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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
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 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
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 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 reply 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
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