机器学习 番外篇 03 numpy之Fancy indexing

Fancy indexing

import numpy as np

x=np.arange(16)
x

>>>array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15])

x[3:9:2]

>>>array([3, 5, 7])

ind=[3,5,8]
x[ind] # 找出数组中特定的值

>>>array([3, 5, 8])

ind1=np.array([[0,2],[1,3]])
x[ind1]

>>>array([[0, 2],
       [1, 3]])

ind2=np.array([[1,2],[0,3]])
x[ind2]

>>>array([[1, 2],
       [0, 3]])

x[5]=6

x

>>>array([ 0,  1,  2,  3,  4,  6,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15])

x[::-1]

>>>array([15, 14, 13, 12, 11, 10,  9,  8,  7,  6,  6,  4,  3,  2,  1,  0])

ind3=np.array([[1,2],[0,3]])
x[ind3]

>>>array([[1, 2],
       [0, 3]])

x[::-1]
x.reshape(4,-1)
x

>>>array([ 0,  1,  2,  3,  4,  6,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15])

xx=x.reshape(4,-1)

xx

>>>array([[ 0,  1,  2,  3],
       [ 4,  6,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15]])

row=np.array([0,1,2])
col=np.array([1,2,3])
xx[row,col]

>>>array([ 1,  6, 11]) # xx[0,1],xx[1,2],xx[2,3]

xx[:2,col]

>>>array([[1, 2, 3],
       [6, 6, 7]])

col=[True,False,True,True]
xx[1:3,col]

>>>array([[ 4,  6,  7],
       [ 8, 10, 11]])

# numpy.array比较

x

>>>array([ 0,  1,  2,  3,  4,  6,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15])

x<3

>>>array([ True,  True,  True, False, False, False, False, False, False,
       False, False, False, False, False, False, False])

x>3

>>>array([False, False, False, False,  True,  True,  True,  True,  True,
        True,  True,  True,  True,  True,  True,  True])

2*4==12-x

>>>array([False, False, False, False,  True, False, False, False, False,
       False, False, False, False, False, False, False])

xx<6

>>>array([[ True,  True,  True,  True],
       [ True, False, False, False],
       [False, False, False, False],
       [False, False, False, False]])

x

>>>array([ 0,  1,  2,  3,  4,  6,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15])

x[5]=5
x

>>>array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15])

np.sum(x<=3) # 统计小于3的个数

>>>4

np.count_nonzero(x<=5)

>>>6

np.any(x==0)

>>>True

np.all(x>0)

>>>False

xx

>>>array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15]])

np.sum(xx%2==0)

>>>8

np.sum(xx%2==0,axis=1)

>>>array([2, 2, 2, 2])

np.sum(xx%2==0,axis=0)

>>>array([4, 0, 4, 0])

np.all(xx>0,axis=1)

>>>array([False,  True,  True,  True])

np.sum((x>3)&(x<10))

>>>6

np.sum(~(x==0))

>>>15

x[x<6]

>>>array([0, 1, 2, 3, 4, 5])

xx[xx[:,3]%3==0,:]

>>>array([[ 0,  1,  2,  3],
       [12, 13, 14, 15]])

xx[xx[:,3]%3==0]

>>>array([[ 0,  1,  2,  3],
       [12, 13, 14, 15]])