Convert float to string with precision & number of decimal digits specified?
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 tofixed
.When
floatfield
is set tofixed
, 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 <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
}
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();
You can use C++20 std::format
:
#include <format>
int main() {
std::string s = std::format("{:.2f}", 3.14159265359); // s == "3.14"
}
or the fmt::format
function from the {fmt} library, std::format
is based on (godbolt):
#include <fmt/core.h>
int main() {
std::string s = fmt::format("{:.2f}", 3.14159265359); // s == "3.14"
}
where 2
is a precision.
It is not only shorter than using iostreams or sprintf
but also significantly faster and is not affected by the locale.