467,169 Members | 965 Online

# Float and double, precision and scale??

 If someone defines a float and double in C++, does that say anything about how many digits the number will hold at maximum and how many places there will be after the decimal point? Or is this open to interpretation? -- Posted via http://dbforums.com Jul 19 '05 #1
• viewed: 108144
Share:
3 Replies
 Erik2000 wrote in news:30****************@dbforums.com: If someone defines a float and double in C++, does that say anything about how many digits the number will hold at maximum and how many places there will be after the decimal point? Or is this open to interpretation? The class template std::numeric_limits< Type > defined in has a whole bunch of static members that describe the properties of C++'s numeric types. Here's an example: #include #include #include int main() { typedef std::numeric_limits< double > dl; typedef std::numeric_limits< float > fl; using namespace std; cout << "double:\n"; cout << "\tdigits (bits):\t\t" << dl::digits << endl; cout << "\tdigits (decimal):\t" << dl::digits10 << endl; cout << endl; cout << "float:\n"; cout << "\tdigits (bits):\t\t" << fl::digits << endl; cout << "\tdigits (decimal):\t" << fl::digits10 << endl; } I get: double: digits (bits): 53 digits (decimal): 15 float: digits (bits): 24 digits (decimal): 6 on my system. HTH Rob. -- http://www.victim-prime.dsl.pipex.com/ Jul 19 '05 #2
 Erik2000 wrote in news:30****************@dbforums.com: double:digits (bits): 53digits (decimal): 15float:digits (bits): 24digits (decimal): 6on my system. So does this mean that a double on your system can have up to 53 digits No the 53 is bits, computers (well most computers) store floating point values in binary, base 2 not base 10 (decimal). The 15 is the number of decimal digits. i.e. how meny decimal digits can be stored in 53 bits. and 15 digits behind the decimal point? The decimal point doesn't come in to it, hence the term *floating-point*. for example the number 2.3e20 doesn't really have "point" it is 230000000000000000000, also the number 2.3e-20 is 0.0000000000000000023, i.e. the "point" isn't within the digits at all. And for float, 24 digits and 6 digits behind the decimal point? As above 6 decimal digits total or 24 bits (base 2 digits). Rob. -- http://www.victim-prime.dsl.pipex.com/ Jul 19 '05 #3
 Erik2000 wrote:double:digits (bits): 53digits (decimal): 15float:digits (bits): 24digits (decimal): 6on my system. So does this mean that a double on your system can have up to 53 digits and 15 digits behind the decimal point? No it means, that a double uses 53 bits as its mantissa. In decimal this gives an average precission of 15 digits. And for float, 24 digits and 6 digits behind the decimal point? 24 bits for the mantissa which results in usually 6 decimal digits. Note: 100000.4 already uses 7 decimal digits and thus cannot be represented proerly with this float. (Just count the digits and ignore the decimal point). -> Conclusion: float isn't big enough most of the time to do serious calculations, at least not in a technical applications. So forget about float until you know what you do. You might also want to check out: http://docs-pdf.sun.com/800-7895/800-7895.pdf 53 digits is a really big number. 54 is bigger :-) -- Posted via http://dbforums.com -- Karl Heinz Buchegger kb******@gascad.at Jul 19 '05 #4

### This discussion thread is closed

Replies have been disabled for this discussion.