python学习笔记（三）：numpy基础

• Counter函数可以对列表中数据进行统计每一个有多少种
• most_common(10)可以提取前十位

from collections import Counter
a = ['q','q','w','w','w']
count = Counter(a)
count.most_common(1)

[('w', 3)]

count

Counter({'q': 2, 'w': 3})

• pandas中的series对象有一个value_counts方法可以计数

• .fillna()函数可以替换确实值NA

import numpy as np
from numpy.random import randn
data = {i : randn() for i in range(7)}
data

{0: -0.2657989059225722,
 1: -1.2517286143172295,
 2: -0.6360811023039581,
 3: 1.2009891917346602,
 4: 1.7528414640242418,
 5: -0.24155970563487628,
 6: -0.7637924413712933}

• 最近的两个结果保存在_和__中

9*3

27

_

27

%pwd
# 获得当前工作目录

'D:\\Code\\Python\\code'

• 魔法命令
• %time 一条语句的执行时间
• %timeit 执行多次的平均时间

numpy基础

import numpy as np
data1 = [6, 7.5, 8],[2, 0 ,1]
arr1 = np.array(data1)
arr1

array([[6. , 7.5, 8. ],
       [2. , 0. , 1. ]])

arr1.ndim

2

arr1.shape

(2, 3)

arr1.dtype

dtype('float64')

np.zeros((2, 3, 4))

array([[[0., 0., 0., 0.],
        [0., 0., 0., 0.],
        [0., 0., 0., 0.]],

       [[0., 0., 0., 0.],
        [0., 0., 0., 0.],
        [0., 0., 0., 0.]]])

np.ones((3))

array([1., 1., 1.])

np.arange(9)

array([0, 1, 2, 3, 4, 5, 6, 7, 8])

np.eye(3,)

array([[1., 0., 0.],
       [0., 1., 0.],
       [0., 0., 1.]])

• 数组切片后的修改会反映到原始数组上

arr = np.arange(9)
arr2 = arr[5:8]
arr2[:] = 4
arr

array([0, 1, 2, 3, 4, 4, 4, 4, 8])

arr = np.arange(9)
arr2 = arr[5:8].copy()
arr2[:] = 4
arr

array([0, 1, 2, 3, 4, 5, 6, 7, 8])

• 三维数组先是层、行、列

data = np.random.randn(4,3)
data

array([[ 0.7144327 ,  0.87144603,  1.10651404],
       [-0.19509352, -0.01102958,  1.8051039 ],
       [ 0.03106339,  0.83767495,  0.20094192],
       [ 0.96032146, -0.40303045,  1.4522938 ]])

data[[1,2,0],[1,2,0]]
# 取出来的数据为（11）（22）（00）

array([-0.01102958,  0.20094192,  0.7144327 ])

• 不连续提取数据

data[[0,2]][:,[0,2]]
# 跳着取方法一

array([[0.7144327 , 1.10651404],
       [0.03106339, 0.20094192]])

data[np.ix_([0,2],[0,2])]
# 跳着取方法二

array([[0.7144327 , 1.10651404],
       [0.03106339, 0.20094192]])

data.T

array([[ 0.7144327 , -0.19509352,  0.03106339,  0.96032146],
       [ 0.87144603, -0.01102958,  0.83767495, -0.40303045],
       [ 1.10651404,  1.8051039 ,  0.20094192,  1.4522938 ]])

np.sqrt(data)

F:\Anaconda\lib\site-packages\ipykernel_launcher.py:1: RuntimeWarning: invalid value encountered in sqrt
  """Entry point for launching an IPython kernel.

array([[0.84524121, 0.93351274, 1.05190971],
       [       nan,        nan, 1.34354155],
       [0.17624808, 0.91524584, 0.44826546],
       [0.97995993,        nan, 1.20511153]])

np.exp(data)

array([[2.04302734, 2.39036489, 3.02379915],
       [0.82275771, 0.98903102, 6.0806032 ],
       [1.03155089, 2.31098757, 1.22255377],
       [2.61253617, 0.66829175, 4.27290447]])

np.rint(data)
# 四舍五入

array([[ 1.,  1.,  1.],
       [-0., -0.,  2.],
       [ 0.,  1.,  0.],
       [ 1., -0.,  1.]])

np.modf(data)
# 将数据分为小数和整数部分

(array([[ 0.7144327 ,  0.87144603,  0.10651404],
        [-0.19509352, -0.01102958,  0.8051039 ],
        [ 0.03106339,  0.83767495,  0.20094192],
        [ 0.96032146, -0.40303045,  0.4522938 ]]), array([[ 0.,  0.,  1.],
        [-0., -0.,  1.],
        [ 0.,  0.,  0.],
        [ 0., -0.,  1.]]))

np.isnan(data)

array([[False, False, False],
       [False, False, False],
       [False, False, False],
       [False, False, False]])

np.where(data > 0,9,data)

array([[ 9.        ,  9.        ,  9.        ],
       [-0.19509352, -0.01102958,  9.        ],
       [ 9.        ,  9.        ,  9.        ],
       [ 9.        , -0.40303045,  9.        ]])

• axis中0表示竖向求和，1表示横向求和

np.mean(data,axis=1)

array([0.89746426, 0.5329936 , 0.35656009, 0.6698616 ])

np.in1d(data, [1,2,3])
# 查看data中每个元素是否在1，2，3内

array([False, False, False, False, False, False, False, False, False,
       False, False, False])

from numpy.linalg import inv, qr
from numpy.random import randn
x = randn(5,5)
mat = x.dot(inv(x))
# 求逆
mat = np.rint(mat)
mat

array([[ 1.,  0., -0.,  0.,  0.],
       [ 0.,  1., -0.,  0.,  0.],
       [ 0., -0.,  1.,  0.,  0.],
       [-0.,  0., -0.,  1., -0.],
       [-0.,  0.,  0., -0.,  1.]])

np.diag(mat)
# 返回对角线元素

array([1., 1., 1., 1., 1.])

np.random.permutation(mat)
# 返回序列的随机排列

array([[ 0., -0.,  1.,  0.,  0.],
       [-0.,  0., -0.,  1., -0.],
       [ 1.,  0., -0.,  0.,  0.],
       [-0.,  0.,  0., -0.,  1.],
       [ 0.,  1., -0.,  0.,  0.]])

np.random.randint(0,2,12)

array([1, 1, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1])