-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Steven Watanabe Sent: Friday, March 12, 2010 7:23 PM To: boost-users@lists.boost.org Subject: Re: [Boost-users] boost.units converting double tp SI prefix

AMDG

Olaf Peter wrote:

...

is there a way to use boost.units to convert a double automatically to a SI unit prefix, e.g. 10e-3 to 10m or 10e3 to 10k ?? The value isn't know at compile time, so the decision has to be done on runtime.

There is not such a facility. There will soon be support for printing things like 1.0 km instead of 1000 m, but formatting of raw numbers is somewhat beyond the scope of the library.

But I think that you can create your own unit (and quantity) easily, and then you *can* use the very useful auto prefixing system Steven has devised. For example for show this, I've created a entirely useless 'thing' type - dimensionless of course. It could be widget count, (or indeed a unit of currency) with a name and symbol. struct thing_base_unit : boost::units::base_unit { static const char* name() { return("thing"); } static const char* symbol() { return(""); } }; and this constructs a quantity of it (2048.0 - a double by default) quantity t = 2048. * thing_base_unit::unit_type(); and you can then chose to output with a trailing name of your choice cout << name_format << engineering_prefix << t << endl; // 2.048 kilothing and similarly with a symbol (I've defined my symbol to be a "", so I just get the SI prefix. cout << symbol_format << engineering_prefix << t << endl; // 2.048 k Steven has even more cunningly allowed the binary 2^10, 2^20, 2^30 ... prefixes, cout << binary_prefix << t << endl; // "2 Ki" For currency, the multiple (k, M, G) *must precede* the name or symbol of the unit. (*not* Symbol first as one might really want for $, £ € ...) quantity ce = 2048. * euro_base_unit::unit_type(); cout << name_format << engineering_prefix << ce << endl; // 2.048 kiloEUR cout << symbol_format << engineering_prefix << ce << endl; // 2.048 k€ Perhaps this will meet your needs when available? Paul --- Paul A. Bristow Prizet Farmhouse Kendal, UK LA8 8AB +44 1539 561830, mobile +44 7714330204 pbristow@hetp.u-net.com