Python where How to use function

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One 、where Definition of function

where The function is numpy In the library , Usually you need to load first numpy library , Call this function again . There are two basic call syntax for functions , One is ：

import numpy as npnp.where(arry)

here ,np.where Output function arry in “ really ” Coordinates of values (‘ really ’ It can also be understood as non 0). Or say np.where Function from arry Returns elements that meet certain conditions . such as , It will return the index position of the value that meets the specific conditions .

The other is ：

import numpy as npnp.where(cond, x, y)

here ,np.where Function satisfies cond Conditional output x, Not satisfied with output y. In order to make everyone right where A clearer understanding of function definitions , Next, it will be explained with specific examples , It's convenient for you to understand and remember .

Two 、where Function instance

Find out the position index that is greater than a certain number in the sequence

y = np.array([1, 5, 6, 8, 1, 7, 3, 6, 9])print(np.where(y>5))

Get the results ：

(array([2, 3, 5, 7, 8], dtype=int64),)

here ,np.where Function returns a value greater than 5 Index position of .

Greater than... In the sequence 5 take ‘m_5’ Otherwise take ’lq_5’

y = np.array(range(1, 10))print(y)print(np.where(y>5, 'm_5', 'lq_5'))

Get the results ：

[1 2 3 4 5 6 7 8 9]
['lq_5' 'lq_5' 'lq_5' 'lq_5' 'lq_5' 'm_5' 'm_5' 'm_5' 'm_5']

y Is an initial value of 1, The final value is 9, In steps of 1 Equal difference sequence of . here ,np.where Function satisfies y>5 Output ’m_5’, Not satisfied with output ’lq_5’.

Less than in sequence 5 Take one value or take another value

print(np.arange(10))print(np.where(np.arange(10)<5, ' Eat an apple ', ' Eat durian '))

Get the results ：

[0 1 2 3 4 5 6 7 8 9]

[‘ Eat an apple ’ ‘ Eat an apple ’ ‘ Eat an apple ’ ‘ Eat an apple ’ ‘ Eat an apple ’ ‘ Eat durian ’ ‘ Eat durian ’ ‘ Eat durian ’ ‘ Eat durian ’ ‘ Eat durian ’]

here ,np.where Function satisfies np.arange(10) The median value is less than 5 Output ’ Eat an apple ’, Not satisfied with output ’ Eat durian ’.

Yes in the sequence 2 Take one value as the multiple of, otherwise take another value

y = np.array(range(1, 10))print(y)print(np.where(np.mod(y, 2)==0, '2b', 'n_2b'))

Get the results ：

[1 2 3 4 5 6 7 8 9]
['n_2b' '2b' 'n_2b' '2b' 'n_2b' '2b' 'n_2b' '2b' 'n_2b']

here ,np.where Function satisfies y Divide the mean value by 2 Remainder is 0 Output ’2b’, Not satisfied with output ’n_2b’.

Find out what is not in the data frame 0 data ‍

x = np.array([[0, 1, 2], [3, 0, 0], [6, 0, 8]])print(x[np.where(x)])

Get the results ：

[1 2 3 6 8]

here ,np.where The function takes out x Central Africa 0 Count , Generate a new sequence .

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