How to select unique elements

by

Helper method

This method uses the helper:

class Array
  def difference(other)
    h = other.each_with_object(Hash.new(0)) { |e,h| h[e] += 1 }
    reject { |e| h[e] > 0 && h[e] -= 1 }
  end
end

This method is similar to Array#-. The difference is illustrated in the following example:

a = [3,1,2,3,4,3,2,2,4]
b = [2,3,4,4,3,4]

a - b              #=> [1]
c = a.difference b #=> [1, 3, 2, 2]

As you see, a contains three 3’s and b contains two, so the first two 3’s in a are removed in constructing c (a is not mutated). When b contains as least as many instances of an element as does a, c contains no instances of that element. To remove elements beginning at the end of a:

a.reverse.difference(b).reverse #=> [3, 1, 2, 2]

Array#difference! could be defined in the obvious way.

I have found many uses for this method: here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here.

I have proposed that this method be added to the Ruby core.

When used with Array#-, this method makes it easy to extract the unique elements from an array a:

a = [1,3,2,4,3,4]
u = a.uniq          #=> [1, 2, 3, 4]
u - a.difference(u) #=> [1, 2]

This works because

a.difference(u)     #=> [3,4]

contains all the non-unique elements of a (each possibly more than once).

Problem at Hand

Code

class Array
  def uniq_elements(&prc)
    prc ||= ->(e) { e }
    a = map { |e| prc[e] }
    u = a.uniq
    uniques = u - a.difference(u)
    select { |e| uniques.include?(prc[e]) ? (uniques.delete(e); true) : false }
  end
end

Examples

t = [1,2,2,3,4,4,5,6,7,7,8,9,9,9]
t.uniq_elements
  #=> [1,3,5,6,8]

t = [1.0, 1.1, 2.0, 3.0, 3.4, 4.0, 4.2, 5.1, 5.7, 6.1, 6.2]
t.uniq_elements { |z| z.round }
  # => [2.0, 5.1]

More Related Contents:

  1. how to i add objects to an array ? ruby
  2. How do I pick randomly from an array?
  3. How to sum array of numbers in Ruby?
  4. How to check if a value exists in an array in Ruby
  5. Ruby array access 2 consecutive(chained) elements at a time
  6. How to find and return a duplicate value in array
  7. Array#each vs. Array#map
  8. How to split (chunk) a Ruby array into parts of X elements? [duplicate]
  9. How to find an item in array which has the most occurrences [duplicate]
  10. Find indices of elements that match a given condition
  11. Creating matrix with `Array.new(n, Array.new)`
  12. Difference between map and collect in Ruby?
  13. Create two-dimensional arrays and access sub-arrays in Ruby
  14. How do I search within an array of hashes by hash values in ruby?
  15. What is the best way to convert an array to a hash in Ruby
  16. Performance of Arrays and Hashes in Ruby
  17. Find key/value pairs deep inside a hash containing an arbitrary number of nested hashes and arrays
  18. Need to split arrays to sub arrays of specified size in Ruby [duplicate]
  19. In Ruby, why does Array.new(size, object) create an array consisting of multiple references to the same object?
  20. How to chunk an array in Ruby
  21. Ruby – Difference between Array#
  22. Sort an array according to the elements of another array
  23. Convert array of 2-element arrays into a hash, where duplicate keys append additional values
  24. How to count duplicates in Ruby Arrays
  25. How do you add an array to another array in Ruby and not end up with a multi-dimensional result?
  26. How to count identical string elements in a Ruby array
  27. Ruby: filter array by regex?
  28. Ruby array subtraction without removing items more than once
  29. Where is it legal to use ruby splat operator?
  30. How do I remove blank elements from an array?

Leave a Comment