question: referencing a variable with a variable

[knohr]

I come from a perl background. I have gotten in the habit of using
references to variables instead of specifically calling said variable/
class/whatever.

Basically, I am wondering how I would translate the following into
ruby.

example 1:
sub my_sub
{
$choice = shift;
$choices="this|that|foo|bar";
if ( $choice =~ m/($choices)/ ) {&$choice}
else {die "some witty msg"}
}

example 2:

$this = 'var_name';
$var_name = 'i want this to print';
print $$var_name . "\n";

thanks

 

[knohr]

I come from a perl background. I have gotten in the habit of using
references to variables instead of specifically calling said variable/
class/whatever.

Basically, I am wondering how I would translate the following into
ruby.

example 1:
sub my_sub
{
$choice = shift;
$choices="this|that|foo|bar";
if ( $choice =~ m/($choices)/ ) {&$choice}
else {die "some witty msg"}
}

example 2:

$this = 'var_name';
$var_name = 'i want this to print';
print $$var_name . "\n";

thanks

s/print $$var_name/print $$this/

 

[Phrogz]

I come from a perl background. I have gotten in the habit of using
references to variables instead of specifically calling said variable/
class/whatever.

In Ruby, a variable is a reference to an object. While you can create
multiple references to the same object, you cannot create a reference
to a variable. You can create an object that refers to another object:

foo = { bj => [1,2,3] } # Hash with a key that points to an array
bar = { bj => foo } # pointing to the other hash

....but you can still swap out what the variable foo points to and
the bj key in bar will not be updated.


You can, however, look up instance and class variables by name:
@foo1 = "whee"
@foo2 = "la"
answer = '@foo1'
puts instance_variable_get( answer )


I think that's about as close as you're going to get (without using
eval).

 

[misaka]

I come from a perl background. I have gotten in the habit of using
references to variables instead of specifically calling said variable/
class/whatever.

I don't think that symbolic references directly supported in Ruby, but
as Phrogz pointed out there are certain methods to access variables
and methods by using a string representation of their name.

Basically, I am wondering how I would translate the following into
ruby.

example 1:
sub my_sub
{
$choice = shift;
$choices="this|that|foo|bar";
if ( $choice =~ m/($choices)/ ) {&$choice}
else {die "some witty msg"}
}

This one is easy, there's a "send" method that let's you call a method
using a string containing it's name. Here's an abbreviated version of
what you have above:

def my_sub( $choice )
if $choice =~ /foo|bar/
send( $choice )
end
end

example 2:

$this = 'var_name';
$var_name = 'i want this to print';
print $$var_name . "\n";

Looks like this would probably need an eval:

this = 'var_name'
var_name = 'i want this to print'
puts( eval( this ) )

There are methods to list local variables 'local_variables', and
return instance variables, but there isn't really anything to actually
return the value of a local variable so far as I can see.


--Misha

 

[Erik Hollensbe]

I come from a perl background. I have gotten in the habit of using
references to variables instead of specifically calling said variable/
class/whatever.

Basically, I am wondering how I would translate the following into
ruby.

example 1:
sub my_sub
{
$choice = shift;
$choices="this|that|foo|bar";
if ( $choice =~ m/($choices)/ ) {&$choice}
else {die "some witty msg"}
}

example 2:

$this = 'var_name';
$var_name = 'i want this to print';
print $$var_name . "\n";

thanks

Hi Knohr.

There are no soft references in ruby. However, you can use symbols to
accomplish most of what you're trying to do:

choice = "foo"
if choice =~ /foo|bar|baz/
method(choice.to_sym).call
end

See Object#*_variable_get for ways to get at variables.

 

[Robert Dober]

I come from a perl background. I have gotten in the habit of using
references to variables instead of specifically calling said variable/
class/whatever.

Basically, I am wondering how I would translate the following into
ruby.

example 1:
sub my_sub
{
$choice = shift;
$choices="this|that|foo|bar";
if ( $choice =~ m/($choices)/ ) {&$choice}

Sorry if I am asking something stupid here but my $perl days are
long gone now.
You are matching the reference of a sub with a rgexp and then call the
sub you are holding the reference of???

Assuming that you meant something like
if (whatever) {&$choice}
you will write
if whatever choice.call

The interesting difference will be the call of my_sub, which I have to
call my_def in Ruby .

def my_def callable

#now callable could be a lambda
my_def lambda{ puts 42 }
#a method
s="*" * 42
m = s.methodsize)
my_def m

But very probably you want to use something more Rubish.
def my_def ...
yield if something # with many variations available
Proc.new.call if something

The next approach -- my preferred -- will give you a reference to the
block - a little bit like an anonymous sub in perl.

def my_def ..., &blk
blk.call if something
end

example 2:

$this = 'var_name';
$var_name = 'i want this to print';
print $$var_name . "\n";

thanks

this = "var_name"
var_name = "I want #{this} to print"
puts var_name

I am aware of the conceptional difference -- and that was nicely
explained already - but maybe that is all you need

HTH
Robert