Python Set

#python #datastructure #programming #language #efficiency

• They are two types of sets: set and frozenset
  • set: Not hashable, cannot be nested
  • frozenset: Hashable, we can have frozenset inside set
• Take a look on both codes below

found = len(needles & haystack) # (1)

found = 0 # (2)
for n in needless:
	if n in haystack:
		found += 1

• The code (1) has superior performance than the code (2).
• To build a set

>>> s = {1} # faster than
>>> s = set([1,2,3])

• Take a look on setcomp
• Elements inside set need to be Hashable or has the methods __hash__ and __eq__
  • But the check of existence of element is fast
• Sets have memory overhead
• Sorting is not guaranteed in set, adding elements can change the order
• Python Dictionary methods like keys() and items() implement some set methods like: &, |, - and ^.