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digitalmars.D.learn - Create uninitialized dynamic array

reply Igor Shirkalin <mathsoft inbox.ru> writes:
Hello!

Preface:
I need 1G array of ints (or anything else).

Problem:
I want to quickly fill it with my own data and I do not want to 
waste CPU time to fill it with zeros (or some other value).


I do like this:


void main() {
     int[] data;
     // key code:
     data.length = SOMETHING; // how to create an array and leave 
it uninitialized?

     // fill 'data' with some data
     foreach(i, ref v; data) v=i; // an example
}

Is there a pure way to make what I want?
Oct 05 2017
next sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Thursday, 5 October 2017 at 19:59:48 UTC, Igor Shirkalin wrote:

 I want to quickly fill it with my own data and I do not want to 
 waste CPU time to fill it with zeros (or some other value).


You could always just allocate it yourself. Something that large 
is liable to be accidentally pinned by the GC anyway, so I 
suggest:

int[] data;
int* dataptr = cast(int*) malloc(SOMETHING * int.sizeof);
if(dataptr is null) throw new Exception("malloc failed");
scope(exit) free(dataptr);
data = dataptr[0 .. SOMETHING];
// work with data normally here


Just keep in mind it is freed at scope exit there, so don't 
escape slices into it.
Oct 05 2017
parent reply Igor Shirkalin <mathsoft inbox.ru> writes:
On Thursday, 5 October 2017 at 20:19:15 UTC, Adam D. Ruppe wrote:

 On Thursday, 5 October 2017 at 19:59:48 UTC, Igor Shirkalin 
 wrote:

 I want to quickly fill it with my own data and I do not want 
 to waste CPU time to fill it with zeros (or some other value).


 You could always just allocate it yourself. Something that 
 large is liable to be accidentally pinned by the GC anyway, so 
 I suggest:

 int[] data;
 int* dataptr = cast(int*) malloc(SOMETHING * int.sizeof);
 if(dataptr is null) throw new Exception("malloc failed");
 scope(exit) free(dataptr);
 data = dataptr[0 .. SOMETHING];
 // work with data normally here


 Just keep in mind it is freed at scope exit there, so don't 
 escape slices into it.


Thank you, Adam, for pinpoint answer.

Doesn't it mean we have to avoid GC for such large blocks? And 
what if we need a lot blocks with less sizes? I'm from C++ world 
but... I like GC.
Usually the block(s) is scoped with some more complex way, so it 
is good to pass it to GC for management.
Maybe (say LDC) compiler can reject this unuseful initialization 
on simplest cases.

Shortly, I'm still in doubt.
Oct 05 2017
parent Adam D. Ruppe <destructionator gmail.com> writes:
On Thursday, 5 October 2017 at 20:52:00 UTC, Igor Shirkalin wrote:

 Doesn't it mean we have to avoid GC for such large blocks? And 
 what if we need a lot blocks with less sizes?


No, it can work, especially if you are on 64 bit. Just if it is 
trivial I'd malloc it, but if the lifetime is nontrivial, sure, 
GC it.

The GC.malloc function from `import core.memory;` works just like 
malloc, except the garbage collector is aware of it and will 
collect eventually:

http://dpldocs.info/experimental-docs/core.memory.GC.malloc.html


int[] data = (cast(int*) GC.malloc(SOMETHING * int.sizeof, 0, 
typeid(int)))[0 .. SOMETHING];


Note that `typeid` returns the TypeInfo for that type, so you can 
pass it there.

Then the GC will collect if you like.


 Usually the block(s) is scoped with some more complex way, so 
 it is good to pass it to GC for management.


Yeah, that's good too.
Oct 05 2017
prev sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Thursday, 5 October 2017 at 19:59:48 UTC, Igor Shirkalin wrote:

 Is there a pure way to make what I want?


oh i almost forgot about this function too:

http://dpldocs.info/experimental-docs/std.array.uninitializedArray.1.html


import std.array;
double[] arr = uninitializedArray!(double[])(100);
Oct 05 2017
parent Igor Shirkalin <mathsoft inbox.ru> writes:
On Thursday, 5 October 2017 at 21:04:30 UTC, Adam D. Ruppe wrote:

 On Thursday, 5 October 2017 at 19:59:48 UTC, Igor Shirkalin 
 wrote:

 Is there a pure way to make what I want?


 oh i almost forgot about this function too:

 http://dpldocs.info/experimental-docs/std.array.uninitializedArray.1.html


 import std.array;
 double[] arr = uninitializedArray!(double[])(100);


Ha! I saw it some day and forgot too!

And GC.malloc is, I think, what I need.

Thank you!
Oct 05 2017