Python笔记 #07# NumPy 文档地址 & Subsetting 2D Arrays

文档地址：np.array()

1、<class 'numpy.ndarray'>

ndarray 表示 n 维度（n D）数组 （= n 行数组）。

2、打印 array 结构 —— np_array.shape

3、Subsetting 2D Arrays 的两种方式

不管是 arr[1][2] 还是 arr[1,2] 都是可行的，不过教程推荐第二种方式。

4、代码演示 & 一个取子集的小技巧

创建：

# Create baseball, a list of lists
baseball = [[180, 78.4],
            [215, 102.7],
            [210, 98.5],
            [188, 75.2]]

# Import numpy
import numpy as np

# Create a 2D numpy array from baseball: np_baseball
np_baseball = np.array(baseball)

# Print out the type of np_baseball
print(type(np_baseball))

# Print out the shape of np_baseball
print(np_baseball.shape)

打印结构：

# baseball is available as a regular list of lists

# Import numpy package
import numpy as np

# Create a 2D numpy array from baseball: np_baseball
np_baseball =  np.array(baseball)

# Print out the shape of np_baseball
print(np_baseball.shape)

小技巧：

# baseball is available as a regular list of lists

# Import numpy package
import numpy as np

# Create np_baseball (2 cols)
np_baseball = np.array(baseball)

# Print out the 50th row of np_baseball
print(np_baseball[49,:])

# Select the entire second column of np_baseball: np_weight
np_weight = np_baseball[:,1]

# Print out height of 124th player
print(np_baseball[123,0])

5、整体数学运算

# baseball is available as a regular list of lists
# updated is available as 2D numpy array

# Import numpy package
import numpy as np

# Create np_baseball (3 cols)
np_baseball = np.array(baseball)

# Print out addition of np_baseball and updated
print(np_baseball + updated)

# Create numpy array: conversion
conversion = np.array([0.0254, 0.453592, 1])

# Print out product of np_baseball and conversion
print(conversion * np_baseball)