5.3.4 Systems of units

A system of units is a set of units, one for each of the physical dimensions that Antik can handle. A system of units is needed by Antik for printing output and in special cases for converting physical quantities that have a physical dimension defined (e.g. length), but no units. The system of units defined by *system-of-units* is used to determine the units in which the output is printed. There are pre-defined several systems of units, *siu*, *englishu*, *cgsu*:

     ANTIK-USER> (defparameter *length1* #_10_m)
     *LENGTH1*
     ANTIK-USER> *length1*
     #_10.000000000000000_m
     ANTIK-USER> (setf *system-of-units* *englishu*)
     (((1 0 0 0 0 0 0 0 0) . FOOT) ((0 0 0 0 0 0 0 0 1) . RADIAN)
      ((0 0 1 0 0 0 0 0 0) . SLUG) ((0 1 0 0 0 0 0 0 0) . SECOND)
      ((1 -2 1 0 0 0 0 0 0) . POUND-FORCE) ((2 -3 1 0 0 0 0 0 0) . HORSEPOWER)
      ((2 -2 1 0 0 0 0 0 0) . FOOT-POUND)
      ((-1 -2 1 0 0 0 0 0 0) . POUNDS-PER-SQUARE-INCH))
     ANTIK-USER> *length1*
     #_32.808398950131235_ft

So by setting the system of units, you can convert values as needed,

     ANTIK-USER> (setf *system-of-units* *siu*)
     ANTIK-USER> #_12_feet
     #_3.657600000000000d0_m
     ANTIK-USER> (setf *system-of-units* *englishu*)
     ANTIK-USER> #_3_m
     #_9.842519685039370d0_ft

A new system of units may be defined with define-sysunits. One need not define all physical dimensions, but if a unit is needed for a particular dimension and it's not defined, an error will be signalled. For convenience, a system of units may be defined by alteration of an existing system of units. For example, suppose that you wish to have a system of units that is like SI except that lengths are measured in km instead of meters. This will define such a system:

     (define-sysunits *kmu* (km) *siu* "Kilometer system.")

To set the default system of units, use set-default-sysunits. To define a symbol macro to set a default system of units, use setsys. The SI system is the initial default system of units. It may be reset as the system of units with the symbol macro si.

For formatted output of angles, the units (degrees or radians) may be set with the nf option :degrees, so the system of units may be maintained whether degrees or radians are desired on output.