如何按值对哈希进行排序

Question

I was trying to sort a particular hash by values. I came across a way using the method sort_by . But even though I call sort_by on a hash, it returns an array, ie:

a = {}
a[0] = "c"
a[1] = "b"
a[2] = "a"
a.sort_by{|key, value| value}
# => [[2, "a"], [1, "b"], [0, "c"]]

If I try to convert the result into a hash, I end up with a hash sorted on key, hence the whole purpose of sort_by is lost. ie:

a = {}
a[0] = "c"
a[1] = "b"
a[2] = "a"
Hash[*a.sort_by{|key, value| value}.flatten]
# => {0=>"c", 1=>"b", 2=>"a"}

Is there a way I can sort a hash by value and yet get the results back in the form of a Hash?

I am using 1.8.6 ruby

Answer 1

A Hash is a collection of key-value pairs. It is similar to an Array, except that indexing is done via arbitrary keys of any object type, not an integer index. The order in which you traverse a hash by either key or value may seem arbitrary , and will generally not be in the insertion order.

Source: Ruby 1.8.7 docs

• Hash preserves order. It enumerates its elements in the

Source: Ruby 1.9.1 changelog

If you want to use a ordered hash in Ruby 1.8, you should look at ActiveSupport's OrderedHash

You don't need to be in a Rails project, just include ActiveSupport in your project.

Answer 2

You can use ActiveSupport::OrderedHash for Ruby 1.8:

ActiveSupport::OrderedHash implements a hash that preserves insertion order, as in Ruby 1.9

I don't have 1.8.6 running, but this should work:

a = {}
a[0] = "c"
a[1] = "b"
a[2] = "a"

ordered = ActiveSupport::OrderedHash[*a.sort_by{|k,v| v}.flatten]
ordered.keys
# => [2, 1, 0], this order is guaranteed

As noted in the quote above hashes in Ruby 1.9 "enumerate their values in the order that the corresponding keys were inserted" , so this is only needed for Ruby 1.8.

Answer 3

Your description is wrong.

a = {}
a[0] = "c"
a[1] = "b"
a[2] = "a"
Hash[*a.sort_by{|key, value| value}.flatten]

gives:

{
  2 => "a",
  1 => "b",
  0 => "c"
}

which is sorted by value. By the way, your code is redundant. A better way to to this is:

Hash[a.sort_by{|_, value| value}]

or

Hash[a.sort_by(&:last)]

After Seeing Your Edit You are using a version that does not have ordered hash. You cannot control the order of the elements. You cannot do anything with it.