numpy.array 基本操作

import numpy as np 
np.random.seed(0)
x = np.arange(10)
x
"""
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
"""
X = np.arange(15).reshape((3, 5))
X
"""
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
"""

x.ndim      # 1
x.shape     # (10,)
x.size      # 10
X.ndim      # 2
X.shape     # (3, 5)
X.size      # 15

x 　　 　　 # array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
x[-1]  　　# 9
x[:5] 　   # array([0, 1, 2, 3, 4])
x[5:] 　　 # array([5, 6, 7, 8, 9])
x[::2]    # array([0, 2, 4, 6, 8])
x[1::2]   # array([1, 3, 5, 7, 9])
x[::-1]   # array([9, 8, 7, 6, 5, 4, 3, 2, 1, 0])

X
"""
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
"""
X[0,-1]  # 4
X[:2, :3] 
"""
array([[0, 1, 2],
       [5, 6, 7]])
"""
X[:2, ::2] 
"""
array([[0, 2, 4],
       [5, 7, 9]])
"""
X[::-1, ::-1] 
"""
array([[14, 13, 12, 11, 10],
       [ 9,  8,  7,  6,  5],
       [ 4,  3,  2,  1,  0]])
"""
X[0, :]   # array([0, 1, 2, 3, 4])
X[:, 0]   # array([ 0,  5, 10])

X
"""
array([[ 0, 1, 2, 3, 4],
       [ 5, 6, 7, 8, 9],
       [10, 11, 12, 13, 14]])
"""
subX = X[:2, :3] 
subX 
"""
array([[0, 1, 2],
       [5, 6, 7]])
"""
subX[0, 0] = 100
subX
"""
array([[100,   1,   2],
       [  5,   6,   7]])
"""
X
"""
array([[100,   1,   2,   3,   4],
       [  5,   6,   7,   8,   9],
       [ 10,  11,  12,  13,  14]])
"""

X
"""
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
"""
subX = X[:2, :3].copy()
subX[0, 0] = 100
subX
"""
array([[100,   1,   2],
       [  5,   6,   7]])
"""
X
"""
array([[ 0,  1,  2,  3,  4],
       [ 5,  6,  7,  8,  9],
       [10, 11, 12, 13, 14]])
"""

x  # array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
x.shape  # (10,)
x.ndim   # 1

a = x.reshape(1,10) 
a  # array([[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]])
a.ndim  # 2
a.shape  # (1, 10)

b = x.reshape(2, 5) 
b
"""
array([[0, 1, 2, 3, 4],
       [5, 6, 7, 8, 9]])
"""
b.shape  # (2, 5)
b.ndim  # 2

x.reshape(-1, 10) 
"""
array([[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]])
"""
x.reshape(10, -1)
"""
array([[0],
       [1],
       [2],
       [3],
       [4],
       [5],
       [6],
       [7],
       [8],
       [9]])
"""