Python--Numpy简单了解

• Numpy 高效的运算工具
• Numpy的优势
• ndarray属性
• 基本操作
  • ndarray.方法()
  • numpy.函数名()
• ndarray运算
  • 逻辑运算
  • 统计运算
  • 数组间运算
• 合并、分割、IO操作、数据处理

1. Numpy优势

1.1 Numpy介绍 - 数值计算库

• num - numerical 数值化的
• py - python
• ndarray
  • n - 任意个
  • d - dimension 维度
  • array - 数组

1.2 ndarray介绍

import numpy as np
score = np.array([[80, 89, 86, 67, 79],
[78, 97, 89, 67, 81],
[90, 94, 78, 67, 74],
[91, 91, 90, 67, 69],
[76, 87, 75, 67, 86],
[70, 79, 84, 67, 84],
[94, 92, 93, 67, 64],
[86, 85, 83, 67, 80]])

1.3 ndarray与Python原生list运算效率对比

import random
import time
# 生成一个大数组
python_list = []

for i in range(100000000):
python_list.append(random.random())
ndarray_list = np.array(python_list)

# 原生pythonlist求和
t1 = time.time()
a = sum(python_list)
t2 = time.time()
d1 = t2 - t1

# ndarray求和
t3 = time.time()
b = np.sum(ndarray_list)
t4 = time.time()
d2 = t4 - t3

d1= 0.7309620380401611
d2= 0.12980318069458008

1.4 ndarray的优势

底层语言
C语言，解除了GIL

2. 认识N维数组-ndarray属性

2.1 ndarray的属性

• shape
  • ndim ：看看维度
  • size ：看看大小
• dtype
  • itemsize ：一个元素所占大小
• 在创建ndarray的时候，如果没有指定类型
• 默认
  • 整数 int64
  • 浮点数 float64

array([[80, 89, 86, 67, 79],
[78, 97, 89, 67, 81],
[90, 94, 78, 67, 74],
[91, 91, 90, 67, 69],
[76, 87, 75, 67, 86],
[70, 79, 84, 67, 84],
[94, 92, 93, 67, 64],
[86, 85, 83, 67, 80]])

score.shape # (8, 5)
score.ndim # 2
score.size # 40
score.dtype # dtype(\'int64\')
score.itemsize # 8

2.2 ndarray的形状

a = np.array([[1,2,3],[4,5,6]])
b = np.array([1,2,3,4])
c = np.array([[[1,2,3],[4,5,6]],[[1,2,3],[4,5,6]]])

a # array([[1, 2, 3],
b # array([1, 2, 3, 4])
c # array([[[1, 2, 3],[4, 5, 6]],[[1, 2, 3],[4, 5, 6]]])
a.shape # (2, 3)
b.shape # (4,)
c.shape # (2, 2, 3)

2.3 ndarray的类型

type(score.dtype)
<type \'numpy.dtype\'>

# 指定类型
# 创建数组的时候指定类型
np.array([1.1, 2.2, 3.3], dtype=\"float32\")

dtype是numpy是numpy.dtype类型，先看看对数组来说都有哪些类型

3. 基本操作

• adarray.方法()
• np.函数名()
  • np.array()

3.1 生成数组的方法

3.1.1 生成0和1

• np.zeros(shape)
• np.ones(shape)

# 1 生成0和1的数组
np.zeros(shape=(3, 4), dtype=\"float32\")
-----------------------------------------
array([[0., 0., 0., 0.],
[0., 0., 0., 0.],
[0., 0., 0., 0.]], dtype=float32)

np.ones(shape=[2, 3], dtype=np.int32)
-----------------------------------------
array([[1, 1, 1],
[1, 1, 1]], dtype=int32)

3.1.2 从现有数组中生成

• np.array() np.copy() 深拷贝
• np.asarray() 浅拷贝

data1 = np.array(score)
data2 = np.asarray(score)
data3 = np.copy(score)
score[3, 1] = 10000

修改source，data2改变，data1，data3不改变

3.1.3 生成固定范围的数组

• np.linspace(0, 10, 100)

  • [0, 10] 等距离 生成个数

• np.arange(a, b, c)

  • range(a, b, c)
    • [a, b) c是步长

np.linspace(0, 10, 5)
# array([ 0. , 2.5, 5. , 7.5, 10. ])
np.arange(0, 11, 5)
# array([ 0, 5, 10])

3.1.4 生成随机数组

分布状况 - 直方图

• 均匀分布uniform
  • low：float类型，此概率的均值(对应着整个分布的中心centre)
  • scale：float类型，此概率分布的标准差(对应于分布的宽度，scale越大越矮胖，越小越瘦高)
  • size：int or tuple of ints 输出的shape，默认位None，只输出一个值

import matplotlib.pyplot as plt
import numpy as np
data1 = np.random.uniform(low=-1, high=1, size=1000000)
array([-0.49795073, -0.28524454, 0.56473937, ..., 0.6141957 ,
0.4149972 , 0.89473129])
# 1、创建画布
plt.figure(figsize=(20, 8), dpi=80)

# 2、绘制直方图
plt.hist(data1, 1000)

# 3、显示图像
plt.show()

• 正态分布normal
  • low：此概率的均值(对应着整个分布的中心centre)
  • scale：float此概率分布的标准差(对应于分布的宽度，scale越大越矮胖，越小越瘦高)
  • size：int or tuple of ints 输出的shape，默认位None，只输出一个值

# 正态分布
data2 = np.random.normal(loc=1.75, scale=0.1, size=1000000)
# 1、创建画布
plt.figure(figsize=(20, 8), dpi=80)

# 2、绘制直方图
plt.hist(data2, 1000)

# 3、显示图像
plt.show()

3.2 数组的索引、切片

stock_change = np.random.normal(loc=0, scale=1, size=(8, 10))
# 返回结果
array([[-0.03469926, 1.68760014, 0.05915316, 2.4473136 , -0.61776756, -0.56253866, -1.24738637, 0.48320978, 1.01227938, -1.44509723],[-1.8391253 , -1.10142576, 0.09582268, 1.01589092, -1.20262068, 0.76134643, -0.76782097, -1.11192773, 0.81609586, 0.07659056],[-0.74293074, -0.7836588 , 1.32639574, -0.52735663, 1.4167841 , 2.10286726, -0.21687665, -0.33073563, -0.46648617, 0.07926839],[ 0.45914676, -0.78330377, -1.10763289, 0.10612596, -0.63375855,-1.88121415, 0.6523779 , -1.27459184, -0.1828502 , -0.76587891],[-0.50413407, -1.35848099, -2.21633535, -1.39300681, 0.13159471, 0.65429138, 0.32207255, 1.41792558, 1.12357799, -0.68599018],[ 0.3627785 , 1.00279706, -0.68137875, -2.14800075, -2.82895231,-1.69360338, 1.43816168, -2.02116677, 1.30746801, 1.41979011],[-2.93762047, 0.22199761, 0.98788788, 0.37899235, 0.28281886,-1.75837237, -0.09262863, -0.92354076, 1.11467277, 0.76034531],[-0.39473551, 0.28402164, -0.15729195, -0.59342945, -1.0311294 ,-1.07651428, 0.18618331, 1.5780439 , 1.31285558, 0.10777784]])

# 获取第一个股票的前3个交易日的涨跌幅数据
stock_change[0, :3]
# 返回结果
array([-0.03469926, 1.68760014, 0.05915316])

一维、二维、三维的数组如何索引？

# 三维，一维
a1 = np.array([ [[1,2,3],[4,5,6]], [[12,3,34],[5,6,7]]])
# 返回结果
array([[[ 1, 2, 3],
[ 4, 5, 6]],

[[12, 3, 34],
[ 5, 6, 7]]])
# 索引、切片
a1.shape # (2, 2, 3)
a1[1, 0, 2] # 34

# 修改
a1[1, 0, 2] = 100000
# 返回结果
array([[[ 1, 2, 3],
[ 4, 5, 6]],

[[ 12, 3, 100000],
[ 5, 6, 7]]])

3.3 形状修改

• ndarray.reshape(shape) 返回新的ndarray，原始数据没有改变
• ndarray.resize(shape) 没有返回值，对原始的ndarray进行了修改
• ndarray.T 转置 行变成列，列变成行

• ndarray.reshape(shape)返回新的ndarray，原始数据没有改变

# 需求：让刚才的股票行、日期列反过来，变成日期行，股票列
stock_change
# 返回结果
array(
[[-0.03469926, 1.68760014, 0.05915316, 2.4473136 , -0.61776756,
-0.56253866, -1.24738637, 0.48320978, 1.01227938, -1.44509723],
[-1.8391253 , -1.10142576, 0.09582268, 1.01589092, -1.20262068,
0.76134643, -0.76782097, -1.11192773, 0.81609586, 0.07659056],
[-0.74293074, -0.7836588 , 1.32639574, -0.52735663, 1.4167841 ,
2.10286726, -0.21687665, -0.33073563, -0.46648617, 0.07926839],
[ 0.45914676, -0.78330377, -1.10763289, 0.10612596, -0.63375855,
-1.88121415, 0.6523779 , -1.27459184, -0.1828502 , -0.76587891],
[-0.50413407, -1.35848099, -2.21633535, -1.39300681, 0.13159471,
0.65429138, 0.32207255, 1.41792558, 1.12357799, -0.68599018],
[ 0.3627785 , 1.00279706, -0.68137875, -2.14800075, -2.82895231,
-1.69360338, 1.43816168, -2.02116677, 1.30746801, 1.41979011],
[-2.93762047, 0.22199761, 0.98788788, 0.37899235, 0.28281886,
-1.75837237, -0.09262863, -0.92354076, 1.11467277, 0.76034531],
[-0.39473551, 0.28402164, -0.15729195, -0.59342945, -1.0311294 ,
-1.07651428, 0.18618331, 1.5780439 , 1.31285558, 0.10777784]])

stock_change.reshape((10, 8))
# 返回结果
array(
[[-0.03469926, 1.68760014, 0.05915316, 2.4473136 , -0.61776756,
-0.56253866, -1.24738637, 0.48320978],
[ 1.01227938, -1.44509723, -1.8391253 , -1.10142576, 0.09582268,
1.01589092, -1.20262068, 0.76134643],
[-0.76782097, -1.11192773, 0.81609586, 0.07659056, -0.74293074,
-0.7836588 , 1.32639574, -0.52735663],
[ 1.4167841 , 2.10286726, -0.21687665, -0.33073563, -0.46648617,
0.07926839, 0.45914676, -0.78330377],
[-1.10763289, 0.10612596, -0.63375855, -1.88121415, 0.6523779 ,
-1.27459184, -0.1828502 , -0.76587891],
[-0.50413407, -1.35848099, -2.21633535, -1.39300681, 0.13159471,
0.65429138, 0.32207255, 1.41792558],
[ 1.12357799, -0.68599018, 0.3627785 , 1.00279706, -0.68137875,
-2.14800075, -2.82895231, -1.69360338],
[ 1.43816168, -2.02116677, 1.30746801, 1.41979011, -2.93762047,
0.22199761, 0.98788788, 0.37899235],
[ 0.28281886, -1.75837237, -0.09262863, -0.92354076, 1.11467277,
0.76034531, -0.39473551, 0.28402164],
[-0.15729195, -0.59342945, -1.0311294 , -1.07651428, 0.18618331,
1.5780439 , 1.31285558, 0.10777784]])

• ndarray.resize(shape)没有返回值，对原始的ndarray进行了修改

stock_change.shape # (8, 10)
stock_change.resize((10, 8))
stock_change.shape # (10, 8)

• ndarray.T转置 行变成列，列变成行

stock_change.T

3.4 类型修改

• ndarray.astype(type)
• ndarray序列化到本地
• ndarray.tostring()

stock_change.astype(\"int32\")
# 返回结果
array([[ 0, 1, 0, 2, 0, 0, -1, 0, 1, -1],
[-1, -1, 0, 1, -1, 0, 0, -1, 0, 0],
[ 0, 0, 1, 0, 1, 2, 0, 0, 0, 0],
[ 0, 0, -1, 0, 0, -1, 0, -1, 0, 0],
[ 0, -1, -2, -1, 0, 0, 0, 1, 1, 0],
[ 0, 1, 0, -2, -2, -1, 1, -2, 1, 1],
[-2, 0, 0, 0, 0, -1, 0, 0, 1, 0],
[ 0, 0, 0, 0, -1, -1, 0, 1, 1, 0]], dtype=int32)

stock_change.tobytes()
b\'\\x95&\\x99\\xdd\\x19\\xc4\\xa1\\xbfm8\\x88\\x00i\\x00\\xfb?\\x92\\xbc\\x81\\xa1RI\\xae?\\xa2\\x95x&\\x19\\x94\\x03@\\x9f?\\xbev\\xc0\\xc4\\xe3\\xbf\\x87\\xf4H\\x13Q\\x00\\xe2\\xbf\\x9eM\\x85hK\\xf5\\xf3\\xbf\\x17mZ\\xb2\\xe8\\xec\\xde?U\\xca\\xd4\\xdbK2\\xf0?G\\xc6\\xbbD\\x1e\\x1f\\xf7\\xbf\\x9f-\\xb0\\xa5\\x0em\\xfd\\xbf\\x9b\\xd0h\\x9dp\\x9f\\xf1\\xbfyH\\x8e\\xc3\\xd5\\x87\\xb8?\\x1d\\x89v\\xd5\\x16A\\xf0?\\x89Aj-\\xef=\\xf3\\xbf\\xbc\\x8ea/\\xf3\\\\\\xe8?\\x94\\xb8\\xbaJ\\xfd\\x91\\xe8\\xbfv\\xc0\\x92\\xbct\\xca\\xf1\\xbf\\x82\\x82\\x19\\x11u\\x1d\\xea?\\xf2.\\x96Qp\\x9b\\xb3?g\\xed\\xef\\xb0\\x16\\xc6\\xe7\\xbf\\xf2\\xbf!\\x9c\\xbb\\x13\\xe9\\xbf\\x7fv\\x1e\\xbd\\xea8\\xf5?\\x1e \\x9d\\x02\\x1b\\xe0\\xe0\\xbf?\\x99O\\xce%\\xab\\xf6?\\x84;\\xb9\\x11\\xac\\xd2\\x00@p\\xe3\\xa07\\x9d\\xc2\\xcb\\xbfop\\x94\\xc4\\xc5*\\xd5\\xbfN\\x15)\\xca\\xe8\\xda\\xdd\\xbf4\\xa8\\x8b\\xf1\\xeeJ\\xb4?Qd\\x8e\\x1c\\xa9b\\xdd?\\xc8\\x92\\xb6\\x10\\xd3\\x10\\xe9\\xbf\\xf1\\x80\\x87C\\xdd\\xb8\\xf1\\xbf\\x18\\x02B \\x12+\\xbb?Xv\\xb4\\x02\\xc0G\\xe4\\xbf\\xa6,\\x8a\\x02t\\x19\\xfe\\xbf\\xb4\\xc9\\xaf\\x9cG\\xe0\\xe4?wCsj\\xbad\\xf4\\xbf\\xbc\\xb1\\xd5\\xa9\\xa2g\\xc7\\xbf\\xbc\\xc6\\x8d{\\x14\\x82\\xe8\\xbf>\\xf7\\xae\\xc6\\xdd!\\xe0\\xbf\\xacB\\x9c\\x90V\\xbc\\xf5\\xbfb\\xae\\xfa\\x06\\x0e\\xbb\\x01\\xc0_B\\xe1\\x82\\xc1I\\xf6\\xbfw\\x9f\\xb6m\\x18\\xd8\\xc0?\\x93\\xcb\\x8e{\\xf4\\xef\\xe4?\\xfe\\xc1\\xba,\\xd6\\x9c\\xd4?k\\x85)\\xbc\\xd2\\xaf\\xf6?{g\\x82\\xea,\\xfa\\xf1?s}\\xaf\\xad\\xa1\\xf3\\xe5\\xbfD(cM\\xc37\\xd7?(\\x1a\\xff\\xect\\x0b\\xf0?7e\\x80\\xce\\xda\\xcd\\xe5\\xbf\"\\xd5\\xe1\\x03\\x1b/\\x01\\xc0\\x94\\x85?\\xbf\\xb1\\xa1\\x06\\xc0w\\x08\\x14\\xdc\\xff\\x18\\xfb\\xbf\\x9f\\x1eL\\xd2\\xb5\\x02\\xf7?\\xb0-5{Y+\\x00\\xc0;\\xf5<\\x94c\\xeb\\xf4?a\\x8f\\xb1\\xd6u\\xb7\\xf6?%Kr)?\\x80\\x07\\xc0\\x9e\\x1c%\\xedjj\\xcc?F\\xa0C\\t\\xc7\\x9c\\xef?\\xf3\\xc3\\xfd\\x1eiA\\xd8?\\xcc\\x9e\\x84D\\xb4\\x19\\xd2?\\xdd$J\\x10K\"\\xfc\\xbf\\xe6E\\xb3\\x95\\x82\\xb6\\xb7\\xbf\\x0cN\\xa4Z\\xa5\\x8d\\xed\\xbf\\x96\\xdd\\xee\\x1c\\xb3\\xd5\\xf1?\\x05\\x8c\\x12\\xb0\\xbfT\\xe8?/\\xa5\\x1a\\xb9XC\\xd9\\xbf~Z!\\x1ci-\\xd2?\\x1f\\xe4\\xe3\\x83$\"\\xc4\\xbf_&\\xc5\\xc0_\\xfd\\xe2\\xbf\\xbf\\x16\\xac\\x8b\\x81\\x7f\\xf0\\xbf\\xf7\\xba)\\tg9\\xf1\\xbf\\xb7q\\x8c\\xd7\\xda\\xd4\\xc7?\\x98P\\xb7\\xf4\\xaa?\\xf9?\\x8c\\x98P\\xdbt\\x01\\xf5?t\\xd8 -T\\x97\\xbb?\'

3.5 数组的去重

• set()：只能处理一维
• np.unique()

temp = np.array([[1, 2, 3, 4],[3, 4, 5, 6]])
# 返回结果
array([[1, 2, 3, 4],
[3, 4, 5, 6]])
np.unique(temp)
# 返回结果
array([1, 2, 3, 4, 5, 6])

set(temp.flatten()) # 将多维降维成一维，然后用set去重 只能处理一维
# 返回结果
{1, 2, 3, 4, 5, 6}

4. ndarray运算

4.1 逻辑运算

• 布尔索引
• 通用判断函数
  • np.all(布尔值)
    • 只要有一个False就返回False，只有全是True才返回True
  • np.any()
    • 只要有一个True就返回True，只有全是False才返回False
• np.where（三元运算符）
  • np.where(布尔值, True的位置的值, False的位置的值)

stock_change = np.random.normal(loc=0, scale=1, size=(8, 10))
# 返回结果
array([[ 1.46338968, -0.45576704, 0.29667843, 0.16606916, 0.46446682,0.83167611, -1.35770374, -0.65001192, 1.38319911, -0.93415832],[ 0.36775845, 0.24078108, 0.122042 , 1.19314047, 1.34072589,0.09361683, 1.19030379, 1.4371421 , -0.97829363, -0.11962767],[-1.48252741, -0.69347186, 0.91122464, -0.30606473, 0.41598897,0.79542753, -0.01447862, -1.49943117, -0.23285809, 0.42806777],[ 0.39438905, -1.31770556, 1.7344868 , -1.52812773, -0.47703227,-0.3795497 , -0.88422651, 1.37510973, -0.93622775, 0.49257673],[-0.9822216 , -1.09482936, -0.81834523, 0.57335311, 0.97390091,0.05314952, -0.58316743, 0.19264426, 0.02081861, 0.84445247],[ 0.41739964, -0.26826893, -0.70003442, -0.58593912, 0.86546709,-1.30304864, 0.05254567, -1.73976785, -0.43532247, 0.4760526 ],[-0.21739882, 0.52007085, -0.60160491, 0.57108639, 1.03303301,-0.69172579, 1.04716985, -0.22985706, -0.11125069, 0.87722923],[-0.183266 , 0.56273065, 0.29357786, -0.19343363, -1.54547303,-0.31977163, -0.00659025, 0.48160678, 0.88443604, -0.48456825]])
--------------------------------------------------
# 逻辑判断, 如果涨跌幅大于0.5就标记为True 否则为False
stock_change > 0.5
# 返回结果
array([[ True, False, False, False, False, True, False, False, True,False],[False, False, False, True, True, False, True, True, False,False],[False, False, True, False, False, True, False, False, False,False],[False, False, True, False, False, False, False, True, False,False],[False, False, False, True, True, False, False, False, False,True],[False, False, False, False, True, False, False, False, False,False],[False, True, False, True, True, False, True, False, False,True],[False, True, False, False, False, False, False, False, True,False]])
--------------------------------------------------
stock_change[stock_change > 0.5] = 1.1
# 返回结果
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916, 0.46446682,1.1 , -1.35770374, -0.65001192, 1.1 , -0.93415832],[ 0.36775845, 0.24078108, 0.122042 , 1.1 , 1.1 ,0.09361683, 1.1 , 1.1 , -0.97829363, -0.11962767],[-1.48252741, -0.69347186, 1.1 , -0.30606473, 0.41598897,1.1 , -0.01447862, -1.49943117, -0.23285809, 0.42806777],[ 0.39438905, -1.31770556, 1.1 , -1.52812773, -0.47703227,-0.3795497 , -0.88422651, 1.1 , -0.93622775, 0.49257673],[-0.9822216 , -1.09482936, -0.81834523, 1.1 , 1.1 ,0.05314952, -0.58316743, 0.19264426, 0.02081861, 1.1 ],[ 0.41739964, -0.26826893, -0.70003442, -0.58593912, 1.1 ,-1.30304864, 0.05254567, -1.73976785, -0.43532247, 0.4760526 ],[-0.21739882, 1.1 , -0.60160491, 1.1 , 1.1 ,-0.69172579, 1.1 , -0.22985706, -0.11125069, 1.1 ],[-0.183266 , 1.1 , 0.29357786, -0.19343363, -1.54547303,-0.31977163, -0.00659025, 0.48160678, 1.1 , -0.48456825]])

# 判断stock_change[0:2, 0:5]是否全是上涨的
stock_change[0:2, 0:5] > 0
# 返回结果
array([[ True, False, True, True, True],
[ True, True, True, True, True]])
--------------------------------------------------
np.all(stock_change[0:2, 0:5] > 0)
# 返回结果
False
--------------------------------------------------
# 判断前5只股票这段期间是否有上涨的
np.any(stock_change[:5, :] > 0)
# 返回结果
True

# 判断前四个股票前四天的涨跌幅 大于0的置为1，否则为0
temp = stock_change[:4, :4]
# 返回结果
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916],
[ 0.36775845, 0.24078108, 0.122042 , 1.1 ],
[-1.48252741, -0.69347186, 1.1 , -0.30606473],
[ 0.39438905, -1.31770556, 1.1 , -1.52812773]])
--------------------------------------------------
np.where(temp > 0, 1, 0)
# 返回结果
array([[1, 0, 1, 1],
[1, 1, 1, 1],
[0, 0, 1, 0],
[1, 0, 1, 0]])
--------------------------------------------------
temp > 0
# 返回结果
array([[ True, False, True, True],
[ True, True, True, True],
[False, False, True, False],
[ True, False, True, False]])
--------------------------------------------------
np.where([[ True, False, True, True],
[ True, True, True, True],
[False, False, True, False],
[ True, False, True, False]], 1, 0)
# 返回结果
array([[1, 0, 1, 1],
[1, 1, 1, 1],
[0, 0, 1, 0],
[1, 0, 1, 0]])
--------------------------------------------------
# 判断前四个股票前四天的涨跌幅 大于0.5并且小于1的，换为1，否则为0
# 判断前四个股票前四天的涨跌幅 大于0.5或者小于-0.5的，换为1，否则为0
# (temp > 0.5) and (temp < 1)
np.logical_and(temp > 0.5, temp < 1)
# 返回结果
array([[False, False, False, False],
[False, False, False, False],
[False, False, False, False],
[False, False, False, False]])
--------------------------------------------------
np.where([[False, False, False, False],
[False, False, False, False],
[False, False, False, False],
[False, False, False, False]], 1, 0)
# 返回结果
array([[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0]])
--------------------------------------------------
np.where(np.logical_and(temp > 0.5, temp < 1), 1, 0)
# 返回结果
array([[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0],
[0, 0, 0, 0]])
--------------------------------------------------
np.logical_or(temp > 0.5, temp < -0.5)
# 返回结果
array([[ True, False, False, False],
[False, False, False, True],
[ True, True, True, False],
[False, True, True, True]])
--------------------------------------------------
np.where(np.logical_or(temp > 0.5, temp < -0.5), 11, 3)
# 返回结果
array([[11, 3, 3, 3],
[ 3, 3, 3, 11],
[11, 11, 11, 3],
[ 3, 11, 11, 11]])

4.2 统计运算

axis轴的取值并不一定，Numpy中不同的API轴的值不一样，
在这里，axis 0代表行，1代表列

• 统计指标函数
  • min, max, mean, median, var, std
  • np.函数名
  • ndarray.方法名
• 返回最大值、最小值所在位置
  • np.argmax(temp, axis=)
  • np.argmin(temp, axis=)

# 前四只股票前四天的最大涨幅
temp # shape: (4, 4) 0 1
# 返回结果
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916],
[ 0.36775845, 0.24078108, 0.122042 , 1.1 ],
[-1.48252741, -0.69347186, 1.1 , -0.30606473],
[ 0.39438905, -1.31770556, 1.1 , -1.52812773]])
--------------------------------------------------
temp.max(axis=0)# 按列求最大值
# 返回结果
array([1.1 , 0.24078108, 1.1 , 1.1 ])
--------------------------------------------------
np.max(temp, axis=-1)
# 返回结果
array([1.1, 1.1, 1.1, 1.1])
--------------------------------------------------
np.argmax(temp, axis=-1)
# 返回结果
array([0, 3, 2, 2])

5. 数组间运算

5.1 场景

5.2 数组与数的运算

• +-*/

arr = np.array([[1, 2, 3, 2, 1, 4], [5, 6, 1, 2, 3, 1]])
arr / 10
# 返回结果
array([[0.1, 0.2, 0.3, 0.2, 0.1, 0.4],
[0.5, 0.6, 0.1, 0.2, 0.3, 0.1]])

5.3 数组与数组的运算

arr1 = np.array([[1, 2, 3, 2, 1, 4], [5, 6, 1, 2, 3, 1]])
arr2 = np.array([[1, 2, 3, 4], [3, 4, 5, 6]])

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

5.4 广播机制

执行broadcast的前提在于，两个ndarray执行的是element-wise的运算，Broadcast机制的功能是为了方便不同形状的ndarray(numpy库的核心数据结构)进行数学运算

• 维度相等
• shape(其中相对应的一个地方为1)
  广播的原则：如果两个数组的后缘维度（trailing dimension，即从末尾开始算起的维度）的轴长度相符，或其中的一方的长度为1，则认为它们是广播兼容的。广播会在缺失和（或）长度为1的维度上进行。

5.5 矩阵运算

1 什么是矩阵
矩阵matrix 二维数组
矩阵 & 二维数组
两种方法存储矩阵
1）ndarray 二维数组
矩阵乘法：
np.matmul
np.dot
2）matrix数据结构
2 矩阵乘法运算
形状
(m, n) * (n, l) = (m, l)
运算规则
A (2, 3) B(3, 2)
A * B = (2, 2)

# ndarray存储矩阵
data = np.array([[80, 86],
[82, 80],
[85, 78],
[90, 90],
[86, 82],
[82, 90],
[78, 80],
[92, 94]])

# matrix存储矩阵
data_mat = np.mat([[80, 86],
[82, 80],
[85, 78],
[90, 90],
[86, 82],
[82, 90],
[78, 80],
[92, 94]])
type(data_mat)
numpy.matrixlib.defmatrix.matrix

data # (8, 2) * (2, 1) = (8, 1)
np.matmul(data, weights)
array([[84.2],
[80.6],
[80.1],
[90. ],
[83.2],
[87.6],
[79.4],
[93.4]])
np.dot(data, weights)
array([[84.2],
[80.6],
[80.1],
[90. ],
[83.2],
[87.6],
[79.4],
[93.4]])
data_mat * weights_mat
matrix([[84.2],
[80.6],
[80.1],
[90. ],
[83.2],
[87.6],
[79.4],
[93.4]])
data @ weights
array([[84.2],
[80.6],
[80.1],
[90. ],
[83.2],
[87.6],
[79.4],
[93.4]])

6. 合并、分割

6.1 合并

• numpy.hstack(tup)
  
• numpy.vstack(tup)
  
• numpy.concatenate((a1, a2 , ...), axis=0)
  

a = stock_change[:2, 0:4]
b = stock_change[4:6, 0:4]

a
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916],
[ 0.36775845, 0.24078108, 0.122042 , 1.1 ]])

a.shape # (2, 4)

a.reshape((-1, 2))
array([[ 1.1 , -0.45576704],
[ 0.29667843, 0.16606916],
[ 0.36775845, 0.24078108],
[ 0.122042 , 1.1 ]])

b
array([[-0.9822216 , -1.09482936, -0.81834523, 1.1 ],
[ 0.41739964, -0.26826893, -0.70003442, -0.58593912]])

np.hstack((a, b))
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916, -0.9822216 ,
-1.09482936, -0.81834523, 1.1 ],
[ 0.36775845, 0.24078108, 0.122042 , 1.1 , 0.41739964,
-0.26826893, -0.70003442, -0.58593912]])

np.concatenate((a, b), axis=1)
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916, -0.9822216 ,
-1.09482936, -0.81834523, 1.1 ],
[ 0.36775845, 0.24078108, 0.122042 , 1.1 , 0.41739964,
-0.26826893, -0.70003442, -0.58593912]])

np.vstack((a, b))
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916],
[ 0.36775845, 0.24078108, 0.122042 , 1.1 ],
[-0.9822216 , -1.09482936, -0.81834523, 1.1 ],
[ 0.41739964, -0.26826893, -0.70003442, -0.58593912]])

np.concatenate((a, b), axis=0)
array([[ 1.1 , -0.45576704, 0.29667843, 0.16606916],
[ 0.36775845, 0.24078108, 0.122042 , 1.1 ],
[-0.9822216 , -1.09482936, -0.81834523, 1.1 ],
[ 0.41739964, -0.26826893, -0.70003442, -0.58593912]])

6.2 分割

7. IO操作与数据处理

7.1 Numpy读取

data = np.genfromtxt(\"test.csv\", delimiter=\",\")

array([[ nan, nan, nan, nan],
[ 1. , 123. , 1.4, 23. ],
[ 2. , 110. , nan, 18. ],

7.2 如何处理缺失值

两种思路：

• 直接删除含有缺失值的样本
• 替换/插补
  • 按列求平均，用平均值进行填补

来源：https://www.cnblogs.com/Slience-me/p/15149756.html
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