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