Re: How to initialize an immutable array

[Dicebot]

On Friday, 1 March 2013 at 20:05:41 UTC, Sparsh Mittal wrote:
I am making a program which accesses 1D array using for loop 
and then I am parallelizing this with foreach, TaskPool and 
parallel.


The array does not need to change, once initialized. However, 
the parallel version takes more time than serial version, which 
I think may be because compiler is trying to make sure that 
array is properly handled by different threads.


So, is there a way, an array can be made immutable and still 
initialized? Thanks a lot for your time.


immutable arr = [ some, data, that, simple ];

But I suppose your case is somewhat more complex. Probably you 
can provide an example of problem via DPaste 
(http://dpaste.1azy.net) ?

Re: How to initialize an immutable array

[Sparsh Mittal]

Array is really big!


import std.stdio;
import std.datetime;
import std.parallelism;
import std.range;
//int numberOfWorkers = 2; //for parallel;
double my_abs(double n) { return n  0 ? n : -n; }

immutable long DIM = 1024L*1024L *128L;

void main()
{

  double[] signal = new double[DIM+1];

  double temp;


  double sample[2]= [4.1,7.2];



  for(long i=0L; i DIM+1; i++)
  {
signal[i] = (i+ DIM)%7 + (i+DIM+1)%5; // could be any random 
value

  }

  //auto workerPool = new TaskPool(numberOfWorkers); // for 
parallel

  StopWatch sw;
  sw.start(); //start/resume mesuring.


  for (long i=0L; i DIM; i++)
  //foreach(i; workerPool.parallel(iota(0, DIM))) // for parallel
  {

temp =
my_abs(sample[0]-signal[i]) + 
my_abs(sample[1]-signal[i+1]) ;

  }
  //workerPool.finish(); // for parallel

  sw.stop(); //stop/pause measuring.


  writeln( Total time: , (sw.peek().msecs/1000), [sec]);

}

It has both serial and parallel versions. Just comment/uncomment 
as per comments.

Re: How to initialize an immutable array

[FG]

I suppose this:

immutable long DIM = 1024L*1024L *128L;
immutable(double)[] signal = new double[DIM+1];
static this() {
for (long i=0L; i DIM+1; i++) {
signal[i] = (i+DIM)%7 + (i+DIM+1)%5;
}
}
void main()
{ ... }

Re: How to initialize an immutable array

[bearophile]

Sparsh Mittal:

So, is there a way, an array can be made immutable and still 
initialized? Thanks a lot for your time.


There are various ways to do it. One of the safest way to do it 
is to create a mutable array inside a strongly pure function, and 
then when you return it assign it to immutable:



import std.stdio, std.datetime, std.range;

double myAbs(in double n) pure nothrow {
return n  0 ? n : -n;
}

enum long DIM = 1024L * 1024L * 128L;

double[] genSignal() pure nothrow {
auto signal = new double[DIM + 1];

foreach (immutable i; 0 .. DIM + 1) {
signal[i] = (i + DIM) % 7 + (i + DIM + 1) % 5;
}

return signal;
}

void main() {
immutable signal = genSignal();

double sample[2] = [4.1, 7.2];

StopWatch sw;
sw.start;
foreach (immutable i; 0 .. DIM) {
double temp = myAbs(sample[0] - signal[i]) +
  myAbs(sample[1] - signal[i + 1]);
}
sw.stop;

writeln( Total time: , sw.peek.msecs / 1000, [sec]);
}



A less safe way to do it is to use assumeUnique from Phobos.

Bye,
bearophile

Re: How to initialize an immutable array

[Sparsh Mittal]

foreach (immutable i; 0 .. DIM + 1) {


Thanks. However, rdmd gives error on this line:

temp1.d(12): Error: no identifier for declarator immutable(i)

Re: How to initialize an immutable array

[Sparsh Mittal]

On Friday, 1 March 2013 at 20:28:19 UTC, FG wrote:

I suppose this:

immutable long DIM = 1024L*1024L *128L;
immutable(double)[] signal = new double[DIM+1];
static this() {
for (long i=0L; i DIM+1; i++) {
signal[i] = (i+DIM)%7 + (i+DIM+1)%5;
}
}
void main()
{ ... }


Thanks. This gives an error, which I don't know how to resolve:

Error: cannot evaluate new double[](134217729LU) at compile time

Can you please tell.

Re: How to initialize an immutable array

[Sparsh Mittal]

Removing immutable word solves the problem. Thanks.

Re: How to initialize an immutable array

[Sparsh Mittal]

I realized that  access to temp causes bottleneck. On defining 
it inside for loop, it become local and then there is speedup. 
Defining it outside makes it shared, which slows the program.

Re: How to initialize an immutable array

[bearophile]

Sparsh Mittal:


Thanks. However, rdmd gives error on this line:

temp1.d(12): Error: no identifier for declarator immutable(i)


Probably v.2.062 of the D compiler is enough to not see that 
error.


Bye,
bearophile

Re: How to initialize an immutable array

[FG]

On 2013-03-01 22:05, Sparsh Mittal wrote:

On Friday, 1 March 2013 at 20:28:19 UTC, FG wrote:

I suppose this:

immutable long DIM = 1024L*1024L *128L;
immutable(double)[] signal = new double[DIM+1];
static this() {
for (long i=0L; i DIM+1; i++) {
signal[i] = (i+DIM)%7 + (i+DIM+1)%5;
}
}
void main()
{ ... }


Thanks. This gives an error, which I don't know how to resolve:

Error: cannot evaluate new double[](134217729LU) at compile time

Can you please tell.



Oh, sorry. Was typing without thinking. :)
Here's a working sample (with long changed to uint):


import std.stdio, std.datetime, std.parallelism, std.range;
double my_abs(double n) { return n  0 ? n : -n; }
immutable uint DIM = 1024 * 1024 * 128;
immutable(double)[] signal;

static this() {
auto temp = new double[DIM+1];
for (uint i = 0; i  DIM + 1; i++)
temp[i] = (i + DIM) % 7 + (i + DIM + 1) % 5;
signal = cast(immutable)temp;
}

void main()
{
double temp;
double sample[2] = [4.1,7.2];
StopWatch sw;
sw.start();
for (uint i = 0; i  DIM; i++)
{
temp = my_abs(sample[0]-signal[i])
+ my_abs(sample[1]-signal[i+1]);
}
sw.stop();
writeln( Total time: , (sw.peek().msecs/1000), [sec]);
}