Just got through debugging a line of code which uses dynamic array.  It boiled to to my use of [].  How should I "D think" about slice[]?  The run time error seems to say the the length of [] is zero.   I was assuming that [] meant "the entirety" of the array.

In short, is there anytime that one would want to use "slice[] = something" syntax?I

//waste[] = waste[0..$-1]; // object.Error@(0): Array lengths don't match for copy: 0 != 1

    waste = waste[0..$-1]; // works

On 09/25/2019 12:06 PM, WhatMeWorry wrote:

> I was
> assuming that [] meant "the entirety" of the array.

Assuming we're talking about D slices, Yes. (It could be a user-defined type with surprisingly different semantics.)

> In short, is there anytime that one would want to use "slice[] =
> something" syntax?I

That changes element values.

> //waste[] = waste[0..$-1]; // object.Error@(0): Array lengths don't
> match for copy: 0 != 1
>
>      waste = waste[0..$-1]; // works

That makes slice refer to a different set of elements. In that example, the slice does not include the last element anymore.

Ali

On Wednesday, 25 September 2019 at 19:25:06 UTC, Ali Çehreli wrote:
> On 09/25/2019 12:06 PM, WhatMeWorry wrote:
>
> > I was
> > assuming that [] meant "the entirety" of the array.
>
> Assuming we're talking about D slices, Yes. (It could be a user-defined type with surprisingly different semantics.)
>
> > In short, is there anytime that one would want to use
> "slice[] =
> > something" syntax?I
>
> That changes element values.

Ok.  But which element(s)?  In my specific case, I was using []. Is

waste[] = waste[0..$-1];

even semantically meaningful?  Because the LDC compiler had no problem compiling it.


>
> > //waste[] = waste[0..$-1]; // object.Error@(0): Array lengths
> don't
> > match for copy: 0 != 1
> >
> >      waste = waste[0..$-1]; // works
>
> That makes slice refer to a different set of elements. In that example, the slice does not include the last element anymore.
>
> Ali

On Wednesday, 25 September 2019 at 20:36:47 UTC, WhatMeWorry wrote:
> Ok.  But which element(s)?  In my specific case, I was using []. Is
>
> waste[] = waste[0..$-1];
>
> even semantically meaningful?  Because the LDC compiler had no problem compiling it.

`waste[]` is just shorthand for `waste[0..$]`. Assigning to a slice means copying the contents of another array into the array that slice refers to. If the lengths of the source and destination don't match, you get an error. Since `waste[0..$]` and `waste[0..$-1]` can never have the same length, you will always get an error if you try to assign one to the other.

Source: https://dlang.org/spec/arrays.html#array-copying

On 25.09.19 22:36, WhatMeWorry wrote:
> On Wednesday, 25 September 2019 at 19:25:06 UTC, Ali Çehreli wrote:
>> On 09/25/2019 12:06 PM, WhatMeWorry wrote:
[...]
>> > In short, is there anytime that one would want to use
>> "slice[] =
>> > something" syntax?I
>>
>> That changes element values.
> 
> Ok.  But which element(s)?

All of them. For example, `slice[] = 42;` sets all elements to 42. And `slice[] = another_slice[];` replaces all elements of `slice` with copies of `another_slice`'s elements.

>  In my specific case, I was using []. Is
> 
> waste[] = waste[0..$-1];
> 
> even semantically meaningful?  Because the LDC compiler had no problem compiling it.

It's equivalent to this:

----
waste[0] = waste[0..$-1][0];
waste[1] = waste[0..$-1][1];
...
waste[waste.length - 2] = waste[0..$-1][waste.length - 2];
waste[waste.length - 1] = waste[0..$-1][waste.length - 1];
----

So it basically does nothing. It just copies `waste`'s elements over themselves.

Except that the last line makes an out-of-bounds access. That's an error that may be detected during compilation or at run time. Or if you're telling the compiler to optimize too aggressively, it might go unnoticed.