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 & number of decimal digits?

For example: 3.14159265359 -> "3.14"

Answer 1

A typical way would be to use stringstream:

#include <iomanip> #include <sstream>  double pi = 3.14159265359; std::stringstream stream; stream << std::fixed << std::setprecision(2) << 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.

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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 <array> #include <charconv>  double pi = 3.14159265359; std::array<char, 128> 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 } 

Answer 2

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 << std::fixed << std::setprecision(2) << number; std::string mystring = ss.str();