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can i concatenate various value to form a unique key

by Anonymous Monk
on Jul 03, 2009 at 06:07 UTC ( #776946=perlquestion: print w/replies, xml ) Need Help??

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I have to define a hash whose key is defined by various values, so can i concatinate the values to get a unique key? Is this the right way or some other option available in perl?
my $key = join ("",$value1,$value2,$value3,$value4,$value5); my @value= @{$Config{$key}};

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Re: can i concatenate various value to form a unique key
by missingthepoint (Friar) on Jul 03, 2009 at 06:41 UTC
    so can i concatinate the values to get a unique key?

    Maybe. Consider this:

    $value1 = "a"; $value2 = "b"; $value3 = "c"; $value4 = "de"; $value5 = "f"; my $key = join("", $value1,$value2,$value3,$value4,$value5); print $key; $value3 = "cd"; $value4 = "e"; $key = join("", $value1,$value2,$value3,$value4,$value5); print $key;

    So if you need different keys in situations like above, no. Perhaps you could generate a string that encodes the structure of the separate values, e.g.:

    my $str = "v1=$value1,v2=$value2,v3=$value3,v4=$value4,v5=$value5"; # use $str for key, or perhaps generate hash of $str and use that

    Ca3n w2e ple6ase st4op doi5ng th4is?
Re: can i concatenate various value to form a unique key
by ambrus (Abbot) on Jul 03, 2009 at 08:07 UTC

    Sure, just use pack "(J/A)*", @strings instead of join "", @strings to concatenate the strings to a unique key. (On older perls, use I instead of J.)

Re: can i concatenate various value to form a unique key
by Marshall (Canon) on Jul 03, 2009 at 09:26 UTC
    It is completely fine to make a hash key from other strings. like:
    #!/usr/bin/perl -w use strict; my %hash; $hash{'my_mother=Nice,my_brother=Great;3;45;987;'}='True'; foreach my $key (keys %hash) { print "$key VALUE =$hash{$key}\n"; } __END__ prints: my_mother=Nice,my_brother=Great;3;45;987; VALUE =True
    I would argue that this is not the best way:
    my $key = join ("",$value1,$value2,$value3,$value4,$value5);
    my $key ="$value1$value2$value3$value4$value5";
    if say $value3 doesn't exist, then,
    $key = "$value1$value2$value4$value5";
    Oooops!...maybe a problem!!!

    $hash{"$value1;$value2;$value3;$value4;$value5"} = 123;
    is "better" (provided that ";" doesn't occur in the $values).
    The reason for the ";" is so that there will always be a unique value even if one of the $value vars is ""!.

      hoo so is it possible to have mutiple values as key whose combination will be unique? and I dont have to download any CPAN modules for the same also?
        The short answer is yes. A hash key can be any string as long as that string is unique. The "classic example" is printing a file and omitting duplicate lines that have been seen before....
        #!usr/bin/perl -w use strict; my %seen; while (<DATA>) { print unless $seen{$_}++; } #Prints: #1 2 this is a line with 1 and 2 #3 a line with just 3 #5,6 five before 6 #5,6 five before six (different) #6,5 #blah __DATA__ 1 2 this is a line with 1 and 2 3 a line with just 3 1 2 this is a line with 1 and 2 3 a line with just 3 5,6 five before 6 5,6 five before six (different) 6,5 blah blah
        In general, I would not concatenate keys together. Well, until you have learned multi-dimensional structs, I would say that concatenating 2 things and not more is "ok".

        There are some advanced techniques where say 8 dimensions can be combined into a single hash key, but those situations are seldom and not applicable for normal code (a way to describe state tables and a way to simplify horrifically complex logic statements).

Re: can i concatenate various value to form a unique key
by JavaFan (Canon) on Jul 03, 2009 at 10:17 UTC
    As explained else where in the thread, that has the potential to not be unique.

    It's a little known feature that Perl actually supports autoconcatenation of strings used as hash keys, joining them together using $;, which by default contains "\034" which is rare enough that it's unlikely unwanted duplicates are formed. So, you can write:

    my @value = @{$Config{$value1,$value2,$value3,$value4,$value5}};
    without having to join them yourself.

    This was how people did multilevel hashes using perl4.

    For more details, see the perlvar entry about $;.

      i am initialize these variable with some default value and then will the same problem exists? and for the default key combination also hash have some value like
      my $value1 = 'x'; my $value2 = 'x'; my $value3 = 'x'; my $value4 = 'x'; ---- #code where actual value will be assigened to the variables and only $ +value2 didnt get assigned to any value so $value2='x' itself ---- my @value = @{$Config {'hai','x','there','bye'}};
      if my has have a value for thsi key also will it be an issue?
Re: can i concatenate various value to form a unique key
by citromatik (Curate) on Jul 03, 2009 at 07:50 UTC

    I don't know exactly what you are trying to address (sounds like an XY Problem to me). You can use any string as a key in a hash, as long as it is unique in the hash, but building a hash whose keys are the concatenated values of other keys sounds like a design problem to me, maybe other data structures should be used instead

    Could you please give a step backwards and elaborate a bit more the kind of problem you are trying to solve and why do you think that such a hash is a good option?


      Hi, I am planning to have a hash like
      %hash = {g1 => [start_date1,end_date2] g2 => [start_date1,end_date2] };
      but i dont have a unique value of g1 and g2.. i will get unique value with combination of some values like value1 value2 value3 ..value5 so i planned to derive g with concatenation of these values assuming that a hash cannot have mutiple values of as a key. I thik the background is explained better now.
        At the risk of getting into more trouble in this thread, I do not think that this will work out well for you. There can be very good reasons to concatenate strings into a single hash key, but once we are talking about 5 FIVE dimensions, this doesn't make sense. I apologize for not reading this thread carefully from the beginning.

        sounds like you need an Array of Hash.

        each hash like: start => "some start date", end => "some end date", parm1 => "some data1", parm2 => "some data2", parm3 => "some data3",

        Maybe I am wrong, but contrary to the others who have replied already, I think you are misinterpreting your problem (sorry if I misinterpreted you! :).

        Do you want the following:

        %hash = ( g1 => [value1, value2, value3], g2 => [value4, value5, value6...] );

        i.e. do you want each key to have multiple values associated? If this is the case you need a reference to an array containing all the values (see perlref). You already have that situation in the example you give: g1 => [$start_date1,$end_date1], so, to concatenate more values, you will need a reference to an AoA (array of arrays). Again, sorry if I totally misinterpreted your problem, but if you think I am close, let me know and we will elaborate it a little more


Re: can i concatenate various value to form a unique key
by SuicideJunkie (Vicar) on Jul 03, 2009 at 14:01 UTC
    Ensuring uniqueness is quite easy, but you definitely don't want to concatenate. It sounds like you want a HoHoHoHoHoA.
    See: perlref and use Data::Dumper to make it easy to see the deep structure of your variable via print Dumper \%hash.

    For a config type example:
    my %config; my $inDir = $config{directories}{input}; my $tempDir = $config{directories}{temp}; my $outDir = $config{directories}{output}; my $bgColor = $config{colors}{background}; ...

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