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Re^5: Dereferencing a Hash of Arrays

by stefbv (Curate)
on Jun 14, 2011 at 08:21 UTC ( [id://909561]=note: print w/replies, xml ) Need Help??


in reply to Re^4: Dereferencing a Hash of Arrays
in thread Dereferencing a Hash of Arrays

From the perlvar man page: $_ is "The default iterator variable in a foreach loop if no other variable is supplied."

Using double quotes is equivalent with a join operation that uses a space as EXPR.

join(' ', @$_)

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Re^6: Dereferencing a Hash of Arrays
by jwkrahn (Abbot) on Jun 14, 2011 at 12:16 UTC
    Using double quotes is equivalent with a join operation that uses a space as EXPR.

    Using double quotes is equivalent with a join operation that uses the variable $" as EXPR, where $" normally contains a single space character.

Re^6: Dereferencing a Hash of Arrays
by toro (Beadle) on Jun 14, 2011 at 20:19 UTC

    Stefby, I am actually familiar with $_ in general, but in this specific case I don't understand where its value comes from. I'm guessing it's because $_ can handle lists and say for cannot? But, why?

    Here is my understanding of what's going on in your answer:

    1. values %alphabet spits out a list;
    2. $_ iterates through that list (but why wasn't my original say for iterating through the list?)
    3. @ dereferences the array (again, why didn't my original @{...} dereference it?)
    4. "   " joins the output with $".
    Would you mind correcting / clarifying the above? Thank you for your help so far.

      $_ iterates through that list (but why wasn't my original say for iterating through the list?)

      @ dereferences the array (again, why didn't my original @{...} dereference it?)

      It was iterating through the list and it did dereference , but the number 2 is not an array reference, so the list was empty :) strict noticed this problem, so it ended your program . This is why we love strict.

      Whatever text you're learning from, you need to go back and do the exercises you skipped :) the only way to absorb and remember is by typing it yourself :)

      $ perl -E " say for [1..4],[5..8] " ARRAY(0x3f8adc) ARRAY(0x9b9eac)
      aww, array references stringify like that, to get at the values, we need to dereference

      Trying it your way

      $ perl -E " say for @{ [1..4],[5..8] }" 5 6 7 8
      Aww, it only printed the contents of the last reference. To print both, we need to iterate over a list of reference, not a list of values
      $ perl -E " say @$_ for [1..4],[5..8] " 1234 5678
      Oh look, the numbers are all joined together, thats how say works
      $ perl -E " say 1,2,3,4; say 5..8; " 1234 5678
      I know, we'll put it in double quotes string, that will improve it
      $ perl -E " say qq{@$_} for [1..4],[5..8] " 1 2 3 4 5 6 7 8
      Now for the brackets
      $ perl -E " say qq{[@$_]} for [1..4],[5..8] " [1 2 3 4] [5 6 7 8]
      Now for the comma by changing the $", the LIST_SEPARATOR (the \" is for my shell, cmd.exe )
      $ perl -E " local $\"=q{,}; say qq{[@$_]} for [1..4],[5..8] " [1,2,3,4] [5,6,7,8]
      Even if we employ $" properly by limiting the scope of the changes with local, its better to use join function
      $ perl -E " say q{[}, join( q{,}, @$_ ), q{]} for [1..4],[5..8] " [1,2,3,4] [5,6,7,8]
      The values function is context sensitive
      $ perl -E " %f = 6..9; say for values %f; " 9 7 $ perl -E " %f = 6..9; say for scalar values %f; " 2
      Using @{} puts values %f in scalar context, just like the scalar function. This makes the values function return the number of values in %f; The number of values is not an array reference.

      To learn more about context see Tutorials: Context in Perl: Context tutorial, or see the free book Modern Perl, a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.

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