Re: [Bug: 21.4.19] Bruised floating point numbers from string-to-int

Executive summary: It's just a bad idea to print floating point numbers in their natural representation, i.e., `(format "%s" x)'; alway use `(format "%f" x)' (usually with a precision specifier): (format "%5.3f" (string-to-number "8.180E-1")) => "0.818" Plain "%f" has some default precision, but it's probably too wide for most tables (format "%f" (string-to-number "8.180E-1")) => "0.818000" Welcome to floating point! All hope abandon, ye who enter here. Please note that *all* floating values are unsightly, containing a 53-bit mantissa where IEEE 754 doubles are used (about 16 digits, as in your example). As a matter of historical practice, most standard libraries truncate trailing zeros when printing. VC++ apparently applies some kind of heuristic to "simplify" trailing nines as well. However, there are always going to be examples that produce what you consider bad behavior. In fact, consider (format "%s" (+ 0.125 0.875)) => "1.0" I bet you would want 1.000, no? You just can't win. IEEE 754 standard floating point is specified so that integers of sufficiently low precision will always be represented exactly. However, there's no guarantee that *any* fraction will be represented exactly, by design. (And of course an integer with more precision than the floating point representation provides is not represented exactly.) This implies that it is not possible to round-trip string->number->string. The point is to allow efficient *calculations* on various architectures. "Printed representation is not my job, mon!" Your architecture evidently uses a binary representation, so "most" decimal fractions cannot be represented exactly. vin> Interestingly, on my windows native XEmacs (both 21.4 and vin> 21.5), compiled with Visual C++, I get: vin> (string-to-int "8.180E-1") 0.818 vin> On my Cygwin XEmacs (both 21.4 and 21.5), compiled with gcc, vin> I get: vin> (string-to-int "8.180E-1") 0.8179999999999999 Yup. `string-to-int' is an alias for `string-to-number', which is (ultimately) just a wrapper around the platform's printf implementation. What you get is what you get. -- School of Systems and Information Engineering http://turnbull.sk.tsukuba.ac.jp University of Tsukuba Tennodai 1-1-1 Tsukuba 305-8573 JAPAN Ask not how you can "do" free software business; ask what your business can "do for" free software.