What is the syntax for deleting an array element if you don't know its index?

Question

You can use the adverb :delete in Perl 6 to delete an array element if you know its index:

my @letters = <a b c>; @letters[0]:delete; say @letters
# OUTPUT: «[(Any) b c]␤»

However, you can't do that if you don't know the index:

my @letters = <a b c>; $_:delete if $elem eq 'a' for @letters
#ERROR! → Variable '$_:delete' is not declared

If you declare the loop variable is rw still the same problem:

my @letters = <a b c>; for @letters -> $l is rw { $l:delete if $l eq 'a' }; say @letters
#ERROR! → Variable '$l:delete' is not declared

There does not seem to be another way of deleting array elements. The equivalent for Perl 5's delete points, in fact, to that adverb. You could use splice but once again you would have to know the index. The implementation seems to be this function, DELETE-POS, which effectively needs to know the array index.

So the question is, as in the title, is there a way of deleting an array (or, for that matter, associative array) element if you have the handle of just the element itself, not its index?

Answer 1

Depends on what you call delete. Internally, on arrays, doing a DELETE-POS binds the element in the array to nqp::null, which will discard any container living at that position. At the HLL level, this is represented by the type of the array:

my Str @letters = <a b c>;
@letters[1]:delete;
say @letters;   # [a (Str) c]

However you can achieve the same effect by assigning Nil:

my Str @letters = <a b c>;
@letters[1] = Nil;
say @letters;   # [a (Str) c]

In this case, the container at that element stays the same, you just let it revert to its "natural" state. And if you're content with that type of deletion, you can also use that in your loop:

my Str @letters = <a b c>;
$_ = Nil if $_ eq "b" for @letters;
say @letters;   # [a (Str) c]

When I said: revert to its natural state, this also includes any default value:

my Str @letters is default<foo> = <a b c>;
$_ = Nil if $_ eq "b" for @letters;
say @letters;   # [a foo c]

If you want to really eradicate the elements from the list without keeping the existing elements at their positions, you can of course use grep:

my @letters = <a b c>;
@letters .= grep: * ne "b";
say @letters;  # [a c]

But that does not seem like what you intended.