Numpy 数组的切片操作

实例+解释如下（表格）：关键是要明白python中数组的下标体系。一套从左往右，一套从右往左。

 1 import numpy as np
 2 import sys
 3
 4 def main():
 5     A=np.array([1,2,3,4,5,6,7,8,9,0])
 6     B=A.copy()
 7     print A
 8     print 'same as print A,',A[:]
 9     print 'same as print A,',A[::1]
10     print 'reverse A,',A[::-1]
11     print 'print all elements with 2 interval,',A[::2]
12     print 'print elements from the 4th element with 2 interval,',A[3::2]
13     print 'print elements from the last 4th element with 2 interval,',A[-3::2]
14     print 'print elements from the 2nd to 5th element with 2 interval,',A[1:4:2]
15     print 'reverse print all elements with 2 interval,',A[::-2]
16     print 'reverse print elements from the 4th element with 2 interval,',A[3::-2]
17     print 'reverse print elements from the last 4th element with 2 interval,',A[-3::-2]
18     #print A[a:b:c]: a and b determine range of slide. c determine the interval and direction (forth or reverse)
19     print A[:-3:-1]
20     print A[:3:1]
21     print A[:3:-1]
22     C=A[::-1]
23     C[0]=10
24     print 'A is also changed as how does c change,',A
25     B[9]=0
26     print 'A will not be affected when B change,',A
27     print 'array slide from second element to the last one,',A[1:]
28     print 'array slide from last second element to the first one,',A[:-1]
29
30
31
32 if __name__ == "__main__":
33     main()

A=np.array([1,2,3,4,5,6,7,8,9,0])	1	2	3	4	5	6	7	8	9	0
下标系一Index(Left to Right)
下标系二Index(Right to Left)	-10	-9	-8	-7	-6	-5	-4	-3	-2	-1
A[a:b:c]	a和b   确定：切片操作的范围(from a to b, exclude b.)， C 确定具体切片的pace和方向（例如：1表示Left to Right，pace=1. -1表示Right to   Left，pace=1.）。
实例：A[:3:1]	output: 1,2,3.   注：Index 3处对应的数组值4。1表示从左往右，步的大小是1。所以输出1，2，3
实例：A[:-3:-1]	output:0,9   注：Index -3处对应的数组值8。-1表示从右往左，步的大小是1。所以输出0，9
实例：A[:3:-1]	output:0,9,8,7,6,5   注：Index 3处对应的数组值4。-1表示从右往左，步的大小是1。所以输出0，9，8，7，6，5
实例：A[:-3:1]	output:1,2，3，4，5，6，7   注：Index -3处对应的数组值8。1表示从左往右，步的大小是1。所以输出1，2，3，4，5，6，7
实例：A[-3::-1]	output:8,7,6,5,4,3,2,1   注：Index -3处对应的数组值8。-1表示从右往左，步的大小是1。所以输出8,7,6,5,4,3,2,1