From: James Edward Gray II <james@...>
Date: 2006-12-07T00:47:16+09:00
Subject: Re: Default values for method arguments

On Dec 6, 2006, at 9:30 AM, paul wrote:

> The Simulated Keyword Arguments you talk about are a little  
> abstract to
> me, but I think I get the idea.

My fault, I did not describe them very well.  Here are the critical  
bits of information:

If a method call ends with some arguments that look like a Hash  
definition (using the fat arrows =>), Ruby collects those and passes  
a single Hash to the method in their place.  You can then work with  
the Hash normally.

> irb(main):001:0> def test(args)
> irb(main):002:1>   args['a'] = 'defaultA' unless args.has_key?('a')
> irb(main):003:1>   args['b'] = 'defaultB' unless args.has_key?('b')
> irb(main):004:1>   args['c'] = 'defaultC' unless args.has_key?('c')
> irb(main):005:1>   puts args.indexes('a', 'b', 'c')
> irb(main):006:1> end
> => nil
> irb(main):007:0> test({'a' => 'someA', 'c' => 'someC'})
> (irb):5: warning: Hash#indexes is deprecated; use Hash#values_at
> someA
> defaultB
> someC
> => nil

Another way to set defaults:

 >> def test(args)
 >>   args = { "a" => "default a",
?>            "b" => "default b",
?>            "c" => "default c" }.merge(args)
 >>   puts args.values_at("a", "b", "c")
 >> end
=> nil
 >> test "a" => "some a", "c" => "some c"
some a
default b
some c
=> nil

> An additional question: do you know a link where I can find more on
> those Simulated Keyword Arguments?

See Collecting Hash Arguments at the bottom of:

http://www.ruby-doc.org/docs/ProgrammingRuby/html/tut_methods.html

> Those colons you use before the variable-names are a little like magic
> to me. (though I checked, without those colons, you're example above
> wouldn't have worked...) :)

Those are just Symbol objects.  Don't lose too much sleep over them  
yet, as you can see from the new example Strings work fine too.

James Edward Gray II