Totally confused over hash behaviour 1

[1DMF]

Hi, I have created an array of hashes like so...

    # Get user details
    my @user = &getSQL("Users","UserID,Role,FullName","1=1","UserID ASC");
       
    foreach my $user (@user)
    {
        $USERS[0]->{$user->{'UserID'}} = $user->{'FullName'};
        $USERS[1]->{$user->{'UserID'}} = $user->{'Role'};
    }

All seem fine so far, however, when i try to access the hashes later on with ....

    foreach (@USERS) {
        foreach my $key (keys $_){
            print "key = $key , value = " . $_->{$key} . "<br>";
        }
    }

I get....

Type of arg 1 to keys must be hash (not scalar dereference)

yet if I do this...

   print "users[0] = " . $USERS[0];

I get ....

users[0] = HASH(0x2c35044)

So I don't get it , it is hash isn't so why won't the foreach work?

cheers,
1DMF

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[rharsh]

I suspect you want something like this:

foreach my $user (@USERS) {
    foreach my $key (keys %{$USERS[$user]}){
        print "key = $key , value = " . $USERS[$user]->{$key} . "<br>";
    }
}

I didn't test it, but that looks about right.

 

[Kirsle]

An array can't contain "hashes", it can contain hash REFERENCES.

Thus, you need to put %{ } around an array element that is a hashref to dereference it as a hash.

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[stevexff]

use Data::Dumper;
# Get user details
    my @user = &getSQL("Users","UserID,Role,FullName","1=1","UserID ASC");

print Dumper(@user);

will give you a graphic representation of the structure; from there you should be able to figure out how to dereference it to do what you want.


Steve

[small]"Every program can be reduced by one instruction, and every program has at least one bug. Therefore, any program can be reduced to one instruction which doesn't work." (Object:erlDesignPatterns)[/small]

 

[1DMF]

Thus, you need to put %{ } around an array element that is a hashref to dereference it as a hash.

I don't understand, I don't always need to do this....

my @rs = &getSQL("Contacts","Title,FirstName,LastName,CompanyName,[Membership Date]","(DATEADD(day,45,[Membership Date]) >= GETDATE()) AND ContactTypeID = 'Member' AND (MembershipLevel = 'Adviser' OR MembershipLevel = 'Trainee')","[Membership Date] DESC");

my $i=0;

for(@rs){
    $i++;
    #format date
    $_->{'Membership Date'} = substr($_->{'Membership Date'},0,10);
    my @dte = split(/-/,$_->{'Membership Date'});
    $_->{'Membership Date'} = "$dte[2]/$dte[1]/$dte[0]";
    $_->{'ID'} = $i;
    $_->{'FirstName'} =~ s/'/\\'/g;
    $_->{'LastName'} =~ s/'/\\'/g;
    $_->{'CompanyName'} =~ s/'/\\'/g;
}

I can write that where @rs is an array of hashes without having to use a percent sign anywhere.

I can't get my head round when a dereference is required as it doesn't always seem to require one?

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[1DMF]

well if I uses data::dumper on @USERS i get..

$VAR1 = { 'Christopher' => 'Christopher Smith', 'Craig' => 'Craig Jones' }; $VAR2 = { 'Christopher' => 'Admin', 'Craig' => 'Admin' };

which is exactly what I expect to see and array of hashes.

So what's the problem with my foreach and hash:keys command?

I'm lost over all this, if I loop and array of hashes normally I don't need %{} wrapped around $_ ,

"In complete darkness we are all the same, only our knowledge and wisdom separates us, don't let your eyes deceive you."

"If a shortcut was meant to be easy, it wouldn't be a shortcut, it would be the way!"

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[1DMF]

oh well if i wrap $_ in %{}, it works fine....

foreach (@USERS) {        
    foreach my $key (keys %{$_}){            
        print "key = $key , value = " . $_->{$key} . "<br>";        
    }    
}

So got there in the end.

Bit unsure why $_->{'hash key'} works, but keys $_ doesn't.

does '->' dereference first automatically ?

"In complete darkness we are all the same, only our knowledge and wisdom separates us, don't let your eyes deceive you."

"If a shortcut was meant to be easy, it wouldn't be a shortcut, it would be the way!"

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[Kirsle]

When you do a foreach keys on a hash, you always use the % sign, like "foreach (keys %myhash)", not the $ sign like when you actually get a key from it, $myhash{key}.

Same for @, foreach (@array), not foreach($array)

So when you use a reference in either context you need to dereference it with %{} or @{}.

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[Kirsle]

As for Data:umper, the difference between this

print Dumper(@array);
print Dumper(@arrayref);

is that the output is this for regular array:

$VAR1 = "dog";
$VAR2 = "cat";
$VAR3 = "mouse";

and this for an array reference:

$VAR1 = [
   "dog",
   "cat",
   "mouse",
];

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perl -e '$|=$i=1;print" oo\n<|>\n_|_";x:sleep$|;print"\b",$i++%2?"/":"_";goto x;'

 

[Kirsle]

er,

-- print Dumper(@arrayref);
++ print Dumper($arrayref);

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perl -e '$|=$i=1;print" oo\n<|>\n_|_";x:sleep$|;print"\b",$i++%2?"/":"_";goto x;'

 

[1DMF]

Thanks for clearing that up Kirsle

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