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: clojure@googlegroups.com [mailto:clojure@googlegroups.com] On Behalf Of Glen Fraser Sent: 05 February 2014 13:17 To: clojure@googlegroups.com 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. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
