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ruby hash per-value defaults

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Here’s a quick tip for assigning default values with a ruby hash. It’s well publicized that you can set an overall default (i think this is called “default assignment”) for the hash with the .default method like this (stolen directly from the rubydocs):

h = Hash.new                            #=> {}
   h.default                               #=> nil
   h.default(2)                            #=> nil

   h = Hash.new("cat")                     #=> {}
   h.default                               #=> "cat"
   h.default(2)                            #=> "cat"

   h = Hash.new {|h,k| h[k] = k.to_i*10}   #=> {}
   h.default                               #=> 0
   h.default(2)                            #=> 20</pre>

But you can also set per-key defaults using the or-operator. if an assigned value is false, or nil, you’ll get the default value. See below:

ruby-1.9.1-p378 > x = {}
=> value: {}
ruby-1.9.1-p378 > x[:y] = "y"
=> value: "y"
ruby-1.9.1-p378 > x[:y]
=> value: "y"
ruby-1.9.1-p378 > x[:y] = "y" || "noty"
=> value: "y"
ruby-1.9.1-p378 > x[:y]
=> value: "y"
ruby-1.9.1-p378 > x[:y] = nil || "noty"
=> value: "noty"
ruby-1.9.1-p378 > x[:y] = false || "noty"
=> value: "noty"
ruby-1.9.1-p378 > x[:y] = "" || "noty"
=> value: ""

… Note that or-assignment doesn’t work in this case:

ruby-1.9.1-p378 > x[:y] = "" ||= "noty"
SyntaxError: (irb):19: syntax error, unexpected tOP_ASGN, expecting $end
x[:y] = "" ||= "noty"
^
from /home/jcran/.rvm/rubies/ruby-1.9.1-p378/bin/irb:17:in `<main>'
ruby-1.9.1-p378 >
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Written by jcran

July 19, 2010 at 4:12 PM

Posted in Uncategorized

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