Thanks for the tip. After reading your comment, I looked and discovered
the Java library called StrictMath, and tried it (replacing Math/cos and
Math/sin by the StrictMath versions). I did indeed get different results
than with the regular library, but unfortunately still not the same answer
as in other languages. I guess the Java implementation(s) are indeed
different. It's not a big deal for me, just something I found confusing,
wondering if I'd done something wrong.
Thanks,
Glen.
On Wednesday, February 5, 2014 4:06:31 PM UTC+1, Jon Harrop wrote:
>
>
>
> IIRC, Java provides unusual trigonometric functions which, I’m guessing,
> Clojure is using. I think the Java ones are actually more accurate (and
> slower) so you may well find the answer obtained on the JVM is more precise
> than the others.
>
>
>
> Cheers,
>
> Jon.
>
>
>
> *From:* clo...@googlegroups.com <javascript:> [mailto:
> clo...@googlegroups.com <javascript:>] *On Behalf Of *Glen Fraser
> *Sent:* 05 February 2014 13:17
> *To:* clo...@googlegroups.com <javascript:>
> *Subject:* Confused by Clojure floating-point differences (compared to
> other languages)
>
>
>
> (sorry if you received an earlier mail from me that was half-formed, I hit
> send by accident)
>
>
>
> Hi there, I'm quite new to Clojure, and was trying to do some very simple
> benchmarking with other languages. I was surprised by the floating-point
> results I got, which differed (for the same calculation, using doubles)
> compared to the other languages I tried (including C++, SuperCollider, Lua,
> Python).
>
>
>
> My benchmark iteratively runs a function 100M times: g(x) <-- sin(2.3x) +
> cos(3.7x), starting with x of 0.
>
>
>
> In the other languages, I always got the result *0.0541718*..., but in
> Clojure I get *0.24788989*.... I realize this is a contrived case, but
> -- doing an identical sequence of 64-bit floating-point operations on the
> same machine should give the same answer. Note that if you only run the
> function for about ~110 iterations, you get the same answer in Clojure (or
> very close), but then it diverges.
>
>
>
> I assume my confusion is due to my ignorance of Clojure and/or Java's math
> library. I don't think I'm using 32-bit floats or the "BigDecimal" type (I
> even explicitly converted to double, but got the same results, and if I
> evaluate the *type* it tells me *java.lang.Double*, which seems right).
> Maybe Clojure's answer is "better", but I do find it strange that it's
> different. Can someone explain this to me?
>
>
>
> Here are some results:
>
>
>
> *Clojure: ~23 seconds*
>
> (defn g [x] (+ (Math/sin (* 2.3 x)) (Math/cos (* 3.7 x))))
>
> (loop [i 100000000 x 0] (if (pos? i) (recur (dec i) (g x)) x))
>
> ;; final x: *0.24788989279493556 **(???)*
>
>
>
> *C++ (g++ -O2): ~4 seconds*
>
> double g(double x) {
>
> return std::sin(2.3*x) + std::cos(3.7*x);
>
> }
>
> int main() {
>
> double x = 0;
>
> for (int i = 0; i < 100000000; ++i) {
>
> x = g(x);
>
> }
>
> std::cout << "final x: " << x << std::endl;
>
> return 0;
>
> }
>
> // final x: *0.0541718*
>
>
>
> *Lua: ~39 seconds*
>
> g = function(x)
>
> return math.sin(2.3*x) + math.cos(3.7*x)
>
> end
>
>
>
> x = 0; for i = 1, 100000000 do x = g(x) end
>
> -- Final x: *0.054171801051906*
>
>
>
> *Python: ~72 seconds*
>
> def g(x):
>
> return math.sin(2.3*x) + math.cos(3.7*x)
>
>
>
> x = 0
>
> for i in xrange(100000000):
>
> x = g(x)
>
>
>
> # Final x: *0.05417180105190572*
>
>
>
> *SClang: ~26 seconds*
>
> g = { |x| sin(2.3*x) + cos(3.7*x) };
>
> f = { |x| 100000000.do{ x = g.(x) }; x};
>
> bench{ f.(0).postln };
>
> // final x: *0.054171801051906* (same as C++, Lua, Python; different from
> Clojure)
>
>
>
> Thanks,
>
> Glen.
>
>
>
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