Bug #2004: Setting Hash default to an array hides the hash keys

• Status: Rejected
• Assignee: -
• ruby -v: ruby 1.8.7 (2008-08-11 patchlevel 72) [i686-darwin9.5.0]

[totoro (Alan Stebbens)]

=begin
Setting the default value of a hash to an array causes the keys and values methods results to be empty.

It may be counter-intuitive and not actually useful, but the assignment of the default value to an array causes all values to be same array object.

So, the little sums and subtractions end up applying to the same array object, no matter what the hash key is.

However, even though the hash keys and values appear to be empty, they are clearly still there.

% cat bug1.rb
#!/usr/bin/env ruby

h = Hash.new [0, 0]
d = ['a', 'b', 'c']
puts h

d.each_with_index{|k,x|
h[k][0] += x
h[k][1] -= x
}

puts "h.inspect = #{h.inspect}"
puts "h.keys = #{h.keys.inspect}"
puts "h.values = #{h.values.inspect}"

d.each{|k| puts "#{k} = #{h[k].inspect}"}

---

$ ruby bug1.rb

h.inspect = {}
h.keys = []
h.values = []
a = [3, -3]
b = [3, -3]
c = [3, -3]
=end

[totoro (Alan Stebbens)]

• File good1.rb good1.rb added

=begin
This is the same code, except without an array default value (using code to initialize a default for each key):

$ cat good1.rb
#!/usr/bin/env ruby

h = Hash.new
d = ['a', 'b', 'c']
puts h

d.each_with_index{|k,x|
h[k] = [0,0] unless h.key? k
h[k][0] += x
h[k][1] -= x
}

puts "h.inspect = #{h.inspect}"
puts "h.keys = #{h.keys.inspect}"
puts "h.values = #{h.values.inspect}"

d.each{|k| puts "#{k} = #{h[k].inspect}"}

---

ruby good1.rb

h.inspect = {"a"=>[0, 0], "b"=>[1, -1], "c"=>[2, -2]}
h.keys = ["a", "b", "c"]
h.values = [[0, 0], [1, -1], [2, -2]]
a = [0, 0]
b = [1, -1]
c = [2, -2]

=end

[nobu (Nobuyoshi Nakada)]

=begin
Completely expected behavior.
=end

[totoro (Alan Stebbens)]

=begin
It is expected that having a default value of an array should cause h.keys and h.values to be empty, even when there are keys and values?

Why?
=end

[nobu (Nobuyoshi Nakada)]

=begin
Hi,

At Thu, 27 Aug 2009 13:59:17 +0900,
Alan Stebbens wrote in [ruby-core:25162]:

It is expected that having a default value of an array
should cause h.keys and h.values to be empty, even when there
are keys and values?

You don't assign any key at all. The default value is not
automatically assigned. You may want to use default_proc
instead.

--
Nobu Nakada

=end

[marcandre (Marc-Andre Lafortune)]

• Status changed from Open to Rejected

=begin
Closing since this it the official behavior, as explained in detail in Hash.new, #default and #default=
=end