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comp.lang.ruby

How to modify the param value in block

Infinit Blue

4/8/2008 4:08:00 PM

For example I hv the following code:

def test
n = 1
yield(n)
puts n
end

--------
test {|x| x = x + 1; puts x}

the output is:
2
1

How can I modify the parameter x in block?

Appreciate for your help.
--
Posted via http://www.ruby-....
4 Answers

Brian Adkins

4/8/2008 4:15:00 PM

0

On Apr 8, 12:08 pm, Infinit Blue <infinitb...@gmail.com> wrote:
> For example I hv the following code:
>
> def test
> n = 1
> yield(n)
> puts n
> end
>
> --------
> test {|x| x = x + 1; puts x}
>
> the output is:
> 2
> 1
>
> How can I modify the parameter x in block?
>
> Appreciate for your help.
> --
> Posted viahttp://www.ruby-....

brian@airstream:~$ irb
irb(main):001:0> def test
irb(main):002:1> n = 1
irb(main):003:1> n = yield(n)
irb(main):004:1> puts n
irb(main):005:1> end
=> nil
irb(main):006:0> test {|x| x += 1; puts x; x }
2
2
=> nil

Infinit Blue

4/8/2008 4:21:00 PM

0


>
> brian@airstream:~$ irb
> irb(main):001:0> def test
> irb(main):002:1> n = 1
> irb(main):003:1> n = yield(n)
> irb(main):004:1> puts n
> irb(main):005:1> end
> => nil
> irb(main):006:0> test {|x| x += 1; puts x; x }
> 2
> 2
> => nil

how can I force x pass by ref ? then I can change the value of x in the
block and no need to hv a return value and assign statement.
--
Posted via http://www.ruby-....

Florian Gilcher

4/8/2008 4:30:00 PM

0

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Hash: SHA1


On Apr 8, 2008, at 6:08 PM, Infinit Blue wrote:
> For example I hv the following code:
>
> def test
> n = 1
> yield(n)
> puts n
> end
>
> --------
> test {|x| x = x + 1; puts x}
>
> the output is:
> 2
> 1
>
> How can I modify the parameter x in block?
>
> Appreciate for your help.
> --
> Posted via http://www.ruby-....
>

Hi,

actually, something like this code would work if you would use a
'real' object:

- -----
class A
attr_accessor :name
end

def test
a = A.new
a.name = "foo"
yield(a)
puts a.inspect
end

test {|x| x.name = "bar" }
#=> a.name is "bar"
- -----


But, if you reassign b with a new object, this will not work:

- -----
class A
attr_accessor :name
end

def test
a = A.new
a.name = "foo"
yield(a)
puts a.inspect
end
test {|x| x = A.new; x.name = "bar" }
- -----

The first example changes x "in place", while the second operates on a
totally different x.

Fixnums in Ruby are so called "immediate objects" and do show
subtle differences in their behaviour. They are immutable (there
is only one 1), so 1+1 is a different object. This makes it impossible
to change the value of a Fixnum in place. (This is - by the way - the
reason why i++ does not work in ruby)

As you are not able to do that, it is impossible to "modify"
a fixnum parameter. (because you only option is reassigning)

Because of this, i would not talk about "modifying the parameter" but
about
"modifying the internal state of a parameter".

Regards,
Florian Gilcher
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Infinit Blue

4/9/2008 3:19:00 AM

0


>
> As you are not able to do that, it is impossible to "modify"
> a fixnum parameter. (because you only option is reassigning)
>
> Because of this, i would not talk about "modifying the parameter" but
> about
> "modifying the internal state of a parameter".
>
> Regards,
> Florian Gilcher

Got it. Thanks for your great inside explanation!
--
Posted via http://www.ruby-....