slice

How can we make a class represent itself as a slice when appropriate? This didn't work: class MyThing(object): def __init__(self, start, stop, otherstuff): self.start = start self.stop = stop self.otherstuff = otherstuff def __index__(self): return slice(self.start, self.stop) Expected output: >>> thing = MyThing(1, 3, 'potato') >>> 'hello world'[thing] 'el' Actual output: TypeError: __index__ returned non-(int,long) (type slice) Inheriting from slice doesn't work either. MisterMiyagi TLDR: It's impossible to make custom classes replace slice for builtins types such as list and tuple . The _

If I want to enumerate an array (say for a map() function where I would need to use the index of an element together with its value), I could use enumerate() function. E.g.: import Foundation let array: [Double] = [1, 2, 3, 4] let powersArray = array.enumerate().map() { pow($0.element, Double($0.index)) } print("array == \(array)") print("powersArray == \(powersArray)") // array == [1.0, 2.0, 3.0, 4.0] // powersArray == [1.0, 2.0, 9.0, 64.0] <- As expected Now, if I want to use some sub-sequence from the array, I could use a slice , and it would allow me to use just the same indices as I would

What is the best way to "slice" an NSArray from the end, rather than the beginning, of the array (for example, finding the subarray containing the last few elements of a NSArray of unknown length)? In Python, you can use negative indices to accomplish this, e.g.: new_list = old_list[-5:-3] What's the most natural way to do this in Objective-C? There's nothing to match Python's nice syntax for this, but you could do: NSUInteger count = [myArray count]; NSArray * slice = [myArray subarrayWithRange:(NSRange){count-n, n}]; You could also write up a category for NSArray , something like: @interface

I have a list that looks like this: lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] I'd like to generate a filtered list that looks like this: filtered_lst = [2, 6, 7, 9, 10, 13] Does Python provide a convention for custom slicing. Something such as: lst[1, 5, 6, 8, 9, 12] # slice a list by index Use operator.itemgetter() : from operator import itemgetter itemgetter(1, 5, 6, 8, 9, 12)(lst) Demo: >>> from operator import itemgetter >>> lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] >>> itemgetter(1, 5, 6, 8, 9, 12)(lst) (2, 6, 7, 9, 10, 13) This returns a tuple; cast to a list with list

Suppose I want the first element, the 3rd through 200th elements, and the 201st element through the last element by step-size 3, from a list in Python. One way to do it is with distinct indexing and concatenation: new_list = old_list[0:1] + old_list[3:201] + old_list[201::3] Is there a way to do this with just one index on old_list ? I would like something like the following (I know this doesn't syntactically work since list indices cannot be lists and since Python unfortunately doesn't have slice literals ; I'm just looking for something close): new_list = old_list[[0, 3:201, 201::3]] I can

I have a 3D-numpy array of a gray image, which looks something like this: [[[120,120,120],[67,67,67]]...] Obviously I have every R G and B the same because it is a gray image - this is redundent. I want to get a new 2D array which looks like: [[120,67]...] Which means to take every pixel's array [x,x,x] to just the value x How can I do that? If the shape of your ndarray is (M, N, 3), then you can get an (M, N) gray-scale image like this: >>> gray = img[:,:,0] 来源： https://stackoverflow.com/questions/14365029/numpy-3d-image-array-to-2d