perl_traps

These are some things you should understand about Perl datastructures (and if
you learn something there, maybe it's time to read all the perltut* from the
perl distribution).

* Perl works with values instead of references so a hashes and arrays can be
  empty, never undefined (only a scalar can be undef)
* Arrays and hashes can store lists, * , is the operator to create a list.
* Perl takes carre of the context (scalar or list) which is a very expressive
  way to "do what i mean" (which is a Perl motto).
* in a scalar context, an array returns the number of elements
* a boolean context *is* a scalar context
* sigils means "what i get" and not "where it comes from"

so:

* defined( @data ) is a perl non-sense
* @my_array = undef actually means $my_array[0] = undef;
* @my_array = (undef, 'a') actually means

    $my_array[0] = undef;
    $my_array[1] = 'a';

* @dict{qw< foo bar >} = (undef, 'a') actually means

    $dict{foo} = undef;
    $dict{bar} = 'a';

* @dict{qw< foo bar >} = @dict{qw< bar foo >} exchanges the values of
  $dict{foo} and $dict{bar}

* if you want to know if the array is empty, just use a scalar context

    if ( @data ) {
    }
    if ( %dict ) {
    }

* $#my_array is the bound, not the size

    @my_array == 1+ $#my_array

* negative subscripts are working but not reverse range

    $my_array[ $#my_array ] # is actually
    $my_array[ -1 ]
    $my_array[ $#my_array -1 ] # is actually
    $my_array[ -2 ]


* call a sub using & is a way to share @_ with the caller! (very powerfull but tricky for most of us). If you don't understand or don't want that, please don't call using &.

* remember for and map loops are side-effects, even if you declare a variable

    @data = ( 1..3 );
    for my $d ( @data ) { $d++ }

    # so @data is now 2,3,4

* about booleans: 

    $dict{key} = undef;
    exists  $dict{key} # true
    defined $dict{key} # false
    0   # false
    1   # true
    '0' # false (converted to 0)
    '1' # true  (converted to 1)
    ''  # false (empty string)
    length '0' # true (converted to 1)

* perl autovivification

if you challenge perl on nested datastructures, it will create everything
needed to anwser correctly so:

    if ($my_hash{'level1'}{'level2'}) {...}

needed to anwser correctly so even if $my_hash{level1} was empty, it will be
filled with a { level2 => undef } hashref in the mean to reply false.