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I have a couple of variables, like:

my $thing_added = "test1";
my $otherthing_added = "test2";


I also have an array:

@array = ("thing", "otherthing", "no_such_variable");


Now, I would like to go through every element in the array, and check if
there is a corresponding "<array_element>_added" variable.


For example, for "thing", I would like to check if there is a
$thing_added variable. This should evaluate true, as I defined that
variable earlier.

Similarly, for "otherthing", I would like to check if there is a
$otherthing_added variable. This one should also evaluate true.

For "no_such_variable" element, a check for $no_such_variable_added
should evaluate false, as this variable was not defined.


I tried using "eval" to construct a variable out of yet another variable
value and a "_added" word, but haven't been successful.

Any hints?


--
Tomasz Chmielewski
http://wpkg.org

Tomasz Chmielewski wrote:
> I have a couple of variables, like:
>
> my $thing_added = "test1";
> my $otherthing_added = "test2";
>
>
> I also have an array:
>
> @array = ("thing", "otherthing", "no_such_variable");
>
>
> Now, I would like to go through every element in the array, and check if
> there is a corresponding "<array_element>_added" variable.
>
>
> For example, for "thing", I would like to check if there is a
> $thing_added variable. This should evaluate true, as I defined that
> variable earlier.
>
> Similarly, for "otherthing", I would like to check if there is a
> $otherthing_added variable. This one should also evaluate true.
>
> For "no_such_variable" element, a check for $no_such_variable_added
> should evaluate false, as this variable was not defined.
>
>
> I tried using "eval" to construct a variable out of yet another variable
> value and a "_added" word, but haven't been successful.
>
> Any hints?

Every time you want to use variables as part of variable names, you're
better off using a hash.

Usually you save your life by using a hash as you usually get stoned to
death by asking these kinds of questions here.

Usually,

Josef

--
These are my personal views and not those of Fujitsu Siemens Computers!
Josef Möllers (Pinguinpfleger bei FSC)
If failure had no penalty success would not be a prize (T. Pratchett)
Company Details: http://www.fujitsu-siemens.com/imprint.html

Quoth Tomasz Chmielewski <>:
> I have a couple of variables, like:
>
> my $thing_added = "test1";
> my $otherthing_added = "test2";
>
>
> I also have an array:
>
> @array = ("thing", "otherthing", "no_such_variable");
>
>
> Now, I would like to go through every element in the array, and check if
> there is a corresponding "<array_element>_added" variable.

perldoc -q "variable name" explains why this is a bad idea (it's
actually impossible for 'my' variables, short of using deep magic like
PadWalker). What are you actually trying to do?

Ben

--
We do not stop playing because we grow old;
we grow old because we stop playing.

Tomasz Chmielewski wrote:
> I have a couple of variables, like:
>
> my $thing_added = "test1";
> my $otherthing_added = "test2";
>
>
> I also have an array:
>
> @array = ("thing", "otherthing", "no_such_variable");
>
>
> Now, I would like to go through every element in the array, and check if
> there is a corresponding "<array_element>_added" variable.
>
>
> For example, for "thing", I would like to check if there is a
> $thing_added variable. This should evaluate true, as I defined that
> variable earlier.
>
> Similarly, for "otherthing", I would like to check if there is a
> $otherthing_added variable. This one should also evaluate true.
>
> For "no_such_variable" element, a check for $no_such_variable_added
> should evaluate false, as this variable was not defined.
>
>
> I tried using "eval" to construct a variable out of yet another variable
> value and a "_added" word, but haven't been successful.
>
> Any hints?
>
>
$ perl -e '$x=foo; $foo=1; print defined($$x)?"Yes\n":"No\n"'
Yes

$ perl -e '$x=foo; $foo=0; print defined($$x)?"Yes\n":"No\n"'
Yes

$ perl -e '$x=foo; $bar=0; print defined($$x)?"Yes\n":"No\n"'
No

But IMHO you really really should be using hashes. Something not
entirely unlike:

my $thingies{thing_added} = "test1";
my $thingies{otherthing_added} = "test2";
...
for (@array) {
if (defined($thingies{$_})) {
...
}
}
(untested - caveat emptor)

I'm sure there's a FAQ that advises against the $$x kind of crazy
foolishness.

--
RGB

Quoth RedGrittyBrick <>:
>
> $ perl -e '$x=foo; $foo=1; print defined($$x)?"Yes\n":"No\n"'
> Yes

~% perl -le'
use strict;
my $x = "foo";
my $foo = 1;
print defined($$x) ? "Yes" : "No";'
Can't use string ("foo") as a SCALAR ref while "strict refs" in use
at -e line 5

~% perl -le'
my $x = "foo";
my $foo = 1;
print defined($$x) ? "Yes" : "No";'
No

> I'm sure there's a FAQ that advises against the $$x kind of crazy
> foolishness.

perldoc -q "variable name"

There's also 'use strict', which forbids it entirely.

Ben

--
I've seen things you people wouldn't believe: attack ships on fire off
the shoulder of Orion; I watched C-beams glitter in the dark near the
Tannhauser Gate. All these moments will be lost, in time, like tears in rain.
Time to die.

Josef Moellers schrieb:
> Tomasz Chmielewski wrote:

(...)

>> Any hints?
>
> Every time you want to use variables as part of variable names, you're
> better off using a hash.
>
> Usually you save your life by using a hash as you usually get stoned to
> death by asking these kinds of questions here.

Ha ha, I take it as a good advice for the future then


--
Tomasz Chmielewski
http://wpkg.org

Ben Morrow schrieb:
> Quoth Tomasz Chmielewski <>:
>> I have a couple of variables, like:
>>
>> my $thing_added = "test1";
>> my $otherthing_added = "test2";
>>
>>
>> I also have an array:
>>
>> @array = ("thing", "otherthing", "no_such_variable");
>>
>>
>> Now, I would like to go through every element in the array, and check if
>> there is a corresponding "<array_element>_added" variable.
>
> perldoc -q "variable name" explains why this is a bad idea (it's
> actually impossible for 'my' variables, short of using deep magic like
> PadWalker). What are you actually trying to do?

I have a couple of such "ifs", and they differ only slightly:


if ($product_id_added == 0 && length $product_id) {
&add_params("product_id", $product_id, $lun, $driver);
}


if ($vendor_id_added == 0 && length $vendor_id) {
&add_params("vendor_id", $vendor_id, $lun, $driver);
}



I thought of replacing these all "ifs" which differ only slightly by
going through array elements, and replacing each part.

my @array = ("product_id", "vendor_id", ...)

But it looks like I have to rethink that approach.


--
Tomasz Chmielewski
http://wpkg.org

Tomasz Chmielewski <> wrote:
>I have a couple of variables, like:
>my $thing_added = "test1";
>my $otherthing_added = "test2";
>
>I also have an array:
>@array = ("thing", "otherthing", "no_such_variable");
>
>Now, I would like to go through every element in the array, and check if
>there is a corresponding "<array_element>_added" variable.
[...]
>Any hints?

Short answer: don't do that, it is A Bad Idea (TM).
Longer answer: you are looking for symbolic references. Please see the
history of this NG as well as the FAQ ("variable as a variable name")
why that is not a good idea.
Yet longer answer: Use a hash, dude, that's what they are for.

my %added;
$added{'thing'} = 'test1';
$added{'otherthing'} = 'test2';
.....
if (exists($added{'thing'})) {
print "Someone has added 'thing' to \%added";
}

jue

Tomasz Chmielewski <> writes:

> I have a couple of such "ifs", and they differ only slightly:
>
>
> if ($product_id_added == 0 && length $product_id) {
> &add_params("product_id", $product_id, $lun, $driver);
> }
>
>
> if ($vendor_id_added == 0 && length $vendor_id) {
> &add_params("vendor_id", $vendor_id, $lun, $driver);
> }
>
>
>
> I thought of replacing these all "ifs" which differ only slightly by
> going through array elements, and replacing each part.
>
> my @array = ("product_id", "vendor_id", ...)
>
> But it looks like I have to rethink that approach.

You should. If you choose the right datastructure, you can probably
reduce the amount of code a lot more than if you "just" use symbolic
references here and there.

Also, it looks like you're using 2 flags to mark if something was
added. You won't need to if you're using a hash:

my %added;

# add a vendor id
$added{vendor} = 2;

# don't do anything with products

for my $key (qw(vendor products foo)) {
next unless exists $added{$key};
add_params("${key}_id",$added{$key},...)
}

also: IMHO it's bad style to use the &foo() construct to call
functions. foo() is a little shorted and has less potential problems.


--
Joost Diepenmaat | blog: http://joost.zeekat.nl/ | work: http://zeekat.nl/

Joost Diepenmaat schrieb:
> Tomasz Chmielewski <> writes:
>
>> I have a couple of such "ifs", and they differ only slightly:
>>
>>
>> if ($product_id_added == 0 && length $product_id) {
>> &add_params("product_id", $product_id, $lun, $driver);
>> }
>>
>>
>> if ($vendor_id_added == 0 && length $vendor_id) {
>> &add_params("vendor_id", $vendor_id, $lun, $driver);
>> }
>>
>>
>>
>> I thought of replacing these all "ifs" which differ only slightly by
>> going through array elements, and replacing each part.
>>
>> my @array = ("product_id", "vendor_id", ...)
>>
>> But it looks like I have to rethink that approach.
>
> You should. If you choose the right datastructure, you can probably
> reduce the amount of code a lot more than if you "just" use symbolic
> references here and there.
>
> Also, it looks like you're using 2 flags to mark if something was
> added. You won't need to if you're using a hash:
>
> my %added;
>
> # add a vendor id
> $added{vendor} = 2;
>
> # don't do anything with products
>
> for my $key (qw(vendor products foo)) {
> next unless exists $added{$key};
> add_params("${key}_id",$added{$key},...)
> }

Thanks for the tip.


> also: IMHO it's bad style to use the &foo() construct to call
> functions. foo() is a little shorted and has less potential problems.

Besides being "bad style", what potential problems can it make? (code
readability? something else?)


--
Tomasz Chmielewski
http://wpkg.org