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digitalmars.D.learn - How to set array length for multidimensional static arrays

reply Namal <sotis22 mail.ru> writes:
I understand that I cannot pass a variable to the static array 
like in C++, and have to use dynamic arrays. But how can I set 
the length for them without using a loop?
Jan 31 2016
parent reply Namal <sotis22 mail.ru> writes:
On Monday, 1 February 2016 at 07:41:33 UTC, Namal wrote:

 I understand that I cannot pass a variable to the static array 
 like in C++, and have to use dynamic arrays. But how can I set 
 the length for them without using a loop?


I mean std::vector in C++, not array.
Jan 31 2016
next sibling parent cym13 <cpicard openmailbox.org> writes:
On Monday, 1 February 2016 at 07:42:56 UTC, Namal wrote:

 On Monday, 1 February 2016 at 07:41:33 UTC, Namal wrote:

 I understand that I cannot pass a variable to the static array 
 like in C++, and have to use dynamic arrays. But how can I set 
 the length for them without using a loop?


 I mean std::vector in C++, not array.


I'm not sure I understand well because I know nothing about C++ 
types, so I'll assume you mean dynamic array.

Dynamic arrays are sized at initialization, resized when needed 
(when adding new elements) and can also be explicitely sized to 
some size:

     auto arr = [1, 2];

     arr ~= 3;
     assert(arr == [1, 2, 3]);
     assert(arr.length == 3);

     arr.length = 5;
     assert(arr == [1, 2, 3, 0, 0]);
     assert(arr.length == 5);

Note that new elements are set to the initial value of array 
elements (here int.init).

You can read more about this behaviour in the documentation: 
http://dlang.org/spec/arrays.html
Jan 31 2016
prev sibling parent reply Jonathan M Davis via Digitalmars-d-learn writes:
On Monday, February 01, 2016 07:42:56 Namal via Digitalmars-d-learn wrote:

 On Monday, 1 February 2016 at 07:41:33 UTC, Namal wrote:

 I understand that I cannot pass a variable to the static array
 like in C++, and have to use dynamic arrays. But how can I set
 the length for them without using a loop?


 I mean std::vector in C++, not array.


I'm not sure what you're trying to do exactly. static arrays cannot be
resized. They're a fixed size that's known at compile time. And they're
nothing like std::vector at all. e.g.

   int[42] arr;

That array has 42 elements and will always have 42 elements. And you can't
do something like

    int[x] arr;

unless x is known at compile time (e.g. it's an enum).

Now, if you want something like std::vector, what you're probably looking
for is either dynamic arrays or std.container.Array. And dynamic arrays can
be allocated pretty much just like you'd do an array in C++ or Java. e.g.

    auto arr = new int[](42);

Or you can set its length directly. e.g.

    arr.length = 42;

If the new length is shorter than the current length, then it would be
identical to slicing the array and reassigning it to the original. e.g.

    arr = arr[0 .. 42];

And if the new length is longer, then each of the new elements is set to the
init value of the of the element type.

Or you can append elements with ~=, which would be like using push_back.
e.g.

    arr ~= 12;

And if you want to reserve a particular capacity for the dynamic array like
you might do with std::vector, then just use reserve. e.g.

    arr.reserve(42);

If you haven't yet, you really should read this article:

http://dlang.org/d-array-article.html

It really should help you understand dynamic arrays in D.

However, be warned that its terminology is a bit off. It uses the term
dynamic array for the GC-managed block of memory that a typical dynamic
array refers to (per the language spec, T[] is a dynamic array regardless of
what kind of memory it refers to, and a GC-managed block of memory has no
official term). Similarly, it uses the term slice for T[] rather than
dynamic array, and while a non-null dynamic array is a slice of memory of
some kind, the term slice is used for a lot more in D than just arrays.
Still, while its terminology is a bit wrong, the article is very good and
really a must read for anyone who wants to understand dynamic arrays in D.

- Jonathan M Davis
Feb 01 2016
parent reply Namal <sotis22 mail.ru> writes:
Sorry guys that I didn't express myself well. I also mixed some 
stuff up. What I wanted to ask is this, in c++ this is valid

	int x = 3;
	int y = 10;
	int arr [x][y];

x,y are known at the compile time and arr is a static array. I 
can't do that in D so what is the best way to declare an array of 
that size?
Feb 01 2016
next sibling parent Daniel Kozak via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
V Mon, 01 Feb 2016 11:15:40 +0000
Namal via Digitalmars-d-learn <digitalmars-d-learn puremagic.com>
napsáno:


 Sorry guys that I didn't express myself well. I also mixed some 
 stuff up. What I wanted to ask is this, in c++ this is valid
 
 	int x = 3;
 	int y = 10;
 	int arr [x][y];
 
 x,y are known at the compile time and arr is a static array. I 
 can't do that in D so what is the best way to declare an array of 
 that size?


You can do that:
immutable x = 3;
immutable y = 10;
int[y][x] arr;
Feb 01 2016
prev sibling parent reply Jonathan M Davis via Digitalmars-d-learn writes:
On Monday, February 01, 2016 11:15:40 Namal via Digitalmars-d-learn wrote:

 Sorry guys that I didn't express myself well. I also mixed some
 stuff up. What I wanted to ask is this, in c++ this is valid

   int x = 3;
   int y = 10;
   int arr [x][y];

 x,y are known at the compile time and arr is a static array. I
 can't do that in D so what is the best way to declare an array of
 that size?


If x and y are known at compile time, then you can declare a static array
using them for dimensions. e.g.

    enum x = 3;
    enum y = 10;
    int[y][x] arr;

But x and y must be something that it is evaluated by the compiler at
compile time - e.g. an enum or a static variable. A local variable that just
so happens to be directly initialized (like in your example) won't work.

If x and y are _not_ known at compile time, then you can't use the to
declare a static array. You'll have to use a dynamic array. e.g.

    auto arr = new int[][](x, y);

- Jonathan M Davis
Feb 01 2016
parent reply Namal <sotis22 mail.ru> writes:
On Monday, 1 February 2016 at 12:12:00 UTC, Jonathan M Davis 
wrote:

 On Monday, February 01, 2016 11:15:40 Namal via 
 Digitalmars-d-learn wrote:

 Sorry guys that I didn't express myself well. I also mixed 
 some stuff up. What I wanted to ask is this, in c++ this is 
 valid

   int x = 3;
   int y = 10;
   int arr [x][y];

 x,y are known at the compile time and arr is a static array. I 
 can't do that in D so what is the best way to declare an array 
 of that size?


 If x and y are known at compile time, then you can declare a 
 static array using them for dimensions. e.g.

     enum x = 3;
     enum y = 10;
     int[y][x] arr;

 But x and y must be something that it is evaluated by the 
 compiler at compile time - e.g. an enum or a static variable. A 
 local variable that just so happens to be directly initialized 
 (like in your example) won't work.

 If x and y are _not_ known at compile time, then you can't use 
 the to declare a static array. You'll have to use a dynamic 
 array. e.g.

     auto arr = new int[][](x, y);

 - Jonathan M Davis


Thanks alot, I didn't know that way with new.
Feb 01 2016
next sibling parent Daniel Kozak via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:
V Mon, 01 Feb 2016 12:19:10 +0000
Namal via Digitalmars-d-learn <digitalmars-d-learn puremagic.com>
napsáno:


 On Monday, 1 February 2016 at 12:12:00 UTC, Jonathan M Davis 
 wrote:

 On Monday, February 01, 2016 11:15:40 Namal via 
 Digitalmars-d-learn wrote:  

 Sorry guys that I didn't express myself well. I also mixed 
 some stuff up. What I wanted to ask is this, in c++ this is 
 valid

   int x = 3;
   int y = 10;
   int arr [x][y];

 x,y are known at the compile time and arr is a static array. I 
 can't do that in D so what is the best way to declare an array 
 of that size?  


 If x and y are known at compile time, then you can declare a 
 static array using them for dimensions. e.g.

     enum x = 3;
     enum y = 10;
     int[y][x] arr;

 But x and y must be something that it is evaluated by the 
 compiler at compile time - e.g. an enum or a static variable. A 
 local variable that just so happens to be directly initialized 
 (like in your example) won't work.

 If x and y are _not_ known at compile time, then you can't use 
 the to declare a static array. You'll have to use a dynamic 
 array. e.g.

     auto arr = new int[][](x, y);

 - Jonathan M Davis  


 
 Thanks alot, I didn't know that way with new.


you can use this too:
auto arr = new int[y][x];
Feb 01 2016
prev sibling parent Jonathan M Davis via Digitalmars-d-learn writes:
On Monday, February 01, 2016 13:22:23 Daniel Kozak via Digitalmars-d-learn
wrote:

 V Mon, 01 Feb 2016 12:19:10 +0000
 Namal via Digitalmars-d-learn <digitalmars-d-learn puremagic.com>
 napsáno:


 On Monday, 1 February 2016 at 12:12:00 UTC, Jonathan M Davis
 wrote:

 On Monday, February 01, 2016 11:15:40 Namal via
 Digitalmars-d-learn wrote:

 Sorry guys that I didn't express myself well. I also mixed
 some stuff up. What I wanted to ask is this, in c++ this is
 valid

   int x = 3;
   int y = 10;
   int arr [x][y];

 x,y are known at the compile time and arr is a static array. I
 can't do that in D so what is the best way to declare an array
 of that size?


 If x and y are known at compile time, then you can declare a
 static array using them for dimensions. e.g.

     enum x = 3;
     enum y = 10;
     int[y][x] arr;

 But x and y must be something that it is evaluated by the
 compiler at compile time - e.g. an enum or a static variable. A
 local variable that just so happens to be directly initialized
 (like in your example) won't work.

 If x and y are _not_ known at compile time, then you can't use
 the to declare a static array. You'll have to use a dynamic
 array. e.g.

     auto arr = new int[][](x, y);

 - Jonathan M Davis


 Thanks alot, I didn't know that way with new.


 you can use this too:
 auto arr = new int[y][x];


True, but that's not the same thing. It creates a dynamic array of static
arrays instead of a static array of dynamic arrays. e.g.

    auto a = new int[][](5, 10);
    auto b = new int[5][10];
    auto c = new int[5][](10);
    writeln(typeof(a).stringof);
    writeln(typeof(b).stringof);
    writeln(typeof(c).stringof);

prints

    int[][]
    int[5][]
    int[5][]

And in the cases where the inner element is a static array, its length has
to be known compile time, whereas the lengths of the dynamic arrays don't.

On a side note, I really wish that putting the size inside of the brackets
was illegal for dynamic arrays to make what's going on clearer, but instead,
the outer layer can go either in the brackets or in the parens, whereas
where the other layers go changes them between dynamic arrays and static
arrays.

- Jonathan M Davis
Feb 01 2016