CODEWITHSUNDEEP
ruby
Lobna Khaled
Lobna Khaled

Reputation: 29

How to sum the largest two elements in an array

What is the appropriate method to get the sum of the two largest values in array using Ruby?

I know array.sort and array.max and inject(:+) but I don't know how to combine them.

Upvotes: 1

Views: 6144

Answers (5)

the Tin Man
the Tin Man

Reputation: 160551

It's always good to have some benchmarks to see what is the fastest way to accomplish something:

require 'active_support/core_ext/enumerable'
require 'fruity'

ARRAY = (0..1000).to_a.shuffle

compare do

  adriano { ARRAY.max(2).reduce(:+)       }
  mori    { ARRAY.sort.last(2).sum        }
  edgars  { ARRAY.sort[-2..-1].inject(:+) }

end


# >> Running each test 64 times. Test will take about 1 second.
# >> adriano is faster than edgars by 50.0% ± 10.0%
# >> edgars is similar to mori

Upvotes: 1

user3923122
user3923122

Reputation:

You could try something like this

array = [13, 2, 21, 36, 4, 9]

array.reverse.first(2).reduce(:+)

In this method we are reverse sorting the array, which is sorting it from highest to lowest. This puts the largest numbers first in the array. reduce method returns the sum of these numbers. In this case, the sum of 36 and 21 is 57.

57

Upvotes: 1

Adriano Mitre
Adriano Mitre

Reputation: 330

I would strongly favor the simpler and faster:

array.max(2).reduce(:+)

Sorting is unnecessary, it is enough to scan the array updating a max-heap of size K. Using Array#max(K) you achieve O(N x log(K)) asymptotic time complexity versus O(N x log(N)) of sorting.

Upvotes: 6

Edgars Jekabsons
Edgars Jekabsons

Reputation: 2853

[1,2,3,4].sort[-2..-1].inject(:+)

If you are looking for a more efficient solution, you will be better with this one:

[1,2,3,4].inject([0, 0]) do |largest, el|
  if largest[0] < el
    largest[1] = largest[0]
    largest[0]  = el
  elsif largest[1] < el
    largest[1] = el
  end
  largest
end.inject(:+)

If you are familiar with algorithm complexity, than you can see that the first one is nlogn to n^2, depending on the Ruby sorting algorithm, where the second solution is just n as it goes through the array once.

Upvotes: 2

Mori
Mori

Reputation: 27779

[9, 4, 5, 1].sort.last(2).sum
=> 14

Upvotes: 4

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