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digitalmars.D.learn - how to initialize immutable 2 dim array

reply Michal Minich <michal.minich gmail.com> writes:
I have global static array and I want it to initialize in module 
constructor, but I get error. I there way to do it without using enum.

immutable int[5][5] arr;

static this () {
   arr = new int[5][5]; // Error: slice arr[] is not mutable
}
Oct 31 2010
parent reply Stewart Gordon <smjg_1998 yahoo.com> writes:
On 31/10/2010 17:10, Michal Minich wrote:
 I have global static array and I want it to initialize in module
 constructor, but I get error. I there way to do it without using enum.

So you want to initialise it with data acquired at runtime, but make it immutable once initialised?
 immutable int[5][5] arr;

 static this () {
     arr = new int[5][5]; // Error: slice arr[] is not mutable
 }

You have a more fundamental problem here. arr is a static array, so you can't reference-assign it. Best you can do is to either: - initialise it statically, using CTFE or template metaprogramming to do the calculations, if they don't depend on getting external data - use a dynamic array instead, and use .idup to populate it - create a mutable pointer to it in the module constructor int[5][5]* marr = cast(int[5][5]*) arr; and use that to populate the array. But I'm not sure whether this leads to UB, and I still wish the means of casting away const or immutable were explicit. Stewart.
Oct 31 2010
next sibling parent reply Michal Minich <michal.minich gmail.com> writes:
On Sun, 31 Oct 2010 17:46:29 +0000, Stewart Gordon wrote:

 On 31/10/2010 17:10, Michal Minich wrote:
 I have global static array and I want it to initialize in module
 constructor, but I get error. I there way to do it without using enum.

So you want to initialise it with data acquired at runtime, but make it immutable once initialised?
 immutable int[5][5] arr;

 static this () {
     arr = new int[5][5]; // Error: slice arr[] is not mutable
 }

You have a more fundamental problem here. arr is a static array, so you can't reference-assign it. Best you can do is to either: - initialise it statically, using CTFE or template metaprogramming to do the calculations, if they don't depend on getting external data - use a dynamic array instead, and use .idup to populate it - create a mutable pointer to it in the module constructor int[5][5]* marr = cast(int[5][5]*) arr; and use that to populate the array. But I'm not sure whether this leads to UB, and I still wish the means of casting away const or immutable were explicit. Stewart.

I would rather use CTFE, but seems it isn't possible to work with 2 dim arrays at compile time. Also I need it to be static array, otherwise i would need to rewrite and test more code. I like the simple workaround with using mutable pointer, but I'm getting access violation error even when arr is mutable ... and really don't understand why... :( int[256][256] arr; static this () { int[256][256]* pArr = &arr; for (int y = 0; y <= 255; y++) for (int x = 0; x <= 255; x++) pArr[x][y] = x * y; // Access violation when x = 2 and y = 0 }
Oct 31 2010
parent reply bearophile <bearophileHUGS lycos.com> writes:
Michal Minich:

 I like the simple workaround 
 with using mutable pointer, but I'm getting access violation error even 
 when arr is mutable ... and really don't understand why... :(
 
 int[256][256] arr;
 
 static this () {
 
     int[256][256]* pArr = &arr;
         
     for (int y = 0; y <= 255; y++)
         for (int x = 0; x <= 255; x++)
             pArr[x][y] = x * y; // Access violation when x = 2 and y = 0
 }

You need to put a bit more thought on your code: import std.stdio: writeln; int[256][256] arr; static this () { auto pArr = &arr; foreach (y; 0 .. (*pArr)[0].length) foreach (x; 0 .. (*pArr).length) (*pArr)[x][y] = x * y; } void main() { writeln(arr); } (I have just fixed your code locally, I don't know what you are doing and why you are using a pointer to a fixed sized array, it's an unusual idiom in D.) Bye, bearophile
Oct 31 2010
parent reply bearophile <bearophileHUGS lycos.com> writes:
Michal Minich:

 I need to use this workaround because global array I want to initialize 
 in module constructor is immutable.

A pointer to a immutable array is immutable. So you aren't improving the code, just making it more obfuscated. So there is zero need to use a pointer. Immutable data is not meant to be modified, a compiler may move your empty array in read only memory. This is bad. Some people have recently discussed about this problem and a bug report was proposed. Until some language-supported solution is found, the simpler solution for this problem may be to use a static mutable array...
 is there some performance overhead compared to using  
 for (int x = 0; x <= 255; x++) ?

In theory there is no difference. In practice there is no difference with dmd only if you use the -O compiler switch. So it's a compiler implementation thing, and you need to look at the produced asm if you want to be sure with a different compiler. Bye, bearophile
Oct 31 2010
parent bearophile <bearophileHUGS lycos.com> writes:
 Some people have recently discussed about this problem and a bug report was
proposed. Until some language-supported solution is found, the simpler solution
for this problem may be to use a static mutable array...

http://d.puremagic.com/issues/show_bug.cgi?id=5147
Oct 31 2010
prev sibling next sibling parent Michal Minich <michal.minich gmail.com> writes:
On Sun, 31 Oct 2010 14:53:42 -0400, bearophile wrote:

 import std.stdio: writeln;
 
 int[256][256] arr;
 
 static this () {
     auto pArr = &arr;
     foreach (y; 0 .. (*pArr)[0].length)
         foreach (x; 0 .. (*pArr).length)
             (*pArr)[x][y] = x * y;
 }
 
 void main() {
     writeln(arr);
 }
 
 (I have just fixed your code locally, I don't know what you are doing
 and why you are using a pointer to a fixed sized array, it's an unusual
 idiom in D.)
 
 Bye,
 bearophile

(*pArr)[x][y] = .. fixes it! Thank you! I need to use this workaround because global array I want to initialize in module constructor is immutable.
Oct 31 2010
prev sibling next sibling parent Michal Minich <michal.minich gmail.com> writes:
On Sun, 31 Oct 2010 19:13:33 +0000, Michal Minich wrote:

     foreach (y; 0 .. (*pArr)[0].length)


also good tip on the foreach syntax, it is much cooler :) is there some performance overhead compared to using for (int x = 0; x <= 255; x++) ?
Oct 31 2010
prev sibling parent Michal Minich <michal.minich gmail.com> writes:
On Sun, 31 Oct 2010 17:21:50 -0400, bearophile wrote:

 Some people have recently discussed about this problem and a bug report
 was proposed. Until some language-supported solution is found, the
 simpler solution for this problem may be to use a static mutable
 array...

http://d.puremagic.com/issues/show_bug.cgi?id=5147

Thank you for the bug report. Voted.
Oct 31 2010