# How to modify the param value in block

**URL:** <https://www.ruby-forum.com/t/how-to-modify-the-param-value-in-block/134186>\
**Category:** Ruby\
**Created:** [April 8, 2008, 6:08pm UTC](https://www.ruby-forum.com/t/how-to-modify-the-param-value-in-block/134186 "2008-04-08T18:08:16Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![infinit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/76d3ee/32.png) [@infinit](https://www.ruby-forum.com/u/infinit)\
**Post date:** [April 8, 2008, 6:08pm UTC](https://www.ruby-forum.com/t/how-to-modify-the-param-value-in-block/134186/1 "2008-04-08T18:08:16Z")

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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.

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<div class="post-metadata">

**Author:** ![infinit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/76d3ee/32.png) [@infinit](https://www.ruby-forum.com/u/infinit)\
**Post date:** [April 8, 2008, 6:15pm UTC](https://www.ruby-forum.com/t/how-to-modify-the-param-value-in-block/134186/2 "2008-04-08T18:15:34Z")

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On Apr 8, 12:08 pm, Infinit B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> the output is:  
> 2  
> 1
> 
> How can I modify the parameter x in block?
> 
> ## Appreciate for your help.
> 
> Posted viahttp://www.ruby-forum.com/.

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

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<div class="post-metadata">

**Author:** ![infinit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/76d3ee/32.png) [@infinit](https://www.ruby-forum.com/u/infinit)\
**Post date:** [April 8, 2008, 6:21pm UTC](https://www.ruby-forum.com/t/how-to-modify-the-param-value-in-block/134186/3 "2008-04-08T18:21:00Z")

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> 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.

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<div class="post-metadata">

**Author:** ![infinit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/76d3ee/32.png) [@infinit](https://www.ruby-forum.com/u/infinit)\
**Post date:** [April 8, 2008, 6:31pm UTC](https://www.ruby-forum.com/t/how-to-modify-the-param-value-in-block/134186/4 "2008-04-08T18:31:29Z")

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On Apr 8, 2008, at 6:08 PM, Infinit B. wrote:

> the output is:  
> 2  
> 1
> 
> How can I modify the parameter x in block?
> 
> ## Appreciate for your help.
> 
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

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 G.  
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<div class="post-metadata">

**Author:** ![infinit](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/76d3ee/32.png) [@infinit](https://www.ruby-forum.com/u/infinit)\
**Post date:** [April 9, 2008, 5:18am UTC](https://www.ruby-forum.com/t/how-to-modify-the-param-value-in-block/134186/5 "2008-04-09T05:18:57Z")

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> 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 G.

Got it. Thanks for your great inside explanation!