I need help on keeping the precision of a double
. If I assign a literal to a double, the actual value was truncated.
int main() {
double x = 7.40200133400;
std::cout << x << "\n";
}
For the above code snippet, the output was 7.402
Is there a way to prevent this type of truncation? Or is there a way to calculate exactly how many floating points for a double
? For example, number_of_decimal(x)
would give 11, since the input is unknown at run-time so I can't use setprecision()
.
I think I should change my question to: How to convert a double to a string without truncating the floating points. i.e.
#include <iostream>
#include <string>
#include <sstream>
template<typename T>
std::string type_to_string( T data ) {
std::ostringstream o;
o << data;
return o.str();
}
int main() {
double x = 7.40200;
std::cout << type_to_string( x ) << "\n";
}
The expected output should be 7.40200 but the actual result was 7.402. So how can I work around this problem? Any idea?
Due to the fact the float
and double
are internally stored in binary, the literal 7.40200133400
actually stands for the number 7.40200133400000037653398976544849574565887451171875
...so how much precision do you really want? :-)
#include <iomanip>
int main()
{
double x = 7.40200133400;
std::cout << std::setprecision(51) << x << "\n";
}
And yes, this program really prints 7.40200133400000037653398976544849574565887451171875!