std::string to float (via std::stof) precision

Question

I am trying to figure this out, the market data returns money values as a string that is 8 places after the digit long.

money = "124.19000540"

I need this to be 124.19, any idea how to achieve this?

std::stof(money) = 124.19000244

How to overcome this?

Answer 1

Floating point types are not good for holding money values. If you're content with rounding to the cent, and storing money as an integer number of cents (which is one of the simplest solutions), you could do this:

long numCents = static_cast<long>(100 * std::stof(money))

This will do "truncating" rounding, which always rounds down. If you'd like to do rounding "to the nearest cent", try:

long numCents = static_cast<long>(100 * std::stof(money) + 0.5)

As others mentioned, you may want to go for a fixed point or decimal library instead of something this simple.

Answer 2

You can use setprecision

#include <iostream> //for std::fixed
#include <sstream> //for std::stringstream
#include <iomanip> //for std::setprecision


int main()
{
  std::string money;
  std::cout << "Enter amount: ";
  getline(std::cin, money);

  std::stringstream out;
  out << std::fixed << std::setprecision(2) << std::stof(money);
  float fmoney = std::stof(out.str());
  std::cout << "Result: " << fmoney << std::endl;

  return 0;
}

Execution Output:

Enter amount: 124.19000540
Result: 124.19

Answer 3

Instead of std::stof(), which converts the string to a float, you should use std::stod(), which converts it to a double (with higher precision).

But as others also said, even this way you often cannot get exactly the same digits, due to how floating point numbers are represented. When printing through std::cout, you can use << std::setprecision(<precision>) to control the number of digits to print.