The following is my console input/output.
Please enter a real number: -23486.33 Characters checked: 9
Thank you. The real number you entered is -23486.3
The value I entered is -23486.33, but yet cout prints it as -23486.3. The relevant code is below:
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
// Function prototype (declaration)
string readDouble();
bool isValidDouble(string);
int main()
{
string value;
double number;
value = readDouble();
while (!isValidDouble(value)) {
cout << "The number you entered is not a valid integer." << endl;
value = readDouble();
}
number = atof(value.c_str());
cout << "Thank you." << endl
<< "The real number you entered is " << number << endl;
}
When debugging, I check the value of number right after the method call atof(value.c_str())l;
. Number is shown to have a value of -23486.33. So what happens between that and the print out by cout? In no part of my code do I set the precision of cout or make it fixed.
If you have any questions, please let me know.
Have you tried
std::cout << std::setprecision(2) << number;
look at: http://www.cplusplus.com/reference/iomanip/setprecision/
You can set the precision to the maximum limit for double.
The code snippet is here:
#include <iostream>
#include <limits>
#include <iomanip>
using namespace std;
double number = ... // your double value.
cout << setprecision(numeric_limits<double>::digits10) << number << endl;
Set a precision when you output a double and keep precision explicitly when you compare them.
When you convert a string presentation of a DEC number to a double(float point number presentation), the data in the memory might not be mathematically equal to the string presentation. It's the best approximation by a float point number presentation, and vise versa.
-23486.3 is displayed because std::cout
prints only 6 digits by default.
To print back a number entered from standard input (convertion text → floating number → text), you can use set_precision
with digits10
as precision:
double d = -23486.33;
int precision = std::numeric_limits<double>::digits10;
std::cout << std::setprecision(precision) << d << std::endl;
This displays:
-23486.33
To print a number with full precision (usually for convertion floating number → text → floating number), you can use set_precision
with max_digits10
as precision:
double d = -23486.33;
int precision = std::numeric_limits<double>::max_digits10;
std::cout << std::setprecision(precision) << d << std::endl;
This displays:
-23486.330000000002
Here the printed number is not the same because -23486.33 doesn't have an exact representation in IEEE encoding (expressed in base 2 instead of base 10).
For more details with digits10
and max_digits10
, you can read:
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.