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comp.lang.ruby

Dynamic Variables

Marc Heiler

7/1/2008 9:46:00 AM

Is there any way in ruby to create dynamic variables?

counter = 52
box#{counter} = "cat and dogs"

There would be then a new variable
box52

Note that this is not a question whether someone should do it or not -
this is solely whether it is doable or not. Until today I thought it is
somehow possible with eval but I failed, so I assume it is not possible.
--
Posted via http://www.ruby-....
11 Answers

Igal Koshevoy

7/1/2008 9:52:00 AM

0

Marc Heiler wrote:
> Is there any way in ruby to create dynamic variables?
>
> counter = 52
> box#{counter} = "cat and dogs"
>
> There would be then a new variable
> box52
>
> Note that this is not a question whether someone should do it or not -
> this is solely whether it is doable or not. Until today I thought it is
> somehow possible with eval but I failed, so I assume it is not possible.
>
irb(main):010:0> counter = 52
=> 52
irb(main):011:0> eval "box#{counter} = 'cat and dogs'"
=> "cat and dogs"
irb(main):012:0> box52
=> "cat and dogs"

-igal

Calamitas

7/1/2008 10:12:00 AM

0

On Tue, Jul 1, 2008 at 11:52 AM, Igal Koshevoy <igal@pragmaticraft.com> wrote:
> Marc Heiler wrote:
>>
>> Is there any way in ruby to create dynamic variables?
>>
>> counter = 52
>> box#{counter} = "cat and dogs"
>>
>> There would be then a new variable
>> box52
>>
>> Note that this is not a question whether someone should do it or not -
>> this is solely whether it is doable or not. Until today I thought it is
>> somehow possible with eval but I failed, so I assume it is not possible.
>>
>
> irb(main):010:0> counter = 52
> => 52
> irb(main):011:0> eval "box#{counter} = 'cat and dogs'"
> => "cat and dogs"
> irb(main):012:0> box52
> => "cat and dogs"

Now try the same thing in plain ruby without irb. Never trust irb on
things like these.

Dynamic variables as created by eval live in a separate, eval-specific
scope and can only be accessed through eval. This is a consequence of
Ruby deciding at "compile" time what's a local variable and what's
not. So, if you have code like this:

eval "a = 5"
p a

then in "p a", a is determined to be a method call before that code is
even run. The a is accessible through eval though:

eval "a = 5"
p eval("a")

Peter

Frederick Cheung

7/1/2008 10:12:00 AM

0


On 1 Jul 2008, at 10:52, Igal Koshevoy wrote:

> Marc Heiler wrote:
>> Is there any way in ruby to create dynamic variables?
>>
>> counter = 52
>> box#{counter} = "cat and dogs"
>>
>> There would be then a new variable
>> box52
>>
>> Note that this is not a question whether someone should do it or
>> not -
>> this is solely whether it is doable or not. Until today I thought
>> it is
>> somehow possible with eval but I failed, so I assume it is not
>> possible.
>>
> irb(main):010:0> counter = 52
> => 52
> irb(main):011:0> eval "box#{counter} = 'cat and dogs'"
> => "cat and dogs"
> irb(main):012:0> box52
> => "cat and dogs"
>
Although that doesn't work outside of irb. (although you can change
the value of an existing local variable like that)
See also http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/a954f8aaf698a0b9/e1c761b71b63b82e?#e1c761...

Fred

Igal Koshevoy

7/1/2008 12:00:00 PM

0

Frederick Cheung wrote:
>
> On 1 Jul 2008, at 10:52, Igal Koshevoy wrote:
>
>> irb(main):010:0> counter = 52
>> => 52
>> irb(main):011:0> eval "box#{counter} = 'cat and dogs'"
>> => "cat and dogs"
>> irb(main):012:0> box52
>> => "cat and dogs"
>>
> Although that doesn't work outside of irb. (although you can change
> the value of an existing local variable like that)
> See also
> http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/a954f8aaf698a0b9/e1c761b71b63b82e?#e1c761...
>
>
Thanks for catching the mistake and providing the link.


So there really is no way to get the "box52" variable directly? It's
only available within the context of an eval?

When I run the code below directly (not from IRB), the interpreter is
clearly seeing the "box52" local variable and keeping its state around,
but won't let me access it without wrapping an eval around it, which
seems wrong:

counter = 52
p local_variables # => ["counter"]
eval "box#{counter} = 'cat and dogs'"
p local_variables # => ["counter", "box52"]
p eval("box52") # => "cat and dogs"
p box52 # => undefined local variable or method `box52' for main:Object
(NameError)


I say "seems wrong" because I'm used to the following behavior:

# Python
counter = 52
eval(compile('box%s = "cat and dogs"' % counter, '<string>', 'exec'))
print box52 # => "cat and dogs"

# Perl
$counter = 52;
eval("\$box$counter = 'cat and dogs'");
print $box52, "\n"; # => "cat and dogs"

-igal

Gregory Seidman

7/1/2008 12:50:00 PM

0

On Tue, Jul 01, 2008 at 06:45:48PM +0900, Marc Heiler wrote:
> Is there any way in ruby to create dynamic variables?
>
> counter = 52
> box#{counter} = "cat and dogs"
>
> There would be then a new variable
> box52
>
> Note that this is not a question whether someone should do it or not -
> this is solely whether it is doable or not. Until today I thought it is
> somehow possible with eval but I failed, so I assume it is not possible.

I'm going to give you exactly what you didn't ask for: why this is a bad
idea. This is a discussion about Perl rather than Ruby, but it remains
relevant: http://perl.plover.com/varva...

See the subsequent parts of the series for further discussion.

--Greg

Martin Boese

7/1/2008 2:17:00 PM

0

'Object' has a method to define instance variables:

irb(main):001:0> counter = 52
irb(main):002:0> self.instance_variable_set("@box#{counter}", 'cat and dogs')
=> "cat and dogs"
irb(main):003:0> @box52
=> "cat and dogs"


martin


On Tuesday 01 July 2008 10:45:48 Marc Heiler wrote:
> Is there any way in ruby to create dynamic variables?
>
> counter = 52
> box#{counter} = "cat and dogs"
>
> There would be then a new variable
> box52
>
> Note that this is not a question whether someone should do it or not -
> this is solely whether it is doable or not. Until today I thought it is
> somehow possible with eval but I failed, so I assume it is not possible.

stephen O'D

7/1/2008 3:40:00 PM

0

On Jul 1, 1:50 pm, Gregory Seidman <gsslist+r...@anthropohedron.net>
wrote:
> On Tue, Jul 01, 2008 at 06:45:48PM +0900, Marc Heiler wrote:
> > Is there any way in ruby to create dynamic variables?
>
> > counter = 52
> > box#{counter} = "cat and dogs"
>
> > There would be then a new variable
> > box52
>
> > Note that this is not a question whether someone should do it or not -
> > this is solely whether it is doable or not. Until today I thought it is
> > somehow possible with eval but I failed, so I assume it is not possible.
>
> I'm going to give you exactly what you didn't ask for: why this is a bad
> idea. This is a discussion about Perl rather than Ruby, but it remains
> relevant:http://perl.plover.com/varva...
>
> See the subsequent parts of the series for further discussion.
>
> --Greg

Here here - I remember reading that article way back - the basic point
of it is that if you want to create a variable on the fly, it probably
means you should be using a Hash.

Charles Oliver Nutter

7/1/2008 4:20:00 PM

0

Igal Koshevoy wrote:
> # Python
> counter = 52
> eval(compile('box%s = "cat and dogs"' % counter, '<string>', 'exec'))
> print box52 # => "cat and dogs"
>
> # Perl
> $counter = 52;
> eval("\$box$counter = 'cat and dogs'");
> print $box52, "\n"; # => "cat and dogs"

In python, local variables are not (necessarily) determined at
parse/compile time. They're slots in a local scope that can grow later on.

In Ruby, this behavior only exists for eval scopes. Ruby 1.8 shares a
single eval scope under any given normal scope across all evals in that
normal scope. Ruby 1.9 instantiates a new eval scope for each eval.

I don't know what Perl does.

- Charlie

Igal Koshevoy

7/1/2008 4:48:00 PM

0

Charles Oliver Nutter wrote:
> Igal Koshevoy wrote:
>> # Python
>> counter = 52
>> eval(compile('box%s = "cat and dogs"' % counter, '<string>', 'exec'))
>> print box52 # => "cat and dogs"
>>
>> # Perl
>> $counter = 52;
>> eval("\$box$counter = 'cat and dogs'");
>> print $box52, "\n"; # => "cat and dogs"
>
> In python, local variables are not (necessarily) determined at
> parse/compile time. They're slots in a local scope that can grow later
> on.
>
> In Ruby, this behavior only exists for eval scopes. Ruby 1.8 shares a
> single eval scope under any given normal scope across all evals in
> that normal scope. Ruby 1.9 instantiates a new eval scope for each eval.
>
> I don't know what Perl does.
Charles, thanks for the clarification about the scoping behaviors.


I suppose if I really want to pretend like I was dynamically injecting a
local variable into a local scope, and a class/instance variable didn't
weren't desirable, then I could resort to:

def set(name, value)
self.class.send(:attr_accessor, name)
self.send("#{name}=", value)
end

counter = 52
set "box#{counter}", "cat and dogs"
p box52 # => "cat and dogs"

-igal

Charles Oliver Nutter

7/1/2008 7:58:00 PM

0

Igal Koshevoy wrote:
> I suppose if I really want to pretend like I was dynamically injecting a
> local variable into a local scope, and a class/instance variable didn't
> weren't desirable, then I could resort to:
>
> def set(name, value)
> self.class.send(:attr_accessor, name)
> self.send("#{name}=", value)
> end
>
> counter = 52
> set "box#{counter}", "cat and dogs"
> p box52 # => "cat and dogs"

Yup, that would work except that for an attr_accessor on self you need
to qualify it with self.box52...so it wouldn't be quite seamless.

- Charlie