在 C++ 中使用 cout 打印浮点和双精度值

Question

For the following code:

#include <iostream>
using namespace std;
class Hall
{
 public:
 double cost;
};
int main()
{
 Hall hall;
 hall.cost=10000.50;
 cout<<hall.cost;
 return 0;
}

Why does this code output 10000.5 and not 10000.50, can someone explain the logic behind this?

Answer 1

can someone explain the logic behind this?

The default behaviour is not show any trailing zeroes.

Answer 2

double (and floating-point variables in general) don't store a specific spelling of the number (base-10 or otherwise). That would be way too inefficient, and the ability to store meaningless zeroes would waste memory.

Instead, the numbers are encoded in a certain binary format . Printing them reproduces the base-10 spelling from binary.

Answer 3

By default, C++ does not display trailing zeros, however the standard library <iomanip> can be used along with std::fixed from <iostream> . Here is an example:

#include <iostream>
#include <iomanip>
int main() {
  double myVar = 123.4;
  std::cout << std::fixed << std::setprecision(2); // 2 trailing digits
  std::cout << myVar << std::endl;
  return 0;
}