I'm trying to subtract an array from another array taking frequency into account, like this:
[1,2,2,2] some_code [1,2] # => [2,2]
What's the easiest way to accomplish this?
Using - removes all occurrences of the elements in the second array:
[1,2,2,2] - [1,2] # => []
a1 = [1,2,2,2]
a2 = [1,2]
a2.each { |e| (idx = a1.find_index e) && (a1.delete_at idx) }
#⇒ [2, 2]
Here we iterate the second array and delete elements from the first one, once per iteration, if those were found.
The first found element will be deleted.
a = [1, 2, 2, 2]
b = [1, 2]
ha = a.each_with_object(Hash.new(0)){|e, h| h[e] += 1}
# => {1=>1, 2=>3}
hb = b.each_with_object(Hash.new(0)){|e, h| h[e] += 1}
# => {1=>1, 2=>1}
(ha.keys | hb.keys).flat_map{|k| Array.new([ha[k] - hb[k], 0].max, k)}
# => [2, 2]
If I understood the problem correctly, that you wish to delete single occurrence of each element of array b from array a, here is one way to do this:
a.keep_if {|i| !b.delete(i)}
#=> [2,2]
PS: Both arrays a and b are mutated by above code, so you may want to use dup to create a copy if you want to retain original arrays.
def subtract arr_a, arr_b
arr_b.each do |b|
idx = arr_a.index(b)
arr_a.delete_at(idx) unless idx.nil?
end
end
Output:
a = [1,2,2,2]
b = [1,2]
subtract a, b
puts "a: #{a}"
# => a: [2, 2]
