Convert float to string with precision number of decimal digits specified

Question

How do you convert a float to a string in C   while specifying the precision  amp  number of decimal digits    For example  3 14159265359 - gt   3 14

User · Answer

Here I am providing a negative example where your want to avoid when converting floating number to strings.

float num=99.463;
float tmp1=round(num*1000);
float tmp2=tmp1/1000;
cout << tmp1 << " " << tmp2 << " " << to_string(tmp2) << endl;

You get

99463 99.463 99.462997

Note: the num variable can be any value close to 99.463, you will get the same print out. The point is to avoid the convenient c++11 "to_string" function. It took me a while to get out this trap. The best way is the stringstream and sprintf methods (C language). C++11 or newer should provided a second parameter as the number of digits after the floating point to show. Right now the default is 6. I am positing this so that others won't wast time on this subject.

I wrote my first version, please let me know if you find any bug that needs to be fixed. You can control the exact behavior with the iomanipulator. My function is for showing the number of digits after the decimal point.

string ftos(float f, int nd) {
   ostringstream ostr;
   int tens = stoi("1" + string(nd, '0'));
   ostr << round(f*tens)/tens;
   return ostr.str();
}

User · Answer

You can use C  20 std  format or the fmt  format function from the  fmt  library  std  format is based on   include  lt fmt core h gt   int main     std  string s   fmt  format  quot    2f  quot   3 14159265359      s     quot 3 14 quot     where 2 is a precision

User · Answer

The customary method for doing this sort of thing is to  print to string   In C   that means using std  stringstream something like   std  stringstream ss  ss  lt  lt  std  fixed  lt  lt  std  setprecision 2   lt  lt  number  std  string mystring   ss str

User · Answer

A typical way would be to use stringstream    include  lt iomanip gt   include  lt sstream gt   double pi   3 14159265359  std  stringstream stream  stream  lt  lt  std  fixed  lt  lt  std  setprecision 2   lt  lt  pi  std  string s   stream str      See fixed     Use fixed floating-point notation      Sets the floatfield format flag for the str stream to fixed       When floatfield is set to fixed  floating-point values are written using fixed-point notation  the value is represented with exactly as many digits in the decimal part as specified by the precision field  precision  and with no exponent part    and setprecision     For conversions of technical purpose  like storing data in XML or JSON file  C  17 defines to chars family of functions   Assuming a compliant compiler  which we lack at the time of writing   something like this can be considered    include  lt array gt   include  lt charconv gt   double pi   3 14159265359  std  array lt char  128 gt  buffer  auto  ptr  ec    std  to chars buffer data    buffer data     buffer size    pi                                 std  chars format  fixed  2   if  ec    std  errc          std  string s buffer data    ptr                 else          error handling

User · Answer

Another option is snprintf   double pi   3 1415926   std  string s 16    0    auto written   std  snprintf  amp s 0   s size       2f   pi   s resize written     Demo  Error handling should be added  i e  checking for written  lt  0

User · Answer

Here a solution using only std  However  note that this only rounds down       float number   3 14159      std  string num text   std  to string number       std  string rounded   num text substr 0  num text find      3     For rounded it yields   3 14   The code converts the whole float to string  but cuts all characters 2 chars after the

[c++] Convert float to string with precision & number of decimal digits specified?

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