string x = "123";
auto c = x.ptr;
c++;
writeln(c[-1]); // 1
writeln(c[-1..0]); //BOOM Range violation


Can I do this / Bug / some mistake ?

On Thursday, 22 February 2018 at 00:13:43 UTC, SrMordred wrote:
> string x = "123";
> auto c = x.ptr;
> c++;
> writeln(c[-1]); // 1
> writeln(c[-1..0]); //BOOM Range violation
>
>
> Can I do this / Bug / some mistake ?

youd have to do
(c-1)[0 .. 1];

On Thursday, 22 February 2018 at 00:13:43 UTC, SrMordred wrote:
> string x = "123";
> auto c = x.ptr;
> c++;
> writeln(c[-1]); // 1

That's only happening because pointers bypass range checks.

> writeln(c[-1..0]); //BOOM Range violation

But with a slice negative indexes are never allowed, even on a pointer.

> But with a slice negative indexes are never allowed, even on a pointer.

> youd have to do
> (c-1)[0 .. 1];

Nice!
Thank you both!

In D Slice article it says "You can even use negative indexes!" so I thought
that the [-1..x] should work too :)

On 2/21/18 7:30 PM, SrMordred wrote:
>> But with a slice negative indexes are never allowed, even on a pointer.
> 
>> youd have to do
>> (c-1)[0 .. 1];
> 
> Nice!
> Thank you both!
> 
> In D Slice article it says "You can even use negative indexes!" so I thought
> that the [-1..x] should work too :)
> 
> 

Hah! I never thought of doing a slice with negative indexes ;)

Note that the statement is about C pointers, so since C doesn't have slicing, it stands to reason that slicing with negative indexes isn't supported.

-Steve

On Thursday, 22 February 2018 at 02:41:30 UTC, Steven Schveighoffer wrote:
> On 2/21/18 7:30 PM, SrMordred wrote:
>>> But with a slice negative indexes are never allowed, even on a pointer.
>> 
>>> youd have to do
>>> (c-1)[0 .. 1];
>> 
>> Nice!
>> Thank you both!
>> 
>> In D Slice article it says "You can even use negative indexes!" so I thought
>> that the [-1..x] should work too :)
>> 
>> 
>
> Hah! I never thought of doing a slice with negative indexes ;)
>
> /SNIP
>
> -Steve

At night I dream about doing something like this

auto pos = haystack.find(needle);
auto something = haystack[pos..+3];      // meaning [pos..pos+3]
auto somethingElse = haystack[pos..-3];  // and [pos..pos-3] respectively

:)

On 22.02.2018 01:26, Adam D. Ruppe wrote:
> On Thursday, 22 February 2018 at 00:13:43 UTC, SrMordred wrote:
>> string x = "123";
>> auto c = x.ptr;
>> c++;
>> writeln(c[-1]); // 1
> 
> That's only happening because pointers bypass range checks.
> 
>> writeln(c[-1..0]); //BOOM Range violation
> 
> But with a slice negative indexes are never allowed, even on a pointer.

Actually, it's slightly more complicated than that. E.g. c[-2..-1] does not trigger the range check and will behave as expected. For slices c[l..r] there is a check whether cast(size_t)l<=cast(size_t)r. The range violation happens because -1 is larger than 0 as an unsigned integer.

On Thursday, 22 February 2018 at 02:41:30 UTC, Steven Schveighoffer wrote:
> Hah! I never thought of doing a slice with negative indexes ;)

Maybe is my past of python:

arr[-3:] to get the last 3 elements for eg.

:)