Why $-reference possible to Perl array?

Question

I am thinking how you can refer to an element in Perl array by the sign $.

Minimum code

my @x = @{ $_[0] }; 

for(my $i=0; $i<$#x; $i++){
    print $x[$i]; 
}

You initialize the array as @x that is an array. You print out each item from the array by $x[$i] in a for loop.

I think it is a bit confusing when you initialize the array by @x and get its size by $#x.

Why can you refer to the Perl array by $x?

***`$#x`*** is ***NOT*** the size of the array. It is the index of the last element of the array. The size of the array is `@x` evaluated in scalar context. Also, `my $x = shift; print $x->[$_] for 0 .. $#$x;` to print the elements of the array without creating a copy. — Sinan Ünür, May 14 '15 at 15:09
@SinanÜnür So the index can be something else than scalar in Perl. Right? — Léo Léopold Hertz 준영, May 14 '15 at 15:11
Indexing imposes scalar context. When you write `$ary[ ... ]` whatever is between `[ ]` is evaluated in scalar context. For example, `$ary[ @ary ]` will ***always*** be `undef` thanks to autovivification. In C, it would access an out-of-bounds memory location and lead to undefined behavior. — Sinan Ünür, May 14 '15 at 15:12
@SinanÜnür So `$#x` should correspond to the size of the array through the last element of the array. Any cases where gaps in numbering? Possible? — Léo Léopold Hertz 준영, May 14 '15 at 15:13
`$#x` is the last index --- ***NOT THE SIZE OF THE ARRAY***. I don't know how much clearer this can be made. — Sinan Ünür, May 14 '15 at 15:15

score 2 · Accepted Answer · answered May 14 '15 at 14:56

Because the perlish way is to have the sigil denoting the type of thing you're working with, rather than being part of the variable name.

$x is a scalar, unrelated to the list @x. However $x[1] is still a scalar - but it's an element from the list @x. (and is unrelated to $x, because clearly - you couldn't pick a single element out of a single element).

$#x is a single value (scalar) so has the $ prefix still.

The same applies to hashes. %hash is the whole hash. $hash{$key} is a single value from that hash. And @hash{@some_keys} is a list of values from that hash.

score 1 · Answer 2 · answered May 14 '15 at 14:55

You're not referring to the array with $x, you're referring to an element of the array with it. Your array is an array of scalar values which you use $ to access.

Please see the perlintro and perldata documentation. It'll cover the "why" of your question.

Why $-reference possible to Perl array?

2 Answers2