Pythonic方式获取序列的第一个和最后一个元素

Question

What is the easiest and cleanest way to get the first AND the last elements of a sequence? Eg, I have a sequence [1, 2, 3, 4, 5] , and I'd like to get [1, 5] via some kind of slicing magic. What I have come up with so far is:

l = len(s)
result = s[0:l:l-1]

I actually need this for a bit more complex task. I have a 3D numpy array, which is cubic (ie is of size NxNxN, where N may vary). I'd like an easy and fast way to get a 2x2x2 array containing the values from the vertices of the source array. The example above is an oversimplified, 1D version of my task.

Answer 1

用这个：

result = [s[0], s[-1]]

Answer 2

This would give you a list of the first and last element in your sequence:

 result = [s[0], s[-1]]

Alternatively, this would give you a tuple

 result = s[0], s[-1]

Answer 3

Since you're using a numpy array, you may want to use fancy indexing:

a = np.arange(27)
indices = [0, -1]
b = a[indices] # array([0, 26])

For the 3d case:

vertices = [(0,0,0),(0,0,-1),(0,-1,0),(0,-1,-1),(-1,-1,-1),(-1,-1,0),(-1,0,0),(-1,0,-1)]
indices = list(zip(*vertices))  #Can store this for later use.
a = np.arange(27).reshape((3,3,3)) #dummy array for testing.  Can be any shape size :)
vertex_values = a[indices].reshape((2,2,2))

I first write down all the vertices (although I am willing to bet there is a clever way to do it using itertools which would let you scale this up to N dimensions ...). The order you specify the vertices is the order they will be in the output array. Then I "transpose" the list of vertices (using zip ) so that all the x indices are together and all the y indices are together, etc. (that's how numpy likes it). At this point, you can save that index array and use it to index your array whenever you want the corners of your box. You can easily reshape the result into a 2x2x2 array (although the order I have it is probably not the order you want).

Answer 4

With the particular case of a (N,N,N) ndarray X that you mention, would the following work for you?

s = slice(0,N,N-1)
X[s,s,s]

Example

>>> N = 3
>>> X = np.arange(N*N*N).reshape(N,N,N)
>>> s = slice(0,N,N-1)
>>> print X[s,s,s]
[[[ 0  2]
  [ 6  8]]
 [[18 20]
  [24 26]]]

Answer 5

>>> from operator import itemgetter
>>> first_and_last = itemgetter(0, -1)
>>> first_and_last([1, 2, 3, 4, 5])
(1, 5)

Answer 6

Why do you want to use a slice? Getting each element with

result = [s[0], s[-1]]

is better and more readable.

If you really need to use the slice, then your solution is the simplest working one that I can think of.

This also works for the 3D case you've mentioned.