Re: using '-1' in array range fails

[p5pRT]

Migrated from rt.perl.org#886 (status was 'resolved')

Searchable as RT886$

[p5pRT]

From @Abigail

No.

The reason it isn't in Perl is that the list 3 .. -1 is empty.

Now, theoretically, one could modify perl such that it recognizes 3 .. -1
and does what you do. But that would be a bad idea.

I once argued with a language developer about this; he wanted to be able
to index arrays with negative numbers (in the same way as Perl does), but
I strongly disagreed. Because in that case, the slice

  array [x .. x - 1]

would be empty for all x, except x == 0, where the slice suddenly is the
entire array. (In this language .. only has meaning in array slices).

Negative indices were eventually dropped; just because of this oddity.

Perl doesn't suffer from this oddity, because .. is a list constructer,
however, with your proposal, it would.

Abigail

[p5pRT]

From @Abigail

On Mon, May 03, 1999 at 07​:10​:54PM +0200, Rob Urban wrote​:

supposing​:

@​foo = \(1\,2\,3\,4\,5\,6\,7\,8\,9\);

then

print $foo\[\-1\];

yields "9";

Shouldn't

@​bar = @​foo\[3\.\.\-1\];

cause @​bar to be (4,5,6,7,8,9) ?

instead, @​bar is empty.

No.

The reason it isn't in Perl is that the list 3 .. -1 is empty.

Now, theoretically, one could modify perl such that it recognizes 3 .. -1
and does what you do. But that would be a bad idea.

I once argued with a language developer about this; he wanted to be able
to index arrays with negative numbers (in the same way as Perl does), but
I strongly disagreed. Because in that case, the slice

  array [x .. x - 1]

would be empty for all x, except x == 0, where the slice suddenly is the
entire array. (In this language .. only has meaning in array slices).

Negative indices were eventually dropped; just because of this oddity.

Perl doesn't suffer from this oddity, because .. is a list constructer,
however, with your proposal, it would.

Abigail