Question

I’m trying to format numbers to a specific number of significant digits using C/C++ and preferably STL. I’ve seen examples of doing this in Javascript (toPrecision()) and .Net, but I can’t find anything on doing this in C/C++. I want to create a function something like this:

std::string toPrecision(double value, int significantDigits) {
    std::string formattedString;
    // magic happens here
    return formattedString;
}

So that it produces results like this:

toPrecision(123.4567, 2) --> "120"
toPrecision(123.4567, 4) --> "123.4"
toPrecision(123.4567, 5) --> "123.45"

Does anyone know a good way to do this? I’m considering dumping the whole number into a string and then just scanning through it to find the non-zero digits and count them off in some intelligent way, but that seems cumbersome.

I could also download the source code to one of the browsers and just see what their toPrecision function looks like, but I think it would take me all day to work through the unfamiliar code. Hope someone can help!

Answer 1

Stolen from another question:

#include <string>
#include <sstream>
#include <cmath>
#include <iostream>

std::string toPrecision(double num, int n) {
    https://stackoverflow.com/questions/202302/rounding-to-an-arbitrary-number-of-significant-digits

    if(num == 0) {
      return "0";
    }

    double d = std::ceil(std::log10(num < 0 ? -num : num));
    int power = n - (int)d;
    double magnitude = std::pow(10., power);
    long shifted = ::round(num*magnitude);

    std::ostringstream oss;
    oss << shifted/magnitude;
    return oss.str();
}

int main() {
  std::cout << toPrecision(123.4567, 2) << "\n";
  std::cout << toPrecision(123.4567, 4) << "\n";
  std::cout << toPrecision(123.4567, 5) << "\n";
}