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It is often desirable to scale a unit
automatically, depending on its value, to keep the integral part in a limited
range, usually between 1 and 999.
For example, using engineering notation prefixes,
"1234.5 m" is more helpfully displayed as "1.234 km" "0.000000001234 m" is more clearly displayed as "1.2345 nanometer".
The iostream manipulators engineering_prefixes
or binary_prefixes
make this
easy.
using boost::units::binary_prefix; using boost::units::engineering_prefix; using boost::units::no_prefix; quantity<length> l = 2.345 * meters; // A quantity of length, in units of meters. cout << engineering_prefix << l << endl; // Outputs "2.345 m". l = 1000.0 * l; // Increase it by 1000, so expect a k prefix. // Note that a double 1000.0 is required - an integer will fail to compile. cout << engineering_prefix << l << endl; // Output autoprefixed with k to "2.345 km". quantity<energy> e = kilograms * pow<2>(l / seconds); // A quantity of energy. cout << engineering_prefix << e << endl; // 5.49902 MJ cout << name_format << engineering_prefix << e << endl; // 5.49902 megaJoule
(The complete set of engineering and scientific multiples is not used (not centi or deci for example), but only powers of ten that are multiples of three, 10^3).
Similarly, the equivalent binary prefixes used for displaying computing kilobytes, megabytes, gigabytes...
These are the 2^10 = 1024, 2^20 = 1 048 576, 2^30 ... multiples.
(See also Prefixes for binary multiples
This scale is specified in IEC 60027-2, Second edition, 2000-11, Letter symbols to be used in electrical technology - Part 2: Telecommunications and electronics).
// Don't forget that the units name or symbol format specification is persistent. cout << symbol_format << endl; // Resets the format to the default symbol format. quantity<byte_base_unit::unit_type> b = 2048. * byte_base_unit::unit_type(); cout << engineering_prefix << b << endl; // 2.048 kb cout << symbol_format << binary_prefix << b << endl; // "2 Kib"
But note that scalar dimensionless values, like int, float and double, are not prefixed automatically by the engineering_prefix or binary_prefix iostream manipulators.
const double s1 = 2345.6; const long x1 = 23456; cout << engineering_prefix << s1 << endl; // 2345.6 cout << engineering_prefix << x1 << endl; // 23456 cout << binary_prefix << s1 << endl; // 2345.6 cout << binary_prefix << x1 << endl; // 23456
You can output the name or symbol of a unit (rather than the most common quantity of a unit).
const length L; // A unit of length (but not a quantity of length). cout << L << endl; // Default length unit is meter, // but default is symbol format so output is just "m". cout << name_format << L << endl; // default length name is "meter".
Note too that all the formatting flags are persistent, so that if you set engineering_prefix, then it applies to all future outputs, until you select binary_prefix, or explicitly switch autoprefix off. You can specify no prefix (the default of course) in two ways:
no_prefix(cout); // Clear any prefix flag. cout << no_prefix << endl; // Clear any prefix flag using `no_prefix` manipulator.
And you can get the format flags for diagnosing problems.
cout << boost::units::get_autoprefix(cout) << endl; // 8 is `autoprefix_binary` from `enum autoprefix_mode`. cout << boost::units::get_format(cout) << endl; // 1 is `name_fmt` from `enum format_mode`.