How can I find the index of the second greatest element in my array?

问题

I'm using Ruby 2.4 and I have an array of numbers:

[23423, 349843, 13123, 29239, 20201, ...]

How can I find the array index corresponding to the second greatest value in the array? You can assume that there are at least two elements in the array.

回答1:

Try this one. a is your array

a.index(a.max(2).last)

回答2:

a = [1,3,1,2]

When 1 and 1 are regarded as the two smallest values of a

def second_largest_not_uniq(a)
  a.each_index.min_by(2) { |i| a[i] }[1]
end

second_largest_not_uniq [1,3,1,2] #=> 2 
second_largest_not_uniq [1]       #=> nil
second_largest_not_uniq []        #=> nil

When 1 and 2 are regarded as the two smallest values of a

def second_largest_uniq(a)
  a.each_index.to_a.uniq { |i| a[i] }.min_by(2) { |i| a[i] }[1]
end

second_largest_uniq [1,3,1,2]     #=> 3
second_largest_uniq [1,1,1]       #=> nil
second_largest_uniq []            #=> nil
second_largest_uniq [1]           #=> nil

回答3:

Try this if you need the value

value = array.max(2).last

Try this if you need the index

index = array.each_with_index.max_by(2, &:first).last.last

How does this work?

each_with_index creates an enumerator with tuples of [element, index]
max_by(2, &:first) finds the two greatest tuples, comparing their first value aka the element
last gets the second-greatest tuple
last unpacks that tuple by getting last value aka the index

Note, this creates O(n) temporary arrays though because we chain the each_with_index enumerator and I would not use this for large arrays in a performance critical codepath.

回答4:

I'd sort the array then use something like:

ary.size - 2

For instance:

ary = 5.times.map{ rand(100) } # => [61, 75, 35, 48, 59]

ary.sort # => [35, 48, 59, 61, 75]

ary.sort[-2] # => 61
ary.size - 2 # => 3
ary.sort[ary.size - 2] # => 61

This does not return the index of the element in the original array.

The index of the second largest element after sorting is always array.size - 2.

If the array has to be in its original order I'd do this:

ary = 5.times.map{ rand(100) } # => [83, 72, 4, 63, 68]
hash = ary.each_with_index.to_h # => {83=>0, 72=>1, 4=>2, 63=>3, 68=>4}
hash.sort[-2] # => [72, 1]

At that point hash.sort[-2] returns the value and its index in the original array. 72 is the value and ary[1] is the index to the value.

来源：https://stackoverflow.com/questions/41703164/how-can-i-find-the-index-of-the-second-greatest-element-in-my-array

标签

arrays

ruby

position