How to Control Float-to-String Conversion Precision and Decimal Digits in C ?

Modifying Float-to-String Conversion Precision and Decimal Digits in C

When converting a float to a string in C , it is often necessary to control the precision and number of decimal digits displayed. Here's how this can be achieved:

Using Stringstream

A common approach is to use a stringstream:

<code class="cpp">#include <iomanip>
#include <sstream>

double pi = 3.14159265359;
std::stringstream stream;
stream << std::fixed << std::setprecision(2) << pi;
std::string s = stream.str();

The fixed flag ensures that a fixed-point notation is used, while setprecision(2) specifies that only two decimal digits should be displayed.

Conversion for Technical Purposes

For technical purposes, such as storing data in XML or JSON, C 17 introduces the to_chars family of functions:

<code class="cpp">#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
}</code>

Here, std::chars_format::fixed ensures fixed-point notation, and 2 specifies the number of decimal digits.

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