python numpy matrix slice and string slice

• Create 4 matrices of 6*6, respectively, the matrix values ​​are all 0, the matrix values ​​are all 1, the matrix values ​​are all specified values, and the matrix values ​​are random;

  import numpy as np
  x1 = np.zeros((6,6))
  x2 = np.ones((6,6))
  x3 = np.full((6,6),255)#可以将255换成别的数字
  x4 = np.random.rand(6,6)
  print(x1)
  print(x2)
  print(x3)
  print(x4)

  operation result:
  

• Read the first and fourth numbers of the second and third rows of matrix x4:

x4 = np.random.rand(6,6)
print(x4[2:4,1:5])

• Read the first and fourth numbers of all rows of matrix x4:

x4 = np.random.rand(6,6)
print(x4[:,1:5])

• Read the 0th and 3rd numbers of all columns of matrix x4

x4 = np.random.rand(6,6)
print(x4[0:4,:]) #包括下标为0的数字，不包括下标为4的数字

• print matrix x4 from back to front

x5 = np.array([(1,2,3),(5,6,8)])
print(x5)
print(x5[::,::-1]) #-1表示步长，也可以设置别的数字
print(x5[::-1])

• Set step size to 2

x5 = np.array([(1,2,3),(5,6,8)])
print(x5[::,::2]) #步长为2

• Take only the column labeled 2

x5 = np.array([(1,2,3),(5,6,8)])
print(x5)
print(x5[:,2]) #逗号前一个冒号表示所有行

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Added:
String slices are very similar to matrix slices:

s = 'abcdefgh'
print(s)
print(s[::-1]) #-1表示步长
print(s[::2])
print(s[0:5])#下标为0的计算入内，下标为5的不计算入内

Output:
abcdefgh
hgfedcba
aceg
abcde