C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  std::ios_base::fixed fails for +(1.79769E+308)          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Andrew Guest Posted: Thu Feb 19, 2004 2:08 pm    Post subject: std::ios_base::fixed fails for +(1.79769E+308) Hi I've got the following piece of code that is failing when I pass it a double value of +(1.79769E+308). If I change the fixed notation to scientific, std::ios_base::scientific, the code succeeds. Can anyone explain why this happens...is it a default precision issue for fixed notation and what is the precision compared to scientific notation. void StringTokenizer::addToken(SVR::Double token) { std::strstream conversionStream; SVR::String literal; conversionStream.setf(std::ios_base::fixed); conversionStream << token; conversionStream >> literal; addLiteral(literal); } Many thanks Andrew [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Billy O'Connor Guest Posted: Fri Feb 20, 2004 1:17 pm    Post subject: Re: std::ios_base::fixed fails for +(1.79769E+308) [email]awylie (AT) netcomuk (DOT) co.uk[/email] (Andrew) writes: Quote: Hi I've got the following piece of code that is failing when I pass it a double value of +(1.79769E+308). If I change the fixed notation to scientific, std::ios_base::scientific, the code succeeds. Can anyone explain why this happens...is it a default precision issue for fixed notation and what is the precision compared to scientific notation. void StringTokenizer::addToken(SVR::Double token) { std::strstream conversionStream; SVR::String literal; conversionStream.setf(std::ios_base::fixed); conversionStream << token; conversionStream >> literal; addLiteral(literal); } This works, modified a bit: void addToken(double token) { std::strstream conversionStream; string literal; conversionStream.setf(std::ios_base::fixed); conversionStream << token; conversionStream >> literal; cout << literal << endl; } int main(void) { addToken(1.79769E+308); return 0; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Fri Feb 20, 2004 5:28 pm    Post subject: Re: std::ios_base::fixed fails for +(1.79769E+308) [email]awylie (AT) netcomuk (DOT) co.uk[/email] (Andrew) wrote in message news:<4f8979bf.0402181241.56a18d95 (AT) posting (DOT) google.com>... Quote: I've got the following piece of code that is failing when I pass it a double value of +(1.79769E+308). What do you mean by fails: it doesn't compile, it generates an error message at runtime, it crashes? Quote: If I change the fixed notation to scientific, std::ios_base::scientific, the code succeeds. While it shouldn't "fail", converting such a value to a fixed notation doesn't make much sense. Unless your goal is to generate a string with a couple of hundred 0's. Quote: Can anyone explain why this happens... Error in the library implementation? Quote: is it a default precision issue for fixed notation and what is the precision compared to scientific notation. Both have a default precision of 6 digits after the decimal. With scientific notation, this means a total of 7 digits, always. With fixed, of course, the actual number of significant digits depends on the actual value -- in your case, you have asked for 314 total digits. A double doesn't have this precision, of course (at least not on any machine I've used), but the library should generate the digits anyway, generating 0's once you've gone beyond the actual precision (typically about 17 digits). (Formally, the C++ standard refers to the C standard for the signification of formatting flags. And the C standard has the somewhat curious sentence that "The number of characters that can be produced by any single conversion shall be at least 4095." Which means that something like "s << std::string( 5000, 'x' )" is perhaps undefined behavior, but what you are trying to do should work, even if it is senseless.) -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Sat Feb 21, 2004 4:11 am    Post subject: Re: std::ios_base::fixed fails for +(1.79769E+308) Andrew wrote: Quote: Hi I've got the following piece of code that is failing when I pass it a double value of +(1.79769E+308). Presumably that is DBL_MAX on your system? Also what do you mean by failing? I don't see any way in which the code can indicate failure, so I guess you mean that the string it produces is incorrect. Quote: If I change the fixed notation to scientific, std::ios_base::scientific, the code succeeds. Can anyone explain why this happens...is it a default precision issue for fixed notation and what is the precision compared to scientific notation. I think it is a bug in your implementation, but without knowing what the failure is it's hard to say. Quote: void StringTokenizer::addToken(SVR::Double token) { std::strstream conversionStream; SVR::String literal; conversionStream.setf(std::ios_base::fixed); conversionStream << token; conversionStream >> literal; addLiteral(literal); } Also, please don't post code that depends on classes or functions that aren't well-known (and preferably part of the standard library). The following program should demonstrate the problem where it exists: #include <cfloat> #include <iostream> #include <ostream> #include <string> #include <strstream> int main() { std::strstream conversionStream; std::string literal; conversionStream.setf(std::ios_base::fixed); conversionStream << DBL_MAX; std::cout.write(conversionStream.str(), conversionStream.pcount()) << 'n'; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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