切片Python列表与切片R矢量

Question

In R I can do this

> a = (10:19)
> a
 [1] 10 11 12 13 14 15 16 17 18 19
> b = c(4,7)
> b
[1] 4 7
>
> a[b]
[1] 13 16
>
> a[-b]
[1] 10 11 12 14 15 17 18 19

I suppose there is an equally elegant way of doing this to Python (2.7) lists, but haven't found yet. I'm particularly interested in the a[-b] bit. Any thoughts?

[edit] a is [10,11,12,13,14,15,16,17,18,19], b is [4,7] (indices into a)

Answer 1

You do this using list comprehensions

[n for n, i in enumerate(a) if i not in b]

Or using numpy:

x = np.arange(10, 20)
y = [2, 7]

x[y]

Answer 2

 a=numpy.array(range(10,20))
 b = [4,7]
 print a[b]
 print a[~numpy.in1d(a,a[b])]

not quite as elegent but meh ... it also wont work if there are duplicate elements in the list ... since its looking at the values in the negation step rather than the indices

Answer 3

You probably want numpy:

import numpy as np
a = np.array(range(10,19))
b = [3,6]
a[b]
=> array([13, 16])
a[[_ for _ in range(len(a)) if _ not in b]]
=> array([10, 11, 12, 14, 15, 17, 18])