From: "funny_falcon (Yura Sokolov)" <funny.falcon@...>
Date: 2013-06-03T18:29:48+09:00
Subject: [ruby-core:55272] [ruby-trunk - Feature #8478] The hash returned by Enumerable#group_by should have an empty array for its default value


Issue #8478 has been updated by funny_falcon (Yura Sokolov).


There are always different ways to work with hash:

    counters = Hash.new{|h,k| 0}
    list.each{|e| counters[e]+=1}
    some_thing_count = counters[some_thing]

compared to

    counters = {}
    list.each{|e| counters[e] = (counters[e]||0) + 1}
    raise "THERE IS NO SOME-THING" unless counters[some_thing]

So that, group_by should not assume user's needs

So, one could assign default proc if he really wants:

    a = [1, 2, 3, "a", "b"]
    g = a.group_by {|o| o.class }
    g.default_proc = proc{|hash, key| hash[key] = [] if key.class==Class }

    puts "Fixnums: #{g[Fixnum].size}"
    puts "Strings: #{g[String].size}"
    puts "Arrays: #{g[Array].size}"
    if g[0]
      puts "Zeroes: #{g[0].size}"
    else
      raise "NO ZEROES!!!!"
    end
----------------------------------------
Feature #8478: The hash returned by Enumerable#group_by should have an empty array for its default value
https://bugs.ruby-lang.org/issues/8478#change-39668

Author: phiggins (Pete Higgins)
Status: Open
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: 
Target version: 


Without this patch, nil checks might need to be done on the return value of Enumerable#group_by:

$ cat test_group_by.rb 
a = [1, 2, 3, "a", "b"]
g = a.group_by {|o| o.class }

puts "Fixnums: #{g[Fixnum].size}"
puts "Strings: #{g[String].size}"
puts "Arrays: #{g[Array].size}"

$ ruby test_group_by.rb 
Fixnums: 3
Strings: 2
test_group_by.rb:6:in `<main>': undefined method `size' for nil:NilClass (NoMethodError)

This patch adds a default value of an empty array to the hash returned by Enumerable#group_by, so the script above will work:

$ ./ruby -I.:lib test_group_by.rb 
Fixnums: 3
Strings: 2
Arrays: 0



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