Unit.h

Classes

Unit -- defines physical units (full description)

class Unit

Interface

Public Members
Unit()
Unit(const Unit &other)
Unit(const String &other)
Unit(const Char *other)
explicit Unit(Char other)
Unit(const Char *other, Int len)
~Unit()
Unit& operator=(const Unit &other)
Bool operator==(const Unit &other) const
Bool operator!=(const Unit &other) const
Bool empty() const
const UnitVal &getValue() const
const String &getName() const
void setValue(const UnitVal &in)
void setName(const String &in)
Private Members
void check()

Description

Review Status

Programs:
Tests:

Synopsis

Physical units are basically used as quantities (see the Quantum class), i.e. a value and a dimension. The Unit class, or one of its subsidaries, will in general not be called separately. The only reason to make use of these classes is to generate additional 'tagged' units, i.e. units with a special name, e.g. 'beam' for a telescope beam, or 'JY', a non-SI name for Jy.

Units

A Unit is a String, and can be defined as either a Unit or a String everywhere where a Unit is required.
If defined as a Unit, the format of the string will be checked for a legal definition and its value will be stored. If defined as a String, the checking and determination of the value will be done each time the string is encountered when a Unit is expected.
Tip The use of a separate Unit variable will give a tremendous speed increase, if compared to using the String representation in e.g. Quantity(5,"deg")
Caution If using an explicit Unit variable (e.g. Unit a("5Bolton/beam")), the check on the legality of the given string, and the conversion to the cached canonical value in the variable 'a', is only done at creation time. This means that if the user changes the value of a unit involved by the putUser() method, the unit using it should be re-created ( a = Unit("5Bolton/beam");).
A unit is a string of one or more fields separated by 'space' or '.' (to indicate multiply) or '/' (to indicate divide). Multiple separators are acted upon (i.e. m//s == m.s). Separators are acted upon left-to-right (i.e. m/s/A == (m/s)/A; use () to indicate otherwise (e.g. m/(s/A))).

A field is a name, or a unit enclosed in (), optionally followed by an, optionally signed, decimal constant.

E.g. m.(m/s)-2 == m-1.s2)

Tip A 'space' or '.' before an opening '(' can be omitted.
A name can consist of case-sensitive letters, '_', ''', ':', '"' and '0' ('0' not as first character). Digits 1-9 are allowed if preceded with an '_'.

Possible legal names are e.g. Jy, R0, R_1, "_2.

Tip
Caution The standard naming conventions for SI units are that they are all in lowercase, unless derived from a person's name, when they start with a capital letter. Notable exceptions are some of the astronomical SI related units (e.g. AU).
A name can be preceded by a (standard) decimal prefix.

A name must be defined in a Unit map before it can be used.

All SI units and some customary units are part of the classes. User defined names can be added by the UnitMap::putUser() function (see the UnitMap class).

Example: km/s/(Mpc.s)2 is identical to km.s-1.Mpc-2.s-2

There are 5 name lists in the UnitMap, which are searched in reverse order:

  1. Defining units: m, kg, s, A, K, cd, mol, rad, sr, _
  2. SI units: including a.o. g, Jy, AU
  3. Customary units: e.g. lb, hp, ly
  4. User defined units: defined by user (e.g. beam, KPH, KM)
  5. Cached units: for speed in operations
All known names can be viewed by running the tUnit test program, or using the MapUnit::list() routine. They are also (at least the 1999/09/15 values) available in the Quanta module documentation.
Caution There is a difference between units without a dimension (non-dimensioned I will call them), and undimensioned units. Non-dimensioned examples are "", "%"; undimensioned examples: "beam", "pixel".

Unit class

The Unit class is not directly based on the String class, but Strings and Units are interchangeable in all Unit and Quantum related calls. (But notice the earlier note on speed if using explicit Strings often.)

To calculate with Units (or Strings representing units), use the UnitVal class. To use dimensioned values, use the Quantum (cq Quantity) class.

Using Unit i.s.o. String will give an immediate check of the legality of the unit string. In addition the UnitVal class contains a check facility to determine the legality of a unit string: 

    Bool UnitVal::check("string");

Example

    #include <aips/Quanta.h>
    // check if a string is a valid unit
    if ( !UnitVal::check("Km") ) { cout << "Invalid unit string " << "Km" << endl;
    // define some units
    String unit1="km/Mpc";
    Unit unit2="uJy/Mpc";
    // define your own unit name
    UnitMap::putUser("my_univ", UnitVal( C::pi, unit2), "My universe param");
    // use the units in model calculations
    Quantity observed( 8.97, "Mmy_univ/a");
    Quantity theory (3.8e-9, "mmy_univ/s");
    if ( ( observed / theory) < 1.) { cout << "Eureka" << endl;

Motivation

Make basis for all dimensioned values the SI system of units

To Do

Member Description

Unit()

Default empty string constructor

Unit(const Unit &other)

Copy constructor

Unit(const String &other)
Unit(const Char *other)
explicit Unit(Char other)
Unit(const Char *other, Int len)

String based constructors.

Thrown Exceptions

~Unit()

Destructor

Unit& operator=(const Unit &other)

* Operators Copy assignment

Bool operator==(const Unit &other) const

Comparisons. Comparisons are done on the basis of the inherent units. I.e. m/s are identical to AU/cy.

Bool operator!=(const Unit &other) const

Bool empty() const

Fast check for "" units

const UnitVal &getValue() const

 Get the unit value

const String &getName() const

Get the unit name

void setValue(const UnitVal &in)

Set the unit value

void setName(const String &in)

Set the unit name

void check()

Check format of unit string

Thrown Exceptions