Looks to me like your Clojure loop runs in the opposite direction (counting
downwards) versus the other languages. Since your code only returns the
result of the last iteration of the loop, it's not too surprising that they
return completely different results -- the last iteration of the Clojure
code is a completely different input than in the other languages.
On Wed, Feb 5, 2014 at 5:17 AM, Glen Fraser <holag...@gmail.com> wrote:
> (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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