Using .length returns incorrect number of elements

[Simen Kjærås]

On Sunday, 19 August 2018 at 16:03:06 UTC, QueenSvetlana wrote:
> On Sunday, 19 August 2018 at 15:53:25 UTC, Chris M. wrote:
>> On Sunday, 19 August 2018 at 15:49:18 UTC, Chris M. wrote:
>>> On Sunday, 19 August 2018 at 15:44:07 UTC, QueenSvetlana 
>>> wrote:
>>>> [...]
>>>
>>> auto appendNumber = appender(arrayofNumbers);
>>>
>>> This returns a separate object. You probably meant to put 
>>> this for the last line
>>>
>>> writeln(appendNumber.length);
>>
>> Whoops
>>
>> writeln(appendNumber.data.length);
>>
>> https://run.dlang.io/is/4aNx1l
>
> New to d programming here :D Bare with me.
>
> The object that is returned by appendNumber.data is an array 
> reflecting the elements I added using appendNumber correct? So 
> I would have the call length on appendNumber.data instead of 
> the original array?

I suggest reading the D tour's page on arrays:
https://tour.dlang.org/tour/en/basics/arrays

In short, D arrays (more correctly called slices) are structs 
that look like this:

struct Slice(T) {
     T* ptr;
     size_t length;
}

As the name Slice indicates, it represents a slice of memory, 
with a beginning address and a length. If you have two int[] 
instances (let's call them A and B) that point to the same data, 
they have their own copy of the ptr and length fields, and 
changing the value of a field in A will not change the 
corresponding field in B.

When you append to A, what's happening* is the length is 
increased, and the appended data is written to the end of the 
memory pointed to. B does not see this, since it only looks at 
the first <length> elements of that block.

--
   Simen

* Plus some checking of the length of the block of memory it's 
pointing to, and possible reallocation if the block isn't big 
enough.