Viktor Cerovski wrote:
> More precisely, if you have M-bits of mantissa, your integer should be
> 2^M .
D'oh! Of course. Thank you.
> Since dlamch reports that it sees 53 bits, that means your integer
> should be somewhere near 1e16. But it is not according to the test!
This is probably be due to J's tolerance. Which, according to:
http://www.jsoftware.com/help/dictionary/d000.htm
defaults to 2^_44 . Your tests seem to support this conclusion:
> two=:1%0.5
> (= >:)two^43
> 0
> (= >:)two^44
> 1
as do these:
(=!.0 >:)two^43
0
(=!.0 >:)two^44
0
Further tests more or less agree with your Lisp examples:
(=!.0 >:)two^52
0
(=!.0 >:)two^53
1
> $ clisp -q
> [1]> (defun test(x)(=(- x 1.0d0)x))
> TEST
> [2]> (test(expt 2.0d0 53))
> NIL
> [3]> (test(expt 2.0d0 54))
> T
But I don't know why J's answer changes at two^53 yet Lisp's at 2^54 .
Maybe the FP representation of 1%0.5 in J isn't the same as 2.0d0 in
Lisp.
-Dan
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