Joe,
Thank you for taking the time to explain this bit of code. I have spent
some time trying to understand the ternary operator (?:) this evening and I
think it is making more sense. In the past I have seen this operator and
moved on in favor of using if/then/else statements but I can see how in this
example it is much simpler to evaluate the EXPR with a ternary than to use
code blocks. I can now see mostly how this expression works in your
example. I see that the first expression says $actual{$_} equal
$register{$_} if this is true then $_ equals the hostname. But, if this
statement is false then I see that we dereference the $_ values in both
Arrays.
When I do run this code though it seems to print out the memory address for
the hostname reference which I think means it is not being dereferenced
correctly but I don't see how it differs form the else part of the print
statement
ARRAY(0x816e00c) differs:actual 164.72.119.175 <-> register 164.72.119.179
ARRAY(0x816e03c) differs:actual 164.72.123.43 <-> register 164.72.21.43
host3 is ok
ARRAY(0x819bed4) differs:actual 164.72.98.89 <-> register 164.72.8.89
host5 is ok
I still have some ways to go with really understanding this but I am closer.
Well again thanks for your help,
-angus
P.s. Your English is probably better than mine; I wouldn't know it wasn't
your first language if you didn't mention it.
-----Original Message-----
From: John Doe [mailto:[EMAIL PROTECTED]
Sent: Thursday, February 02, 2006 3:26 AM
To: beginners@perl.org
Subject: Re: How to compare hashes to find matching keys with conflicting
values.
Angus am Donnerstag, 2. Februar 2006 10.14:
> John,
Hello Angus
> Well that works perfect now my issue is to understand what you have
> provided for me. I see that we are creating an array called res however I
> am not sure what map is doing. I did read the perldoc info on map but I
am
> still confused on how it is being used here. I see from the perldoc that
I
> could use it like this
>
> %register = map {$register($) => $_ @array;
> print "@array\n";
No, this is syntactically incorrect and wouldn't do anything useful.
> This would I think create an array based on the values contained in the
> register hash.
No; in an assignement
<anything> = <anything_else>
the left part is made from the expression on the right.
> But, I would have now way to link them back to the keys
> from register as now they would be numerically ordered in the array
instead
> of being linked to the hostname key.
>
> So I am looking at your example:
> %actual = map ($actual{$_} eq $register{$_})
This is not my example... and syntactically incorrect.
> This I think says each key eq to the other key value.
The part within the braces does a comparison between $actual{$_} and
$register{$_}. Result is true or false.
> Next you use a ? mark which I understand means that the previous
> values are optional,
No, its part of the ternary ?: operator (see perldoc perlop).
my $var = $x ? $y : $z
is the same as
my $var;
if ($x) {
$var=$y;
}
else {
$var=$z;
}
> which
> in this case if the are not equal are they then not included in the $_
> variable. But if they are eq then they are included in the $_ value which
> means they are placed back into the hashes? After this the register hash
> is sorted but I don't understand why.
It's because of the sort (see perldoc -f sort) in the expression. The keys
of
%keys are sorted with it before it goes through the "map pipe".
> The next code block iterates through the res array and unless it finds a
> reference? I did a perldoc on ref and see that it returns a non-empty
> string if EXPR is a reference. So now I think we created an array of
> hashes with the @res and map combo, is that correct?
Ok, let's analyse in detail:
[...]
> my %actual = (
> "host1" => "164.72.119.175",
> "host2" => "164.72.123.43",
> "host3" => "164.72.45.98",
> );
>
> my %register = (
> "host1" => "164.72.119.179",
> "host2" => "164.72.21.43",
> "host3" => "164.72.45.98",
> );
[citation removed, reformatted:]
my @res = map {
( $actual{$_} eq $register{$_} )
? $_
: [ $actual{$_}, $register{$_} ]
} sort keys %register;
The whole expression is to read from bottom to top; %register is the start;
it
is handled in 3 ways each after the other, resulting in @res.
1st, the keys are taken (keys %register).
2nd, the keys are sorted (sort)
3rd, map creates a new structure.
You could image some sort of "pipe" with stations:
entity:@res <- do:map <- do:sort <- do:keys <- entity:%register
Note that @res does nowhere occur on the right side: It's the result;
nothing
is inside it before the expression on the right side of the '=' is
evaluated.
Now to the map part: ... <-- map { <do something> } <-- ... :
It takes something at the right, mangles it, and puts it out on the left.
(Note the {}-braces are part of the map; it's one of it's two syntax).
Within map{}, $_ contains the sorted keys of %register; one by one is
mangeld
by map{}. $_ is a scalar, fed to map on the right, and map returns a scalar
on the left.
The scalar is either the key ( $_ ) or an arrayref [ ... ], depending of the
result of the comparison, taken from after ? or after : .
What the map does here (for every $_ separate) in words:
"Compare the value from %actual keyed by $_ with the value from %register
keyed by $_. If they are equal (means: if the IPs are equal), return the key
(means: hostname); otherwise, return the two different values (means: the
two
different IPs) within an arrayref."
I hope this helps and my english was not too confusing.
Just play around with map to get the feeling. I remember that it is not easy
to understand at the beginning, but if you get the idea, you won't miss it,
because it allows to do complex actions in a very short way.
joe
[nothing new below]
> foreach my $e(@res) {
> unless(ref($e)) {
> print "$e is ok\n";
> }
> else {
> print "$e differs:actual @{[$e->[0]]} <-> register @{[$e->[1]]}\n";
> }
> }
>
>
> -----Original Message-----
> From: John Doe [mailto:[EMAIL PROTECTED]
> Sent: Wednesday, February 01, 2006 2:45 AM
> To: beginners@perl.org
> Subject: Re: How to compare hashes to find matching keys with conflicting
> values.
>
> Sorry for another mail, but there are too many typos:
> > Also untested:
> >
> > # If no conflict, value is hostname.
> > # If conflict, value is arrayref with the two different IPs
> > #
> > my @res = map {
> > ($actual{$_} eq $register{$_})
> > ? $_
> >
> > : [$actual{$_}, $register{$_}]
> >
> > } sort keys %register;
> >
> > foreach my $e (@res) {
> > if (ref($e)) {
>
> This should read
> unless (ref($e)) {
>
> > print "$e is ok\n";
> > }
> > else {
> > print "$e differs: actual @{[$e->[0]]} <-> register @{[$e->[1]]}\n";
> > }
> > }
> >
> > I think this should give you an idea.
> > To look op any perl funktion, type:
>
> up, function
>
> > perldoc -f FUNCTIONNAME
> >
> > The man page for operaters:
>
> operators
>
> Nice day to all :-)
>
> --
> To unsubscribe, e-mail: [EMAIL PROTECTED]
> For additional commands, e-mail: [EMAIL PROTECTED]
> <http://learn.perl.org/> <http://learn.perl.org/first-response>
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
<http://learn.perl.org/> <http://learn.perl.org/first-response>
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
<http://learn.perl.org/> <http://learn.perl.org/first-response>
