public member function
<ios> <iostream>
| get (1) | streamsize precision() const; |
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| set (2) | streamsize precision (streamsize prec); |
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Get/Set floating-point decimal precision
The first form (1) returns the value of the current floating-point precision field for the stream.
The second form (2) also sets it to a new value.
The floating-point precision determines the maximum number of digits to be written on insertion operations to express floating-point values. How this is interpreted depends on whether the floatfield format flag is set to a specific notation (either fixed or scientific) or it is unset (using the default notation, which is not necessarily equivalent to either fixed nor scientific).
For the default locale:
- Using the default floating-point notation, the precision field specifies the maximum number of meaningful digits to display in total counting both those before and those after the decimal point. Notice that it is not a minimum, and therefore it does not pad the displayed number with trailing zeros if the number can be displayed with less digits than the precision.
- In both the fixed and scientific notations, the precision field specifies exactly how many digits to display after the decimal point, even if this includes trailing decimal zeros. The digits before the decimal point are not relevant for the precision in this case.
This decimal precision can also be modified using the parameterized manipulator setprecision.
Parameters
- prec
- New value for the floating-point precision.
streamsize is a signed integral value.
Return Value
The precision selected in the stream before the call.
Example
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// modify precision
#include <iostream> // std::cout, std::ios
int main () {
double f = 3.14159;
std::cout.unsetf ( std::ios::floatfield ); // floatfield not set
std::cout.precision(5);
std::cout << f << '\n';
std::cout.precision(10);
std::cout << f << '\n';
std::cout.setf( std::ios::fixed, std:: ios::floatfield ); // floatfield set to fixed
std::cout << f << '\n';
return 0;
}
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Possible output:
3.1416
3.14159
3.1415900000
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Notice how the first number written is just 5 digits long, while the second is 6, but not more, even though the stream's precision is now 10. That is because precision with the default floatfield only specifies the maximum number of digits to be displayed, but not the minimum.
The third number printed displays 10 digits after the decimal point because the floatfield format flag is in this case set to fixed.
Data races
Accesses (1) or modifies (2) the stream object.
Concurrent access to the same stream object may cause data races.
Exception safety
Basic guarantee: if an exception is thrown, the stream is in a valid state.