Python中list 和numpy.array的区别
1. list可以用append 或者 + 来新增元素或者添加数组，但array不行
2. 大多数array的操作都是elementwise级别的，即对每个元素进行单独处理，而不是视为一个整体再处理，
比如 A + A，得到的结果是A中每个元素的值对应✖️2的结果，
又比如有一个numpy.array A = [1, 2, 3], A + A = [2, 4, 6]。
又比如 A ** 2 = [1, 4, 9]，
这种特性使得numpy.array适合快速对每个元素进行单独处理。
----------------------------------------------------------------
以下是练习代码：
(py3) mbp$ ipython
Python 3.7.4 (default, Aug 13 2019, 15:17:50)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.8.0 -- An enhanced Interactive Python. Type '?' for help.
In [1]: import numpy as np
In [2]: L = [1, 2, 3]
In [3]: A = np.arrar([1, 2, 3])
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
in
----> 1 A = np.arrar([1, 2, 3])
AttributeError: module 'numpy' has no attribute 'arrar'
In [4]: A = np.array([1, 2, 3])
In [5]: for e in L:
...: print e
File "", line 2
print e
^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(e)?
In [6]: for e in L:
...: print (e)
...:
1
2
3
In [7]: for e in A:
...: print (e)
...:
1
2
3
In [8]: L.append(4)
In [9]: L
Out[9]: [1, 2, 3, 4]
In [10]: A.append(4)
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
in
----> 1 A.append(4)
AttributeError: 'numpy.ndarray' object has no attribute 'append'
In [11]: L = L + [5]
In [12]: L
Out[12]: [1, 2, 3, 4, 5]
In [13]: A = A + [4 ,5]
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
in
----> 1 A = A + [4 ,5]
ValueError: operands could not be broadcast together with shapes (3,) (2,)
In [14]: L2 = []
In [15]: for e in L:
...: L2.append(e + e)
...:
In [16]: L2
Out[16]: [2, 4, 6, 8, 10]
In [17]: A + A
Out[17]: array([2, 4, 6])
In [18]: 2*A
Out[18]: array([2, 4, 6])
In [19]: 2*L
Out[19]: [1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
In [20]: L ** 2
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
in
----> 1 L ** 2
TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'int'
In [21]: A **2
Out[21]: array([1, 4, 9])
In [22]: np.sqrt(A)
Out[22]: array([1. , 1.41421356, 1.73205081])
In [23]: np.log(A)
Out[23]: array([0. , 0.69314718, 1.09861229])
In [24]: np.exp(A)
Out[24]: array([ 2.71828183, 7.3890561 , 20.08553692])