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

what's the difference between: 
my @array = (); and my @array;

and 

my %hash = (); and my %hash;

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Re: initialize arrays and hashes
by BrowserUk (Patriarch) on Aug 29, 2013 at 15:07 UTC
    what's the difference

    Absolutely nothing! (Say it again. :) 

    C:\test>perl -MDevel::Peek=Dump -E"my %h; Dump \%h; my %i=(); Dump \%i
+;"
SV = RV(0x9ecb8) at 0x9eca8
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x2e7590
  SV = PVHV(0x2cd310) at 0x2e7590
    REFCNT = 2
    FLAGS = (PADMY,SHAREKEYS)
    ARRAY = 0x0
    KEYS = 0
    FILL = 0
    MAX = 7
    RITER = -1
    EITER = 0x0
SV = RV(0x9ecb8) at 0x9eca8
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x2e7608
  SV = PVHV(0x2cd338) at 0x2e7608
    REFCNT = 2
    FLAGS = (PADMY,SHAREKEYS)
    ARRAY = 0x0
    KEYS = 0
    FILL = 0
    MAX = 7
    RITER = -1
    EITER = 0x0

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Re: initialize arrays and hashes
by Eily (Monsignor) on Aug 29, 2013 at 15:02 UTC

    From a data point of view, none.

    From a reader / programmer point of view, in one case you have explicitly empty elements, in the other it's implied and the focus is on declaration of the variable and its scoping. You could for example use the first syntax as a reminder that your container might still be empty by the end, and the second when you are just defining the variable to give a specific scope, but the data has to be found somewhere else.

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Re: initialize arrays and hashes
by davido (Cardinal) on Aug 29, 2013 at 16:50 UTC

    Perl, unlike C, C++, and other "pay only for what you use" languages, always automatically initializes scalar variables to "undef", or in the case of container variables, to the empty list. In a "Do what I mean" and "autovivification" language like Perl, it just seems to work better that way.


    Dave

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