From: Marnen Laibow-Koser <marnen@...>
Date: 2010-01-18T11:33:17+09:00
Subject: Re: basic question: passing a modifiable argument to a routi

lalawawa wrote:
> Arguments to subroutines 

They're called functions or methods in Ruby.

> seem to be passed by value in ruby

Nope.  They're passed by reference -- or rather, they're object 
references passed by value, like in Java.

> 
> $ irb
> irb(main):001:0> def woof(a)
> irb(main):002:1>   a += 2
> irb(main):003:1> end
> => nil
> irb(main):004:0> b = 5
> => 5
> irb(main):005:0> woof(b)
> => 7
> irb(main):006:0> b
> => 5
> irb(main):007:0>
>

Yes, because += makes a reference to a different object.


> so how do I pass a variable to a routine if I want the routine to
> modify the variable.

Don't.  Instead, assign the return value to the variable, or use a bang! 
method on an object.


>  In C I would say:
> 
> void woof(int *a) {
>     *a += 2;
> }
> int b = 5;
> woof(&b);
> printf("%d\n", b);
> 
> would output 7.

b = woof(b)

Best,
-- 
Marnen Laibow-Koser
http://www.marnen.org
marnen@marnen.org
-- 
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