how to initialize immutable 2 dim array
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
}
Re: how to initialize immutable 2 dim array
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.
Re: how to initialize immutable 2 dim array
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
Re: how to initialize immutable 2 dim array
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.
Re: how to initialize immutable 2 dim array
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
Re: how to initialize immutable 2 dim array
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
Re: how to initialize immutable 2 dim array
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.
