When using the .length property of a dynamic array why does it return the incorrect number of elements after I use the appender?

import std.stdio;
import std.array : appender;

void main()
{
    //declaring a dynamic array
    int [] arrayofNumbers;
    //append an element using the ~= syntax
    arrayofNumbers ~= 1;
    arrayofNumbers ~= 2;
    //print the array
    writeln(arrayofNumbers);

    //Using appender
    auto appendNumber = appender(arrayofNumbers);
    appendNumber.put(10);
    writeln(appendNumber.data);

    writeln(arrayofNumbers.length);

}

Output:

[1, 2]
[1, 2, 10]
2  --- > Should be 3

On Sunday, 19 August 2018 at 15:44:07 UTC, QueenSvetlana wrote:
> When using the .length property of a dynamic array why does it return the incorrect number of elements after I use the appender?
>
> import std.stdio;
> import std.array : appender;
>
> void main()
> {
>     //declaring a dynamic array
>     int [] arrayofNumbers;
>     //append an element using the ~= syntax
>     arrayofNumbers ~= 1;
>     arrayofNumbers ~= 2;
>     //print the array
>     writeln(arrayofNumbers);
>
>     //Using appender
>     auto appendNumber = appender(arrayofNumbers);
>     appendNumber.put(10);
>     writeln(appendNumber.data);
>
>     writeln(arrayofNumbers.length);
>
> }
>
> Output:
>
> [1, 2]
> [1, 2, 10]
> 2  --- > Should be 3

auto appendNumber = appender(arrayofNumbers);

This returns a separate object. You probably meant to put this for the last line

writeln(appendNumber.length);

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

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?

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?

Yes, the .data field gives the array you're working on.

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.