Return maximum of 2D Array

Question

I have a 2D array:

[["user1", 20], ["user2", 30], ["user3", 10]]

The first element of a subarray is the username, and the second one its score.

I need to return the username with the maximum score. For example, here it would return "user2".

Either one user has a higher score, or they all have the same score (in which case it should return "all").

I know how to do this with a bunch of iterations and many lines of code, but I'm trying to find the "best practice way".

What would you advise ?

What's a `user` in this context - `["user1", 20]` or `"user1"`? And is a user's number the number within the string (i.e. `1` in `"user1"`) or the second element (i.e. `20` in `["user1", 20]`)? That being said, I would advice to use a custom `User` class with attributes `:name` and `:number` or at least a hash instead of an array. — Stefan, Jan 10 '16 at 11:51
the first element of a subarray is the username, and the second one its score. The number is the second element of the subarray. — Graham Slick, Jan 10 '16 at 12:11
Maybe because the question was unclear or because you didn't include your own attempt. — Stefan, Jan 10 '16 at 12:18
What the expected result if there is more than one maximum score? — Stefan, Jan 10 '16 at 12:20

score 6 · Accepted Answer · edited Jan 10 '16 at 11:54

6

Check out max_by

users = [["user1", 20], ["user2", 30], ["user3", 10]]

users.max_by{ |user, weight| weight }
#=> ["user2", 30]

# shorthand
users.max_by(&:last)
#=> ["user2", 30]

users.max_by(&:last).first
#=> "user2"

It worth to be mentioned that if you have a number of maximums it will return only first one.

edited Jan 10 '16 at 11:54

Stefan

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answered Jan 10 '16 at 11:10

fl00r

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score 1 · Answer 2 · edited May 23 '17 at 12:15

Simple solution for the given example:

a = [["user1", 20], ["user2", 30], ["user3", 10]]
h = Hash[a].invert
h[h.keys.max]
#=> "user2"

In the general case, where identical maximum numbers will occur, the following solution will return all users sharing the same maximum value (w/ invert function by Nigel, see Swapping keys and values in a hash):

class Hash
  def safe_invert
    self.each_with_object( {} ) { |(key, value), out| ( out[value] ||= [] ) << key }
  end
end

a = [["user1", 20], ["user2", 30], ["user3", 10], ["user4", 30]]
h = Hash[a].safe_invert
h[h.keys.max]
#=> ["user2", "user4"]

Return maximum of 2D Array

2 Answers2