string to double or float cuts off decimal points

Question

I don't know what I am doing wrong but I want to convert a string to a double, using std::stod, but it chops of the decimal points

int main()
{
  auto value = std::string("321415.1342");
  auto val = std::stod(value, 0);
  std::cout << val << '\n';
}

this program produces: 321415. The stof also produces the same result. My locale is listed below if it helps.

LANG=en_US.UTF-8
LANGUAGE=
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"

Change the precision to try to see if there is something after the decimal point `std::cout << std::setprecision(10) << val << std::endl; — Matthieu Brucher, Nov 23 '18 at 19:48
Yes, sir, there is. Should not the default output for double produce a few decimal points ? this is very surprising default behavior. — Ring Zero., Nov 23 '18 at 19:53
The default precision is to output 6 digits, not the scientific format: https://en.cppreference.com/w/cpp/io/manip/setprecision — Matthieu Brucher, Nov 23 '18 at 19:57
The precision fix works but I wonder why its needed. I tried making val a double instead of auto but that didnt have any effect. — Nirvedh Meshram, Nov 23 '18 at 19:59
Doesn't change, the default precision is 6 digits. Nothing to do with where it is. — Matthieu Brucher, Nov 23 '18 at 20:07
I think I was just unlucky as my number had 6 digits before decimal fractions. — Ring Zero., Nov 23 '18 at 20:39

Aykhan Hagverdili · Accepted Answer · 2018-11-23T20:03:06.480

You need to alter the precision. std::setprecision and std::fixed are what you are looking for. You have to include iomanip:

#include <iostream>
#include <string> 
#include <iomanip>  // <- include this

int main()
{
  auto value = std::string("321415.1342");
  auto val = std::stod(value, 0);
  std::cout << std::setprecision(5) << std::fixed << val << std::endl;

  return 0;
}

Output:

321415.13420

See this answer for more on the topic.

string to double or float cuts off decimal points

1 Answers1