Perl pitfalls

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 hash and array 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 care 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 mean "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';

• @dictqw< foo bar > = (undef, 'a') actually means

    $dictfoo = undef;
    $dictbar = 'a';

• @dictqw< foo bar > = @dictqw< bar foo > exchanges the values of $dictfoo and $dictbar

• 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:

    $dictkey = undef;
    exists  $dictkey # true
    defined $dictkey # 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_hashlevel1 was empty, it will be filled with a level2 => undef hashref in the mean to reply false.