c++ - sprintf too many/few decimals -

i must convert decimal numbers using non-scientific (ie. no mantissa/exponent/e's) character string. code looks this:

/*! \brief converts <em>xml schema decimal</em> */ char *todecimal(double val) const {     const size_t nmax = 200;     char *doublestr = new char[nmax];      sprintf(doublestr, "%1.6f", val);      return doublestr; } 

the problem when input val 1 function returns 1.000000 hoping output of 1. also, if change code sprintf(doublestr, "%1.0f", val); correctly outputs 1, if input val changed 0.000005 output 0, , hoping output 0.000005. want output short possible , remove unnessesary 0's. possible sprintf? support 3.4e +/- 38 range function.

it turns out c++ iostreams (specifically, ostringstream) better suited task sprintf.

use std::fixed manipulator disable scientific notation. use std::setprecision specify precision (number of characters after decimal dot). in case, precision of 45 places seems enough represent float numbers.

#include <sstream> #include <string> #include <iostream> #include <iomanip>  std::string convert(double x) {     std::ostringstream buffer;     buffer << std::fixed << std::setprecision(45) << x;     std::string result = buffer.str();      return result; } 

in addition, clean-up result, remove trailing zeros.

    size_t = result.find_last_not_of('0');     if (result[i] != '.')         ++i;     result.erase(i); 

note: clean-up of trailing zeros work numbers representable (like 0.75 or 0.03125): example, number 0.1 converted 0.10000000000000000555111512312578270211815834. 1 use non-constant precision (depending on magnitude of number), tricky right.

instead, it's possible use following ugly (and slow) hack: try converting start of string double, , cut string if result equal initial number.

size_t i; (i = 1; < result.size(); ++i) {     std::istringstream cut(result.substr(0, i));     double temp;     cut >> temp; // verbose syntax fit 1 line      if (temp == x) // using boost::lexical_cast         break; }