Add, delete, modify, and query Numpy array data

This article mainly introduces the addition, deletion, modification and query of Numpy array data. It has certain reference value. Now I share it with you. Friends in need can refer to it

Preparation work:

There are many ways to add, delete, modify, and check. Here are only some commonly used ones.

>>> import numpy as np 
>>> a = np.array([[1,2],[3,4],[5,6]])#创建3行2列二维数组。 
>>> a 
array([[1, 2], 
  [3, 4], 
  [5, 6]]) 
>>> a = np.zeros(6)#创建长度为6的，元素都是0一维数组 
>>> a = np.zeros((2,3))#创建3行2列，元素都是0的二维数组 
>>> a = np.ones((2,3))#创建3行2列，元素都是1的二维数组 
>>> a = np.empty((2,3)) #创建3行2列,未初始化的二维数组 
>>> a = np.arange(6)#创建长度为6的，元素都是0一维数组array([0, 1, 2, 3, 4, 5]) 
>>> a = np.arange(1,7,1)#结果与np.arange(6)一样。第一，二个参数意思是数值从1〜6,不包括7.第三个参数表步长为1. 
a = np.linspace(0,10,7) # 生成首位是0，末位是10，含7个数的等差数列[ 0.   1.66666667 3.33333333 5.   6.66666667 8.33333333 10.  ] 
a = np.logspace(0,4,5)#用于生成首位是10**0，末位是10**4，含5个数的等比数列。[ 1.00000000e+00 1.00000000e+01 1.00000000e+02 1.00000000e+03 1.00000000e+04]

increased

>>> a = np.array([[1,2],[3,4],[5,6]])
>>> b = np.array([[10,20],[30,40],[50,60]])
>>> np.vstack((a,b))
array([[ 1, 2],
  [ 3, 4],
  [ 5, 6],
  [10, 20],
  [30, 40],
  [50, 60]])
>>> np.hstack((a,b))
array([[ 1, 2, 10, 20],
  [ 3, 4, 30, 40],
  [ 5, 6, 50, 60]])

Direct addition of arrays of different dimensions is obviously not allowed. But you can use an n column vector and an m column row vector to construct an n×m matrix

>>> a = np.array([[1],[2]]) 
>>> a 
array([[1], 
  [2]]) 
>>> b=([[10,20,30]])#生成一个list，注意，不是np.array。 
>>> b 
[[10, 20, 30]] 
>>> a+b 
array([[11, 21, 31], 
  [12, 22, 32]]) 
>>> c = np.array([10,20,30]) 
>>> c 
array([10, 20, 30]) 
>>> c.shape 
(3,) 
>>> a+c 
array([[11, 21, 31], 
  [12, 22, 32]])

check

>>> a
array([[1, 2],
  [3, 4],
  [5, 6]])
>>> a[0] # array([1, 2])
>>> a[0][1]#2
>>> a[0,1]#2
>>> b = np.arange(6)#array([0, 1, 2, 3, 4, 5])
>>> b[1:3]#右边开区间array([1, 2])
>>> b[:3]#左边默认为 0array([0, 1, 2])
>>> b[3:]#右边默认为元素个数array([3, 4, 5])
>>> b[0:4:2]#下标递增2array([0, 2])

NumPy’s where function uses

np.where(condition, x, y), No. One parameter is a boolean array, and the second and third parameters can be scalars or arrays.

cond = numpy.array([True,False,True,False]) 
a = numpy.where(cond,-2,2)# [-2 2 -2 2] 
cond = numpy.array([1,2,3,4]) 
a = numpy.where(cond>2,-2,2)# [ 2 2 -2 -2] 
b1 = numpy.array([-1,-2,-3,-4]) 
b2 = numpy.array([1,2,3,4]) 
a = numpy.where(cond>2,b1,b2) # 长度须匹配# [1,2,-3,-4]

Change

>>> a = np.array([[1,2],[3,4],[5,6]]) 
>>> a[0] = [11,22]#修改第一行数组[1,2]为[11,22]。 
>>> a[0][0] = 111#修改第一个元素为111,修改后，第一个元素“1”改为“111”。 
 
>>> a = np.array([[1,2],[3,4],[5,6]]) 
>>> b = np.array([[10,20],[30,40],[50,60]]) 
>>> a+b #加法必须在两个相同大小的数组键间运算。 
array([[11, 22], 
  [33, 44], 
  [55, 66]])

Direct addition of arrays of different dimensions is obviously not allowed. But you can use an n column vector and an m column row vector to construct an n×m matrix

>>> a = np.array([[1],[2]])
>>> a
array([[1],
  [2]])
>>> b=([[10,20,30]])#生成一个list，注意，不是np.array。
>>> b
[[10, 20, 30]]
>>> a+b
array([[11, 21, 31],
  [12, 22, 32]])
>>> c = np.array([10,20,30])
>>> c
array([10, 20, 30])
>>> c.shape
(3,)
>>> a+c
array([[11, 21, 31],
  [12, 22, 32]])

array and the operations of addition, subtraction, multiplication and division of a number, It is equivalent to a broadcast, broadcasting this operation to each element.

>>> a = np.array([[1,2],[3,4],[5,6]]) 
>>> a*2#相当于a中各个元素都乘以2.类似于广播。 
array([[ 2, 4], 
  [ 6, 8], 
  [10, 12]]) 
>>> a**2 
array([[ 1, 4], 
  [ 9, 16], 
  [25, 36]]) 
>>> a>3 
array([[False, False], 
  [False, True], 
  [ True, True]]) 
>>> a+3 
array([[4, 5], 
  [6, 7], 
  [8, 9]]) 
>>> a/2 
array([[0.5, 1. ], 
  [1.5, 2. ], 
  [2.5, 3. ]])

Delete

Method 1:

Use the method in the search, such as a=a[0]. After the operation, there is only one row left for a.

>>> a = np.array([[1,2],[3,4],[5,6]]) 
>>> a[0] 
array([1, 2])

Method 2:

>>> a = np.array([[1,2],[3,4],[5,6]]) 
>>> np.delete(a,1,axis = 0)#删除a的第二行。 
array([[1, 2], 
  [5, 6]]) 
>>> np.delete(a,(1,2),0)#删除a的第二，三行。 
array([[1, 2]]) 
>>> np.delete(a,1,axis = 1)#删除a的第二列。 
array([[1], 
  [3], 
  [5]])

Method 3:

First split, and then assign value according to slice a=a[0].

>>> a = np.array([[1,2],[3,4],[5,6]]) 
>>> np.hsplit(a,2)#水平分割（搞不懂，明明是垂直分割嘛？） 
[array([[1], 
  [3], 
  [5]]), array([[2], 
  [4], 
  [6]])] 
>>> np.split(a,2,axis = 1)#与np.hsplit(a,2)效果一样。 
 
>>> np.vsplit(a,3) 
[array([[1, 2]]), array([[3, 4]]), array([[5, 6]])] 
>>> np.split(a,3,axis = 0)#与np.vsplit(a,3)效果一样。

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