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 :-)
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