From: 7stud -- <bbxx789_05ss@...>
Date: 2009-02-15T03:23:48+09:00
Subject: Re: Code syntax comma usage

Jcm Mz wrote:
> That I can see; Pragmatic Programming Ruby's, index, has reference to
> all sorts of symbols, but I can't find a explanation for a ',' seperator
> such as used in the following:
> 
>  var1 = 1; p var1 #=> 1
>  var2 = 2; p var2 #=> 2
>  var3 = 3; p var3 #=> 3
> 
>  var1, var2, var3 = 5
> 
>  p var1 #=> 5
>  p var2 #=> nil
>  p var3 #=> nil
> 
> What's happening here? 

It's called "parallel assignment".   Here is the basic case:

x, y = 10, 20
puts x   #10
puts y   #20


Then there are rules for dealing with the cases when the number of 
variables on the left side do not equal the number of values on the 
right side.  You are seeing the rule:  if there are more "lvalues" than 
"rvalues", the excess lvalues will have nil assigned to them.

One of the little tricks you can do with parallel assignment is switch 
values in two variables without using a third variable:

str = "hello"
num = 100
str, num = num, str

puts str   #100
puts num   #hello

Normally, if there are more rvalues than lvalues then the excess rvalues 
are discarded, for example:

x, y = 10, 20, 100, 200
puts x   #10
puts y   #20


But you can also do this:

x, y, *remaining = 10, 20, 100, 200

puts x         #10
puts y         #20
p remaining    #[100, 200]

And this is also another way to do parallel assignment:

x, y, z = [10, 20, 30]
puts x   #10
puts y   #20
puts z   #30

In pickaxe2, a detailed description of parallel assignment is on p. 340.

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