c++ long double printing all digits with precision

问题

Regarding my question I have seen a post on here but did not understand since i am new to C++. I wrote a small script which gets a number from user and script prints out the factorial of entered number. Once I entered bigger numbers like 30, script does not print out all the digits.Output is like 2.652528598 E+32 however What I want is exact number 265252859812191058636308480000000. Could someone explain how to get all digits in long double.Thanks in advance

回答1:

You can set the precision of the output stream to whatever you want in order to get your desired results.

http://www.cplusplus.com/reference/ios/ios_base/precision/

Here is an extract from the page, along with a code example.

Get/Set floating-point decimal precision 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).

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.

// 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;
}

Possible output:

3.1416
3.14159
3.1415900000

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.

来源：https://stackoverflow.com/questions/31632753/c-long-double-printing-all-digits-with-precision

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