From: Josh Cheek <josh.cheek@...>
Date: 2010-01-18T11:42:50+09:00
Subject: Re: basic question: passing a modifiable argument to a routine

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On Sun, Jan 17, 2010 at 7:00 PM, lalawawa <usenet@ccjj.info> wrote:

> Arguments to subroutines seem to be passed by value in ruby
>
> $ 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>
>

They are actually pass by reference, as in Java and C++, or arrays in C.

For example
def woof(a)
  a << "e"
end

a = "bcd"
woof(a)
a # => "bcde"

Your example does not reflect this, because of two issues.

The first is that numbers are immutable, so even though you can modify the
objects themselves, you can't modify numbers.
The second is that += does not modify the object itself, it modifies the
label. It is the same as saying a=a+1
This is because of the first reason, since they are immutable you must do
a=a+1 instead of a++

On Sun, Jan 17, 2010 at 7:00 PM, lalawawa <usenet@ccjj.info> wrote:

> so how do I pass a variable to a routine if I want the routine to
> modify the variable.  In C I would say:
>
> void woof(int *a) {
>    *a += 2;
> }
> int b = 5;
> woof(&b);
> printf("%d\n", b);
>
> would output 7.
>

Honestly, I don't ever really find myself in situations where I need to do
this, you may need to rethink your approach.
Regardless, I suppose I can think of three alternatives.

Alternative 1 is to use an instance variable, don't do this trivially, they
will pollute your namespace.
def woof
  @a+=2
end

@a = 2
woof
@a # => 4


Alternative 2 is to have the method return the new value ie, it seems a
little messy, though, since it requires code outside to know that it needs
to assign results in some given manner.

def woof(a)
  a+2
end

a = 2
a = woof(a)
a # => 4


Alternative 3 is to wrap the variable in a mutable object, like Java's
Integer (except that isn't mutable either :P), but I can't seem to come up
with a way of doing this that I don't hate.



If you care to show your example, we might be able to spot a more Ruby
approach.

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