From: "David B. Williams" <david@...>
Date: 2007-09-30T22:56:24+09:00
Subject: Re: which language allows you to change an argument's value?

> And C, Python, and Ruby probably won't let you do that...

From my understanding, in Ruby all argument passing is by value. 
However, the value passed may not be what you expect...  The value that 
is passed is that of the reference, not the value that the reference 
refers to.  So, for example:

def foo(arg)
  arg = 3
end

a = 'Mike'

foo(a)

This will not change the value of a in the top level.  Why?  Because foo 
reassigns the argument reference arg to 3, a different object than 
'Mike.'  However:

def bar(arg)
  arg[0] = 'b'
end

a = 'Mike'

bar(a)

This will change a from 'Mike' to 'bike.'  Why?  Because instead of 
reassigning arg to a new object, the object itself ('Mike') has been 
modified through the method call [].  So, if the method simply reassigns 
the reference, there will be no change in the original (it still refers 
to the same object).  But, if the method modifies that object, the 
original reference will reflect that modification, since it is still 
referencing the now modified object.

It is customary in Ruby to append an exclamation mark (!) to indicate 
methods that actually modify an object in place.  So, bar! would have 
been a more appropriate name for the second method.

Hope that helps.

David B. Williams
http://www.cybersprocket.com
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