Thread overview
.dup vs operation on all elements
Dec 03
Goksan
Dec 03
faissaloo
Dec 03
Goksan
December 03
Are there any differences between these 2 methods of copying elements?

double[] array = [ 1, 20, 2, 30, 7, 11 ];

// Non dup
double[6] bracket_syntax_dup = array;
bracket_syntax_dup[] = array;
bracket_syntax_dup[0] = 50;

// Dup
double[6] normal_dup = array.dup;
normal_dup[0] = 100;

OUTPUT: (array, bracket_syntax_dup and normal_dup respectively):
[1, 20, 2, 30, 7, 11]
[50, 20, 2, 30, 7, 11]
[100, 20, 2, 30, 7, 11]
December 03
Yes it is. The dup version just make an extra copy of array for no reason.



po 3. 12. 2018 21:10 odesílatel Goksan via Digitalmars-d-learn < digitalmars-d-learn@puremagic.com> napsal:

> Are there any differences between these 2 methods of copying elements?
>
> double[] array = [ 1, 20, 2, 30, 7, 11 ];
>
> // Non dup
> double[6] bracket_syntax_dup = array;
> bracket_syntax_dup[] = array;
> bracket_syntax_dup[0] = 50;
>
> // Dup
> double[6] normal_dup = array.dup;
> normal_dup[0] = 100;
>
> OUTPUT: (array, bracket_syntax_dup and normal_dup respectively):
> [1, 20, 2, 30, 7, 11]
> [50, 20, 2, 30, 7, 11]
> [100, 20, 2, 30, 7, 11]
>


December 03
On Monday, December 3, 2018 1:07:24 PM MST Goksan via Digitalmars-d-learn wrote:
> Are there any differences between these 2 methods of copying elements?
>
> double[] array = [ 1, 20, 2, 30, 7, 11 ];
>
> // Non dup
> double[6] bracket_syntax_dup = array;
> bracket_syntax_dup[] = array;
> bracket_syntax_dup[0] = 50;
>
> // Dup
> double[6] normal_dup = array.dup;
> normal_dup[0] = 100;
>
> OUTPUT: (array, bracket_syntax_dup and normal_dup respectively):
> [1, 20, 2, 30, 7, 11]
> [50, 20, 2, 30, 7, 11]
> [100, 20, 2, 30, 7, 11]

dup allocates a new dynamic array and copies the elements of the existing dynamic array to the new one. Calling dup in order to assign to a static array is just needlessly allocating a dynamic array. The contents of the array are going to be copied to the static array regardless, but instead of just copying the elements, if you use dup, you're allocating a new dynamic array, copying the elements into that dynamic array, and then you're copying the elements into the static array. There's no point.

You use dup when you want to copy the elements of a dynamic array instead of simply slicing it. Slicing gives you a new dynamic array that points to exactly the same elements. It's just copying the pointer and the length (meaning that mutating the elements of the new slice will affect the elements in the original array), whereas dup actually allocates a new block of memory for the new dynamic array to be a slice of (copying the elements over in the process), so mutating the elements in the new dynamic array then won't affect the elements in the original.

Regardless, when you create a static array, it's not a slice of anything (since its elements sit directly on the stack), and when assign to it, you're simply copying the elements over.

- Jonathan M Davis



December 03
On Monday, 3 December 2018 at 20:37:22 UTC, Jonathan M Davis wrote:
> On Monday, December 3, 2018 1:07:24 PM MST Goksan via Digitalmars-d-learn wrote:
>> Are there any differences between these 2 methods of copying elements?
>>
>> double[] array = [ 1, 20, 2, 30, 7, 11 ];
>>
>> // Non dup
>> double[6] bracket_syntax_dup = array;
>> bracket_syntax_dup[] = array;
>> bracket_syntax_dup[0] = 50;
>>
>> // Dup
>> double[6] normal_dup = array.dup;
>> normal_dup[0] = 100;
>>
>> OUTPUT: (array, bracket_syntax_dup and normal_dup respectively):
>> [1, 20, 2, 30, 7, 11]
>> [50, 20, 2, 30, 7, 11]
>> [100, 20, 2, 30, 7, 11]
>
> dup allocates a new dynamic array and copies the elements of the existing dynamic array to the new one. Calling dup in order to assign to a static array is just needlessly allocating a dynamic array. The contents of the array are going to be copied to the static array regardless, but instead of just copying the elements, if you use dup, you're allocating a new dynamic array, copying the elements into that dynamic array, and then you're copying the elements into the static array. There's no point.
>
> You use dup when you want to copy the elements of a dynamic array instead of simply slicing it. Slicing gives you a new dynamic array that points to exactly the same elements. It's just copying the pointer and the length (meaning that mutating the elements of the new slice will affect the elements in the original array), whereas dup actually allocates a new block of memory for the new dynamic array to be a slice of (copying the elements over in the process), so mutating the elements in the new dynamic array then won't affect the elements in the original.
>
> Regardless, when you create a static array, it's not a slice of anything (since its elements sit directly on the stack), and when assign to it, you're simply copying the elements over.
>
> - Jonathan M Davis

Then shouldn't the following output false, false, true?
import std.stdio;

  class Programmer
  {
      bool is_confused = false;

      void setConfusion(bool confusion_status)
      {
          is_confused = confusion_status;
      }
  }

  void main()
  {
      Programmer[6] array = new Programmer();

      Programmer[6] bracket_syntax_dup = array;
      bracket_syntax_dup[] = array;

      Programmer[6] normal_dup = array.dup;

      normal_dup[0].setConfusion(true);
      bracket_syntax_dup[0] = new Programmer();

      writeln(array[0].is_confused);
      writeln(bracket_syntax_dup[0].is_confused);
      writeln(normal_dup[0].is_confused);
  }

December 03
On Mon, 03 Dec 2018 21:27:52 +0000, faissaloo wrote:
> Then shouldn't the following output false, false, true?

An object reference is a pointer value. The pointer values are copied. The pointed-at objects are not copied.

Furthermore, the syntax

    Object[6] array = new Object();

only allocates one Object. Each item in the array is an object reference to the same object.
December 03
On Monday, 3 December 2018 at 22:04:25 UTC, Neia Neutuladh wrote:
> On Mon, 03 Dec 2018 21:27:52 +0000, faissaloo wrote:
>> Then shouldn't the following output false, false, true?
>
> An object reference is a pointer value. The pointer values are copied. The pointed-at objects are not copied.
>
> Furthermore, the syntax
>
>     Object[6] array = new Object();
>
> only allocates one Object. Each item in the array is an object reference to the same object.

Oh great, thanks for clearing this up.

I also didn't know about shorthand only allocating one object (the code he shared was a piece of code we were testing with which I mistakenly wrote!)

Thanks!