Ruby 中包含重复元素的集合的交集和并集

Question

我们如何在 Ruby 中获取重复元素的集合的交集和并集。

# given the sets
a = ["A", "B", "B", "C", "D", "D"]
b = ["B", "C", "D", "D", "D", "E"]

# A union function that adds repetitions
union(a, b)
=> ["A", "B", "B", "C", "D", "D", "D", "E"]

# An intersection function that adds repetitions
intersection(a, b)
=> ["B", "C", "D", "D"]

& 和 | 运算符似乎忽略重复和重复，如文档中所写。

# union without duplicates
a | b
=> ["A", "B", "C", "D", "E"]

# intersections without duplicates
a & b
=> ["B", "C", "D"]

Answer 1

def union(a,b)
  (a|b).flat_map { |s| [s]*[a.count(s), b.count(s)].max }
end

union(a,b)
  # => ["A", "B", "B", "C", "D", "D", "D", "E"] 

def intersection(a,b)
  (a|b).flat_map { |s| [s]*[a.count(s), b.count(s)].min }
end

intersection(a,b)
  #=> ["B", "C", "D", "D"]

Answer 2

在的基础上，您可以创建一个临时散列来计算每个数组中每个成员的出现次数：（我已经概括了参数的数量）

def multiplicities(*arrays)
  m = Hash.new { |h, k| h[k] = Array.new(arrays.size, 0) }
  arrays.each_with_index { |ary, idx| ary.each { |x| m[x][idx] += 1 } }
  m
end

multiplicities(a, b)
#=> {"A"=>[1, 0], "B"=>[2, 1], "C"=>[1, 1], "D"=>[2, 3], "E"=>[0, 1]}

实施 union 和 intersection 很简单：

def union(*arrays)
  multiplicities(*arrays).flat_map { |x, m| Array.new(m.max, x) }
end

def intersection(*arrays)
  multiplicities(*arrays).flat_map { |x, m| Array.new(m.min, x) }
end

union(a, b)        #=> ["A", "B", "B", "C", "D", "D", "D", "E"]
intersection(a, b) #=> ["B", "C", "D", "D"]

使用这种方法，每个数组只需遍历一次。

Ruby 中包含重复元素的集合的交集和并集

Intersections and Unions in Ruby for sets with repeated elements

ruby

multiset

set-intersection

set-union