4.19 高级特性之切片

本节主要学习了Python中高级特性的切片操作。例如，定义一个0-99的数列L

L = list(range(100))

要求取出前20个元素并打印出来
传统方法：

L2 = []
for i in range(20):
    L2.append(L[i])
print(L2)
#[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

而如果需要经常此类操作，循环会非常繁琐。对此，Python给出了切片（Slice）操作符，大大简化了操作。
使用切片操作符完成前述问题：

print(L[:20])
#[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

除此之外，切片操作还可以很方便地完成在一个List里取出指定范围的元素的操作。

print(L[1:4]) #print the 2nd to 4th element
#[1, 2, 3]
print(L[:-2]) #print the list except last two element
#[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97]
print(L[-2:]) #print the last two element
#[98, 99]
print(L[::5]) #print one every five
#[0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95]
print(L[10:20:2]) #print one every 2 in [10,20)
#[10, 12, 14, 16, 18]