Skip to content

Latest commit

 

History

History
237 lines (187 loc) · 5.02 KB

索引和切片.md

File metadata and controls

237 lines (187 loc) · 5.02 KB

基本索引

arr = np.arange(10)
arr[5]
>>>5

arr[5:8]
>>>array([5, 6, 7])

arr[5:8] = 12
arr
>>>array([ 0,  1,  2,  3,  4, 12, 12, 12,  8,  9])
arr_slice = arr[5:8]
arr_slice[1]=12345
arr
>>>array([    0,     1,     2,     3,     4,    12, 12345,    12,     8,
           9])
arr_slice[:] = 64
arr
>>>array([ 0,  1,  2,  3,  4, 64, 64, 64,  8,  9])
arr2d = np.array([[1,2,3],[4,5,6],[7,8,9]])
arr2d[2]
>>>array([7, 8, 9])

arr2d[0][2]
>>>3

arr2d[0,2]
>>>3

arr3d = np.array([[[1,2,3],[4,5,6]],[[7,8,9],[10,11,12]]])
arr3d
>>> array([[[ 1,  2,  3],
        [ 4,  5,  6]],

       [[ 7,  8,  9],
        [10, 11, 12]]])
arr3d[0]
>>>array([[1, 2, 3],
       [4, 5, 6]])
old_values = arr3d[0].copy()
arr3d[0] = 42
arr3d
>>>array([[[42, 42, 42],
        [42, 42, 42]],

       [[ 7,  8,  9],
        [10, 11, 12]]])

arr3d[0] = old_values
arr3d
>>>array([[[ 1,  2,  3],
        [ 4,  5,  6]],

       [[ 7,  8,  9],
        [10, 11, 12]]])

arr3d[1,0]
>>>array([7, 8, 9])

数字切片索引

arr[1:6]
>>>array([7, 8, 9])

arr2d
>>>array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])

arr2d[:2]
>>>array([[1, 2, 3],
       [4, 5, 6]])

arr2d[:2,1:]
>>>array([[2, 3],
       [5, 6]])

arr2d[1,:2]
>>>array([4, 5])

arr2d[:,:1]
>>>array([[1],
       [4],
       [7]])

布尔型索引

names = np.array(['Bob','Joe','Will','Bob','Will','Joe','Joe'])
data = np.random.randn(7,4)
names
>>>array(['Bob', 'Joe', 'Will', 'Bob', 'Will', 'Joe', 'Joe'], dtype='<U4')

data
>>>array([[-0.54093662,  0.98533044,  0.82009084,  1.20798381],
       [ 1.82829901,  0.17411663, -1.51303825,  0.22804367],
       [-0.65078152,  0.08313449,  0.83880963, -1.78276309],
       [ 0.61207065,  1.15364584, -0.12602078, -1.09535793],
       [ 0.45050638,  1.58353401, -0.46624268, -0.25089213],
       [-0.14677405,  0.26151823,  1.13177225, -1.47578361],
       [-0.80152908,  0.49052361,  0.22747227, -0.7811803 ]])

names == 'Bob'
>>>array([ True, False, False,  True, False, False, False])

data[names == 'Bob']
>>>array([[-0.54093662,  0.98533044,  0.82009084,  1.20798381],
       [ 0.61207065,  1.15364584, -0.12602078, -1.09535793]])

data[names == 'Bob',2:]
>>>array([[ 0.82009084,  1.20798381],
       [-0.12602078, -1.09535793]])
names != 'Bob'
>>>array([False,  True,  True, False,  True,  True,  True])

data[~(names=='Bob')]
>>>array([[ 1.82829901,  0.17411663, -1.51303825,  0.22804367],
       [-0.65078152,  0.08313449,  0.83880963, -1.78276309],
       [ 0.45050638,  1.58353401, -0.46624268, -0.25089213],
       [-0.14677405,  0.26151823,  1.13177225, -1.47578361],
       [-0.80152908,  0.49052361,  0.22747227, -0.7811803 ]])

mask = (names == 'Bob') | (names == 'Will')
mask
>>>array([ True, False,  True,  True,  True, False, False])

data[mask]
array([[-0.54093662,  0.98533044,  0.82009084,  1.20798381],
       [-0.65078152,  0.08313449,  0.83880963, -1.78276309],
       [ 0.61207065,  1.15364584, -0.12602078, -1.09535793],
       [ 0.45050638,  1.58353401, -0.46624268, -0.25089213]])

data[data<0] = 0
data
>>>array([[0.        , 0.98533044, 0.82009084, 1.20798381],
       [1.82829901, 0.17411663, 0.        , 0.22804367],
       [0.        , 0.08313449, 0.83880963, 0.        ],
       [0.61207065, 1.15364584, 0.        , 0.        ],
       [0.45050638, 1.58353401, 0.        , 0.        ],
       [0.        , 0.26151823, 1.13177225, 0.        ],
       [0.        , 0.49052361, 0.22747227, 0.        ]])

data[names != 'Joe'] = 7
data
>>>array([[7.        , 7.        , 7.        , 7.        ],
       [1.82829901, 0.17411663, 0.        , 0.22804367],
       [7.        , 7.        , 7.        , 7.        ],
       [7.        , 7.        , 7.        , 7.        ],
       [7.        , 7.        , 7.        , 7.        ],
       [0.        , 0.26151823, 1.13177225, 0.        ],
       [0.        , 0.49052361, 0.22747227, 0.        ]])

花式索引

import numpy as np

arr = np.empty((8,4))
for i in range(8):
    arr[i] = i
arr
>>>array([[0., 0., 0., 0.],
       [1., 1., 1., 1.],
       [2., 2., 2., 2.],
       [3., 3., 3., 3.],
       [4., 4., 4., 4.],
       [5., 5., 5., 5.],
       [6., 6., 6., 6.],
       [7., 7., 7., 7.]])

arr[[4,3,0,6]]
>>>array([[4., 4., 4., 4.],
       [3., 3., 3., 3.],
       [0., 0., 0., 0.],
       [6., 6., 6., 6.]])

arr[[-3,-5,-7]]
>>>array([[5., 5., 5., 5.],
       [3., 3., 3., 3.],
       [1., 1., 1., 1.]])
arr = np.arange(32).reshape((8,4))
arr
>>>array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15],
       [16, 17, 18, 19],
       [20, 21, 22, 23],
       [24, 25, 26, 27],
       [28, 29, 30, 31]])

arr[[1,5,7,2],[0,3,1,2]]
>>>array([ 4, 23, 29, 10])

arr[[1,5,7,2],[0,3,1,2]]
>>>array([ 4, 23, 29, 10])

arr[[1,5,7,2]][:,[0,3,1,2]]
>>>array([[ 4,  7,  5,  6],
       [20, 23, 21, 22],
       [28, 31, 29, 30],
       [ 8, 11,  9, 10]])

arr[np.ix_([1,5,7,2],[0,3,1,2])]
>>>array([[ 4,  7,  5,  6],
       [20, 23, 21, 22],
       [28, 31, 29, 30],
       [ 8, 11,  9, 10]])