From: Robert Klemme <bob.news@...>
Date: 2006-01-10T01:58:02+09:00
Subject: Re: hash as paramerter container

Sch�le Daniel wrote:
> Hello all,
>
> I am looking for Ruby equivalent for this Python Code
>
>  >>> def foo(a,b,c):
> ...     print "a is ", a
> ...     print "b is ", b
> ...     print "c is ", c
> ...
>  >>> h = {"c":3, "a":1, "b":2}
>  >>>
>  >>> foo
> <function foo at 0x403d6f7c>
>  >>> foo(**h)
> a is  1
> b is  2
> c is  3
>
> my first try
>
> => {"c"=>333, :b=>2, :a=>1}
> irb(main):017:0> def foo a,b,c
> irb(main):018:1> puts "a is #{a}"
> irb(main):019:1> puts "b is #{b}"
> irb(main):020:1> puts "c is #{c}"
> irb(main):021:1> end
> => nil
> irb(main):022:0> foo *h
> a is c333
> b is b2
> c is a1
> => nil
>
>
> this seem to replace (or substituate) the parameters expected by foo
> in the order of hash. And since hash has no order (in both languages)
> it's a random replacement.
>
> Is there a trick I don't know about?

I don't know Python but I assume that foo(**h) assigns hash entries to
function arguments.  Ruby cannot do this because parameter names are lost
at runtime.  Here's what you can do:

def foo(*a)
  case a.length
    when 1
      a[0].each {|k,v| puts "#{k} is #{v}"}
    when 3
      puts "a is #{a[0]}"
      puts "b is #{a[1]}"
      puts "c is #{a[2]}"
    else
      raise ArgumentError, "wrong number of arguments"
  end
end

>> foo(1,2,3)
a is 1
b is 2
c is 3
=> nil
>> foo(:x=>1, :y=>32, :z=>4)
y is 32
z is 4
x is 1
=> {:y=>32, :z=>4, :x=>1}
>> foo(1,2,3,4)
RuntimeError: Illegal Call
        from (irb):24:in `foo'
        from (irb):29
        from :0

But generally you would decide whether you use a hash or individual
parameters.  You could go through some hoops to retrofit something on Ruby
that will behave approximately like Python does but I won't be fast or
elegant and I guess there would be some limitations.  But Ruby 2 is will
introduce new functionality here (named parameters / arguments); it might
then be possible to do what you know from Python.

HTH

Kind regards

    robert