bug in floating point implementation ?
 Shyjan Mahamud
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Date:   (:) 
From:  Shyjan Mahamud <mahamud@m...> 
Subject:  bug in floating point implementation ? 
hi folks, i'm new to CAML, so i'm not sure if this is a known issue. is there a known bug or difference in the implementation of floating point arithmetic between the toplevel and that for the standalone compiled version. i've run the same exact function from both the toplevel and a standalone program. the numbers come out slightly different. from the toplevel > test sample 1 residue = 0.584302 0 : 0.964812 1 : 0.221230 2 : 0.006142 3 : 0.069594 4 : 0.059595 5 : 0.069939 6 : 0.001882 7 : 0.004956 8 : 0.046719 9 : 0.063383 10 : 0.012646 11 : 0.004402 from the stanalone > test sample 1 residue = 0.570106 0 : 0.952025 1 : 0.229649 2 : 0.023926 3 : 0.010044 4 : 0.036213 5 : 0.052544 6 : 0.005504 7 : 0.010667 8 : 0.054280 9 : 0.079775 10 : 0.079762 11 : 0.058593 all corresponding numbers are supposed to be exactly the same. also, as can be seen the errors accumulate as u go from 0 to 11. both call an SVD routine implemented in C (hence i did have to create my own toplevel linking in the SVD routine, so this is *not* the standard toplevel). i'm running a linux box, RH 5.2. i could provide the code, but it is long and am not sure if it helps anyway. if it helps, the following routine is what gives the values above (except for the residue), excuse the imperative style, it was just a transplant from Numerical Recipes : let svbksb uwv b = let isnan (x : float) = x <> x in let (u, w, v) = uwv in let m = (Array.length u) in let n = (Array.length w) in let sum = ref 0.0 in let tmp = Array.make n 0.0 in let x = Array.make n 0.0 in for j = 0 to n  1 do sum := 0.0; if ((not (isnan w.(j))) && (w.(j) != 0.0)) then begin for i = 0 to m  1 do sum := !sum +. u.(i).(j) *. b.(i) done; sum := !sum /. w.(j) end; tmp.(j) < !sum done; for j = 0 to n  1 do sum := 0.0; for jj = 0 to n  1 do sum := !sum +. v.(j).(jj) *. tmp.(jj) done; x.(j) < !sum done; x;; i'm baffled !!!  shyjan mahamud Carnegie Mellon University