Update 3/2022 There is currently a proposal to add Records and Tuples (basically immutable Objects and Arrays) to Javascript. In that proposal, it offers direct comparison of Records and Tuples using === or !== where it compares values, not just object references AND relevant to this answer both Set and Map objects would use the … Read more
It depends on what you are intending to do with it. Sets are significantly faster when it comes to determining if an object is present in the set (as in x in s), but are slower than lists when it comes to iterating over their contents. You can use the timeit module to see which … Read more
set returns const_iterators (the standard says set<T>::iterator is const, and that set<T>::const_iterator and set<T>::iterator may in fact be the same type – see 23.2.4/6 in n3000.pdf) because it is an ordered container. If it returned a regular iterator, you’d be allowed to change the items value out from under the container, potentially altering the ordering. … Read more
Yes, it is O(2^n) indeed, since you need to generate, well, 2^n possible combinations. Here’s a working implementation, using generics and sets: public static <T> Set<Set<T>> powerSet(Set<T> originalSet) { Set<Set<T>> sets = new HashSet<Set<T>>(); if (originalSet.isEmpty()) { sets.add(new HashSet<T>()); return sets; } List<T> list = new ArrayList<T>(originalSet); T head = list.get(0); Set<T> rest = new … Read more
Edit: Previous solutions for two sets removed. See edit history for details. Here is a way to do it recursively for an arbitrary number of sets: public static Set<Set<Object>> cartesianProduct(Set<?>… sets) { if (sets.length < 2) throw new IllegalArgumentException( “Can’t have a product of fewer than two sets (got ” + sets.length + “)”); return … Read more
You can write an object that behaves like a dict quite easily with ABCs (Abstract Base Classes) from the collections.abc module. It even tells you if you missed a method, so below is the minimal version that shuts the ABC up. from collections.abc import MutableMapping class TransformedDict(MutableMapping): “””A dictionary that applies an arbitrary key-altering function … Read more
The Python itertools page has exactly a powerset recipe for this: from itertools import chain, combinations def powerset(iterable): “powerset([1,2,3]) –> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)” s = list(iterable) return chain.from_iterable(combinations(s, r) for r in range(len(s)+1)) Output: >>> list(powerset(“abcd”)) [(), (‘a’,), (‘b’,), (‘c’,), (‘d’,), (‘a’, ‘b’), (‘a’, ‘c’), (‘a’, ‘d’), (‘b’, ‘c’), (‘b’, … Read more
First off, it’s important to understand that there is no single standard H.264 elementary bitstream format. The specification document does contain an Annex, specifically Annex B, that describes one possible format, but it is not an actual requirement. The standard specifies how video is encoded into individual packets. How these packets are stored and transmitted … Read more
A set is an unordered data structure, so it does not preserve the insertion order. This depends on your requirements. If you have an normal list, and want to remove some set of elements while preserving the order of the list, you can do this with a list comprehension: >>> a = [1, 2, 20, … Read more
The answer is no, but you can use collections.OrderedDict from the Python standard library with just keys (and values as None) for the same purpose. Update: As of Python 3.7 (and CPython 3.6), standard dict is guaranteed to preserve order and is more performant than OrderedDict. (For backward compatibility and especially readability, however, you may … Read more