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Floating point imprecision #1166
Description
Serializing floating point leads to imprecision.
Using this source code:
#include <iostream>
#include "json.hpp"
using json = nlohmann::json;
int main(void)
{
json j;
double a = 1.234;
float b = 1.234;
double c = b;
float d = b;
j["a"] = a;
j["b"] = b;
j["c"] = c;
j["d"] = d;
std::cout << std::setw(4) << j << std::endl;
std::cout << "a=" << a <<std::endl;
std::cout << "b=" << b <<std::endl;
std::cout << "c=" << c <<std::endl;
std::cout << "d=" << d <<std::endl;
return 0;
}
Ouputs:
{
"a": 1.234,
"b": 1.2339999675750732,
"c": 1.2339999675750732,
"d": 1.2339999675750732
}
a=1.234
b=1.234
c=1.234
d=1.234I was excepted every number to be equal to 1.234 as with the std::cout.
I am using g++ on Debian 9:
g++ (Debian 6.3.0-18+deb9u1) 6.3.0 20170516
Reacted by khaledto
Activity
jaredgrubb commentedon Jul 17, 2018
Float is a 32-bit representation, and with 32 bits, the two numbers "1.234" and "1.2339999675750732" have identical representations. Either way is "correct" to print a float. There is no imprecision here, just a representation that is "less pretty".
When cout outputs "c" as "1.234", it is actually lying and printing something less precise than it actually is. If you tell cout you want full precision, then you will get identical results to what nlohmann::json is doing. (add "std::cout.precision(17);" to your program).
jmlemetayer commentedon Jul 17, 2018
Wow very impressive response. Thanks for the explanation.
As I understood the background behind my issue, I close it.
nlohmann commentedon Jul 18, 2018
@jaredgrubb Thanks so much for the detailed response. I am currently overworking the documentation, and this definitely should go in there!