# Dynamic variables in blocks vs constants

**URL:** <https://www.ruby-forum.com/t/dynamic-variables-in-blocks-vs-constants/194602>\
**Category:** Ruby\
**Created:** [September 7, 2010, 7:26pm UTC](https://www.ruby-forum.com/t/dynamic-variables-in-blocks-vs-constants/194602 "2010-09-07T19:26:04Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![jdarklin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/a9a28c/32.png) [@jdarklin](https://www.ruby-forum.com/u/jdarklin)\
**Post date:** [September 7, 2010, 7:26pm UTC](https://www.ruby-forum.com/t/dynamic-variables-in-blocks-vs-constants/194602/1 "2010-09-07T19:26:04Z")

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Hi, I know that dynamic variables created in block it is not accessible  
from outside of that block but I noticed that dynamic constants can be  
accessed and I want to know why it is so.

Sorry for my bad english. For better understanding of my problem I’m  
attaching code below.

%w[magic MAGIC].each do |x|  
eval \<\<-RUBY  
#{x}\_dynamic = 1  
RUBY  
end

And now you can try…

magic\_dynamic  
MAGIC\_dynamic

You can observe it using 1.9 version of ruby.

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<div class="post-metadata">

**Author:** ![jdarklin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/a9a28c/32.png) [@jdarklin](https://www.ruby-forum.com/u/jdarklin)\
**Post date:** [September 7, 2010, 8:35pm UTC](https://www.ruby-forum.com/t/dynamic-variables-in-blocks-vs-constants/194602/2 "2010-09-07T20:35:33Z")

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On Sep 7, 2010, at 1:26 PM, Krzysztof Loch wrote:

> Hi, I know that dynamic variables created in block it is not accessible  
> from outside of that block but I noticed that dynamic constants can be  
> accessed and I want to know why it is so.

Because constants are always defined in reference to a module and not  
in reference to the execution stack/context:

$ irb  
ruby-1.9.2-p0 \> module MyLibrary  
ruby-1.9.2-p0 ?\> MyConstant = 3  
ruby-1.9.2-p0 ?\> 1.times {  
ruby-1.9.2-p0 \> MyOtherConstant = 4  
ruby-1.9.2-p0 ?\> }  
ruby-1.9.2-p0 ?\> end  
=\> 1  
ruby-1.9.2-p0 \>  
ruby-1.9.2-p0 \> p MyLibrary.constants  
[:MyConstant, :MyOtherConstant]  
=\> [:MyConstant, :MyOtherConstant]  
ruby-1.9.2-p0 \>

If there is no explicit module in scope then Ruby tucks constants  
away in the class Object (classes are also modules in Ruby’s object  
model)

$ irb  
ruby-1.9.2-p0 \> Object::Foo  
NameError: uninitialized constant Foo  
from (irb):1  
from irb:17:in `’  
ruby-1.9.2-p0 \> Foo = 3  
=\> 3  
ruby-1.9.2-p0 \> Object::Foo  
=\> 3  
ruby-1.9.2-p0 \>

Constants defined in Object are also accessible via at Ruby’s top level  
scope:

ruby-1.9.2-p0 \> Object::PI = 3.1415927  
=\> 3.1415927  
ruby-1.9.2-p0 \> PI  
=\> 3.1415927  
ruby-1.9.2-p0 \>

Gary

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<div class="post-metadata">

**Author:** ![jdarklin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/a9a28c/32.png) [@jdarklin](https://www.ruby-forum.com/u/jdarklin)\
**Post date:** [September 7, 2010, 8:39pm UTC](https://www.ruby-forum.com/t/dynamic-variables-in-blocks-vs-constants/194602/3 "2010-09-07T20:39:48Z")

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Thank you Gary for clarification.

Best regards!

Gary W. wrote:

> …  
> Gary