (array([6, 7, 8, 9], dtype=int64),)

>>>a=np.reshape(np.arange(20),(4,5))
>>>a 
array([[ 0, 1, 2, 3, 4],
    [ 5, 6, 7, 8, 9],
    [10, 11, 12, 13, 14],
    [15, 16, 17, 18, 19]])
>>>np.where(a>10)
(array([2, 2, 2, 2, 3, 3, 3, 3, 3], dtype=int64),
 array([1, 2, 3, 4, 0, 1, 2, 3, 4], dtype=int64))
Copy after login

for the numpy standard library An introduction to the explanation here:


numpy.where(condition[, x, y])
Copy after login

Based on the condition, the return value comes from x or y.

If.

Parameters:

condition: array, bool value

When True, yield x, otherwise yield y.

x, y: array_like, optional

The shapes of x and y must be the same. When the value in condition is true, the value corresponding to x is returned. If false, the value of y is returned.

Return value:

out : ndarray or tuple of ndarrays

①If the parameters include condition, x and y, the shapes of their three parameters are the same. Then, when the value in condition is true, the value corresponding to x is returned, and if false, y is returned.

② If the parameter is only condition, the return value is the position index where the element value in condition is true. It is returned in the form of a tuple. The element of the tuple is an ndarray array, indicating the index of the position.


>>> np.where([[True, False], [True, True]],
...     [[1, 2], [3, 4]],
...     [[9, 8], [7, 6]])
array([[1, 8],
    [3, 4]])
>>>
>>> np.where([[0, 1], [1, 0]])
(array([0, 1]), array([1, 0]))
>>>
>>> x = np.arange(9.).reshape(3, 3)
>>> np.where( x > 5 )
(array([2, 2, 2]), array([0, 1, 2]))
>>> x[np.where( x > 3.0 )]        # Note: result is 1D.
array([ 4., 5., 6., 7., 8.])
>>> np.where(x < 5, x, -1)        # Note: broadcasting.
array([[ 0., 1., 2.],
    [ 3., 4., -1.],
    [-1., -1., -1.]])
Find the indices of elements of x that are in goodvalues.

>>>
>>> goodvalues = [3, 4, 7]
>>> ix = np.in1d(x.ravel(), goodvalues).reshape(x.shape)
>>> ix
array([[False, False, False],
    [ True, True, False],
    [False, True, False]], dtype=bool)
>>> np.where(ix)
(array([1, 1, 2]), array([0, 1, 1]))
Copy after login

Sample code for two methods

First usage

np.where(conditions,x,y)

if (conditions are established):

Array becomes x

else:

Array Change y


import numpy as np
&#39;&#39;&#39;
x = np.random.randn(4,4)
print(np.where(x>0,2,-2))
#试试效果
xarr = np.array([1.1,1.2,1.3,1.4,1.5])
yarr = np.array([2.1,2.2,2.3,2.4,2.5])
zarr = np.array([True,False,True,True,False])
result = [(x if c else y)
     for x,y,c in zip(xarr,yarr,zarr)]
print(result)

#where()函数处理就相当于上面那种方案

result = np.where(zarr,xarr,yarr)
print(result)

&#39;&#39;&#39;
#发现个有趣的东西
# #处理2组数组
# #True and True = 0
# #True and False = 1
# #False and True = 2
# #False and False = 3

cond2 = np.array([True,False,True,False])
cond1 = np.array([True,True,False,False])
#第一种处理 太长太丑
result = []
for i in range(4):
  if (cond1[i] & cond2[i]):  result.append(0);
  elif (cond1[i]):  result.append(1);
  elif (cond2[i]):  result.append(2);
  else : result.append(3);
print(result)
#第二种 直接where() 很快很方便
result = np.where(cond1 & cond2,0,np.where(cond1,1,np.where(cond2,2,3)))
print(result)
#第三种 更简便(好像这跟where()函数半毛钱的关系都没有
result = 1*(cond1 & -cond2)+2*(cond2 & -cond1)+3*(-(cond1 | cond2)) (没想到还可以这么表达吧)
print(result)
Copy after login

The second usage

where(conditions)

is equivalent to giving the subscript of the array


x = np.arange(16)
print(x[np.where(x>5)])
#输出:(array([ 6, 7, 8, 9, 10, 11, 12, 13, 14, 15], dtype=int64),)

x = np.arange(16).reshape(-1,4)
print(np.where(x>5))

#(array([1, 1, 2, 2, 2, 2, 3, 3, 3, 3], dtype=int64), array([2, 3, 0, 1, 2, 3, 0, 1, 2, 3], dtype=int64))
#注意这里是坐标是前面的一维的坐标,后面是二维的坐标
Copy after login


ix = np.array([[False, False, False],
    [ True, True, False],
    [False, True, False]], dtype=bool)
print(np.where(ix))
#输出:(array([1, 1, 2], dtype=int64), array([0, 1, 1], dtype=int64))
Copy after login

The above is the detailed content of Detailed explanation of the usage of where() function in Python. For more information, please follow other related articles on the PHP Chinese website!

Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

AI Hentai Generator

AI Hentai Generator

Generate AI Hentai for free.

Hot Article

R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)
2 weeks ago By 尊渡假赌尊渡假赌尊渡假赌
Hello Kitty Island Adventure: How To Get Giant Seeds
1 months ago By 尊渡假赌尊渡假赌尊渡假赌
Two Point Museum: All Exhibits And Where To Find Them
1 months ago By 尊渡假赌尊渡假赌尊渡假赌

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

Is there any mobile app that can convert XML into PDF? Is there any mobile app that can convert XML into PDF? Apr 02, 2025 pm 08:54 PM

An application that converts XML directly to PDF cannot be found because they are two fundamentally different formats. XML is used to store data, while PDF is used to display documents. To complete the transformation, you can use programming languages ​​and libraries such as Python and ReportLab to parse XML data and generate PDF documents.

How to control the size of XML converted to images? How to control the size of XML converted to images? Apr 02, 2025 pm 07:24 PM

To generate images through XML, you need to use graph libraries (such as Pillow and JFreeChart) as bridges to generate images based on metadata (size, color) in XML. The key to controlling the size of the image is to adjust the values ​​of the &lt;width&gt; and &lt;height&gt; tags in XML. However, in practical applications, the complexity of XML structure, the fineness of graph drawing, the speed of image generation and memory consumption, and the selection of image formats all have an impact on the generated image size. Therefore, it is necessary to have a deep understanding of XML structure, proficient in the graphics library, and consider factors such as optimization algorithms and image format selection.

Is the conversion speed fast when converting XML to PDF on mobile phone? Is the conversion speed fast when converting XML to PDF on mobile phone? Apr 02, 2025 pm 10:09 PM

The speed of mobile XML to PDF depends on the following factors: the complexity of XML structure. Mobile hardware configuration conversion method (library, algorithm) code quality optimization methods (select efficient libraries, optimize algorithms, cache data, and utilize multi-threading). Overall, there is no absolute answer and it needs to be optimized according to the specific situation.

How to convert XML files to PDF on your phone? How to convert XML files to PDF on your phone? Apr 02, 2025 pm 10:12 PM

It is impossible to complete XML to PDF conversion directly on your phone with a single application. It is necessary to use cloud services, which can be achieved through two steps: 1. Convert XML to PDF in the cloud, 2. Access or download the converted PDF file on the mobile phone.

How to open xml format How to open xml format Apr 02, 2025 pm 09:00 PM

Use most text editors to open XML files; if you need a more intuitive tree display, you can use an XML editor, such as Oxygen XML Editor or XMLSpy; if you process XML data in a program, you need to use a programming language (such as Python) and XML libraries (such as xml.etree.ElementTree) to parse.

Recommended XML formatting tool Recommended XML formatting tool Apr 02, 2025 pm 09:03 PM

XML formatting tools can type code according to rules to improve readability and understanding. When selecting a tool, pay attention to customization capabilities, handling of special circumstances, performance and ease of use. Commonly used tool types include online tools, IDE plug-ins, and command-line tools.

Is there a mobile app that can convert XML into PDF? Is there a mobile app that can convert XML into PDF? Apr 02, 2025 pm 09:45 PM

There is no APP that can convert all XML files into PDFs because the XML structure is flexible and diverse. The core of XML to PDF is to convert the data structure into a page layout, which requires parsing XML and generating PDF. Common methods include parsing XML using Python libraries such as ElementTree and generating PDFs using ReportLab library. For complex XML, it may be necessary to use XSLT transformation structures. When optimizing performance, consider using multithreaded or multiprocesses and select the appropriate library.

What is the function of C language sum? What is the function of C language sum? Apr 03, 2025 pm 02:21 PM

There is no built-in sum function in C language, so it needs to be written by yourself. Sum can be achieved by traversing the array and accumulating elements: Loop version: Sum is calculated using for loop and array length. Pointer version: Use pointers to point to array elements, and efficient summing is achieved through self-increment pointers. Dynamically allocate array version: Dynamically allocate arrays and manage memory yourself, ensuring that allocated memory is freed to prevent memory leaks.

See all articles