From: Robert Klemme <shortcutter@...>
Date: 2007-01-14T19:45:05+09:00
Subject: Re: How does Array.map work for 2D arrays?

On 14.01.2007 11:21, Karthik Nar wrote:
> I was reading this example:
> 
> pt = [ [ "Check", "check" ], [ "Credit card", "cc" ], [ "Purchase
> order", "po" ] ]
> pt.map  {|value| value}
> 
> Displays the 2D Array elements as expected => [["Check", "check"],
> ["Credit card", "cc"], ["Purchase order", "po"]]
> 
> but a two-parameter block suppled to map like below:
> 
>>> pt.map  {|disp, value| value} 
> => ["check", "cc", "po"]
> 
> is able to give me the second element of the 2D array.
> 
> i looked in the ruby doc and on the net for examples, but 2D arrays are
> typically not covered.
> 
> while, i love this functionality, my question is -
> 
> How does this 2 parameter variant really work?
> 
> How does the array.map "figure out" and pass disp to param1 and value to
> param2?

It doesn't really have to do anything at all:

irb(main):001:0> def foo() yield 1,2 end
=> nil
irb(main):002:0> foo {|a| p a}
(irb):2: warning: multiple values for a block parameter (2 for 1)
         from (irb):1
[1, 2]
=> nil
irb(main):003:0> foo {|*a| p a}
[1, 2]
=> nil
irb(main):004:0> foo {|a,b| p a}
1
=> nil
irb(main):005:0> def foo() yield [1,2] end
=> nil
irb(main):006:0> foo {|a| p a}
[1, 2]
=> nil
irb(main):007:0> foo {|*a| p a}
[[1, 2]]
=> nil
irb(main):008:0> foo {|a,b| p a}
1
=> nil

You see in line 6 and 8 how it works.

However, map could also evaluate the block's arity:

irb(main):009:0> def foo(&b) p b.arity; b[1,2] end
=> nil
irb(main):010:0> foo {|a| p a}
1
(irb):10: warning: multiple values for a block parameter (2 for 1)
         from (irb):9
[1, 2]
=> nil
irb(main):011:0> foo {|a,b| p a}
2
1
=> nil
irb(main):012:0> foo {|*a| p a}
-1
[1, 2]
=> nil
irb(main):013:0> def foo(&b) p b.arity; b[[1,2]] end
=> nil
irb(main):014:0> foo {|a| p a}
1
[1, 2]
=> nil
irb(main):015:0> foo {|a,b| p a}
2
1
=> nil
irb(main):016:0> foo {|*a| p a}
-1
[[1, 2]]
=> nil

HTH

	robert