Reduce has dreadful performance?

Russel Winder via Digitalmars-d digitalmars-d at puremagic.com
Thu Jun 18 03:27:30 PDT 2015


On a given machine, the code:

double sequential_loop(const int n, const double delta) {
  auto sum = 0.0;
  foreach (immutable i; 1 .. n + 1) {
    immutable x = (i - 0.5) * delta;
    sum += 1.0 / (1.0 + x * x);
  }
  return 4.0 * delta * sum;
}

runs in about 6.70s. However the code:

double sequential_reduce(const int n, const double delta) {
  return 4.0 * delta * reduce!((double t, int i){immutable x = (i -
0.5) * delta; return t + 1.0 / (1.0 + x * x);})(0.0, iota(1, n + 1));
}

runs in about 17.03s, whilst:

double sequential_reduce_alt(const int n, const double delta) {
  return 4.0 * delta * reduce!"a + b"(
         map!((int i){immutable x = (i - 0.5) * delta; return 1.0 /
(1.0 + x * x);})(iota(1, n + 1)));
}

takes about 28.02s. Unless I am missing something (very possible), this
is not going to create a good advert for D as an imperative language
with declarative (internal iteration) expression.


-- 
Russel.
=============================================================================
Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder at ekiga.net
41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel at winder.org.uk
London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder
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