basic explanation on code

[Ronen Kfir]

#!/usr/bin/perl


%machines =(user1, 'booboo',
user2, 'puter',
user3, 'synta',
user4, 'synta');

for $key (keys %machines)
{
$val=$programs{$key}
if (!defined $supliers{$val})
{
$supliers{$val}=1;
}
else
{
print "duplicate value: $val\n";
print "duplicate value: $val\n";

Cannot understand what is the purpose of $key (keys %machines) ,
$programs{$key} and $supliers{$val} on this script.

Will be grateful to get some help!

R o n e n K f i r
System Administrator
CIT div. Tel Aviv University
Israel
Tel: 972-3-6407416
Fax: 972-3-6405158
cellular: 972-55-405910
E-mail: [email]ronenk@tauex.tau.ac.il[/email]

[Jan Eden]

Hi,

Ronen Kfir wrote:

>#!/usr/bin/perl
>
>
>%machines =(user1, 'booboo',
> user2, 'puter',
> user3, 'synta',
> user4, 'synta');
>
> for $key (keys %machines)
> {
> $val=$programs{$key}
> if (!defined $supliers{$val})
> {
> $supliers{$val}=1;
> }
> else
> {
> print "duplicate value: $val\n";
> print "duplicate value: $val\n";
>
>Cannot understand what is the purpose of $key (keys %machines) ,
>$programs{$key} and $supliers{$val} on this script.

³keys %machines" returns a list of the keys of the %machines hash, in this case

(user1, user 2, user3, user4)

There are two other hashes, %programs and %supliers. Obviously, %pograms should have the same set of keys as %machines.

Now, the for loop assigns the value of $programs{$key} to $val and checks of there is a value defined for $supliers{$val}. If that is the case, it prints the message ""duplicate value..." twice (funny idea), otherwise it assigns 1 to $supliers{$val}.

Now if you run this two or more times, you should always get the duplicate value message since you defined each $suplier{$val} on the first run.

- Jan
--
There's no place like ~/

[Jan Eden]

Hi,

Ronen Kfir wrote:

>Hi Jan,
>I am sorry...
>
>This is the right code:
>
>
>#!/usr/bin/perl
>
>%machines =(user1, 'booboo',
> user2, 'puter',
> user3, 'synta',
> user4, 'synta');
>
> for (keys %machines)
> {
> $val=$machines{$_};
> if (!defined $supliers{$val})
> {
> $supliers{$val}=1;
> # print "hi\n";
> }
> else
> {
> print "duplicate value: $val\n";
>
> }
> }
>
>Seems more reasonable now... What I cant comprehend now is the
>$supliers{$val part. What's inside it , & all the mechanism around
>its action.
>

The %supliers hash must have keys which equal the values of the %machines hash. So the hashes are "tied together" in a way.

The way this is written, the %supliers keys will be booboo, puter and synta. For each of these suppliers, you check if it's value is defined. The idea is to mark the supplier synta as duplicate: when you use the value of $machines{user3}, you assign 1 to $supliers{synta}. On a subsequent looping,$supliers{synta} will be defined, so you get the duplicate message.

Best,

Jan
--
If all else fails read the instructions. - Donald Knuth

[Ronen Kfir]

If I get you correctly the %supliers hash will look like this in this
script:
%supliers =(booboo =>'1',
Puter =>'1',
Synta => 'defined')
.... or am I wrong. I need to see the real print of this hash in order to
understand it...

Thanx a lot!



R o n e n K f i r
System Administrator
CIT div. Tel Aviv University
Israel
Tel: 972-3-6407416
Fax: 972-3-6405158
cellular: 972-55-405910
E-mail: [email]ronenk@tauex.tau.ac.il[/email]



-----Original Message-----
From: Jan Eden [mailto:lists@jan-eden.de]
Sent: Friday, February 13, 2004 4:39 PM
To: Ronen Kfir; Perl Lists
Subject: RE: basic explanation on code

Hi,

Ronen Kfir wrote:

>Hi Jan,
>I am sorry...
>
>This is the right code:
>
>
>#!/usr/bin/perl
>
>%machines =(user1, 'booboo',
> user2, 'puter',
> user3, 'synta',
> user4, 'synta');
>
> for (keys %machines)
> {
> $val=$machines{$_};
> if (!defined $supliers{$val})
> {
> $supliers{$val}=1;
> # print "hi\n";
> }
> else
> {
> print "duplicate value: $val\n";
>
> }
> }
>
>Seems more reasonable now... What I cant comprehend now is the
>$supliers{$val part. What's inside it , & all the mechanism around
>its action.
>

The %supliers hash must have keys which equal the values of the
%machines hash. So the hashes are "tied together" in a way.

The way this is written, the %supliers keys will be booboo, puter and
synta. For each of these suppliers, you check if it's value is defined.
The idea is to mark the supplier synta as duplicate: when you use the
value of $machines{user3}, you assign 1 to $supliers{synta}. On a
subsequent looping, $supliers{synta} will be defined, so you get the
duplicate message.

Best,

Jan
--
If all else fails read the instructions. - Donald Knuth

[Jan Eden]

Ronen Kfir wrote:

>If I get you correctly the %supliers hash will look like this in this
>script:
>%supliers =(booboo =>'1',
> Puter =>'1',
> Synta => 'defined')
>.... or am I wrong. I need to see the real print of this hash in order to
>understand it...
>

Not exactly, all keys in %supliers are assigned the value '1', and are thereby defined. Being defined is the same as having any value other than undef. So %supliers{synta} gets '1' if the loop encounters the first user with the value "synta" (in the %machines hash) and is thereby defined if the loop encounters the second user with the value "synta". Then you get the duplicate message.

Note that the %supliers hash will start out empty, i.e. without key/value pairs. This is a prerequisite for the duplicate check.

Apart from your actual problem, you should always use strict in your scriptto make Perl enforce good programming style and use warnings (if you have a recent Perl distribution) to make Perl tell you about hazardous operations.

HTH,

Jan
--
How many Microsoft engineers does it take to screw in a lightbulb? None. They just redefine "dark" as the new standard.

[Eternius]

<answer>
i agree with you,
but for the sake of read - a - bility I would have defined
the vars here (<---) as they are valid for whole package anyway and not
just for test1();

use strict;

local $a = 1; <---
local $aa = 2; <---

test1();

sub test1
{
test2();
}

sub test2
{
print "a = $a\n";
print "aa = $aa\n";
}
</answer>
Wiggins d'Anconia wrote:

> Eternius wrote:
>

>> Joshua A. Gage wrote:

[snip]

>>
>> use my
>> extract from perldoc -f my :
>> A "my" declares the listed variables to be local
>> (lexically) to the enclosing block, file, or
>> "eval".
>>

>
> In most cases you would be right, but in this case it appears that the
> OP understands what 'local' does, as he is using it inside of an
> embedded function. Do I like that, no, but it is a "correct" usage of
> 'local'. Your suggestion of using 'my' changes how the variables must
> be used so is not equivalent without suggesting the interface to 'test2'
> changes...
>
> sub test1 {
> my $aa = 1;
> my $aaa = 2;
> test2($aa, $aaa);
> }
>
> sub test2 {
> my ($bb, $bbb) = @_;
> print "aa = $bb\n";
> print "aaa = $bbb\n";
> }
>
> Is the above better, in the court of readability and encapsulation, yes,
> in the court of elegance, maybe not...
>
> [url]http://danconia.org[/url]
>
>
>

[Wiggins D'Anconia]

Please bottom post....

Eternius wrote:

> <answer>
> i agree with you,
> but for the sake of read - a - bility I would have defined
> the vars here (<---) as they are valid for whole package anyway and not
> just for test1();
>

But again you have changed the meaning, if you are going to declare them
as package variables at the top then you don't need a 'local' at all and
that should be an 'our', this is even worse for encapsulation and
definitely shows a lack of understanding of 'local'...

> use strict;
>
> local $a = 1; <---
> local $aa = 2; <---
>
> test1();
>
> sub test1
> {
> test2();
> }
>
> sub test2
> {
> print "a = $a\n";
> print "aa = $aa\n";
> }

As an example, try:

use strict;
use warnings;


our $aa = 1;
our $aaa = 2;


print "aa 1st = $aa\n";
print "aaa 1st = $aaa\n";


test1();


print "aa 3rd = $aa\n";
print "aaa 3rd = $aaa\n";


sub test1 {
local $aa = 3;
local $aaa = 4;
test2();
}


sub test2 {
print "aa 2nd = $aa\n";
print "aaa 2nd = $aaa\n";
}

Suggested reading: [url]http://perl.plover.com/FAQs/Namespaces.html[/url]

[url]http://danconia.org[/url]

> Wiggins d'Anconia wrote:
>

> > Eternius wrote:
> >

> >> Joshua A. Gage wrote:

> [snip]

> >>
> >> use my
> >> extract from perldoc -f my :
> >> A "my" declares the listed variables to be local
> >> (lexically) to the enclosing block, file, or
> >> "eval".
> >>

> >
> > In most cases you would be right, but in this case it appears that the
> > OP understands what 'local' does, as he is using it inside of an
> > embedded function. Do I like that, no, but it is a "correct" usage of
> > 'local'. Your suggestion of using 'my' changes how the variables must
> > be used so is not equivalent without suggesting the interface to 'test2'
> > changes...
> >
> > sub test1 {
> > my $aa = 1;
> > my $aaa = 2;
> > test2($aa, $aaa);
> > }
> >
> > sub test2 {
> > my ($bb, $bbb) = @_;
> > print "aa = $bb\n";
> > print "aaa = $bbb\n";
> > }
> >
> > Is the above better, in the court of readability and encapsulation, yes,
> > in the court of elegance, maybe not...
> >
> > [url]http://danconia.org[/url]
> >
> >
> >

>