PhysicalQuantity

Strongly-typed physical quantities

# PhysicalQuantity

I've always wondered why programming languages only implement the number sets used in mathematics—? (int8, uint64, etc.), ? (complex(), etc.), variations of ? (double, single, ,...), etc. To any physicist, it would make sense to have a data type that represents a physical quantity; something that has both a value (double) and a unit of measurement. Preferably, this datatype would behave intuitively for all sorts of standard operations - a Mass times a Length divided by a Time² should give a Force, according to Isaac Newton, yet that same set of operations can not ever equal an Area.

This submission is an implementation of such a datatype.

Preparations:

- put the PhysicalQuantity and PhysicalVectorQuantity directories on the
MATLAB search path, recursively

Example session:

>> L = Length(4, 'm')
L =
4 meters

>> R = Length(2, 'ft')
R =
2 feet

>> A = Area(L*R, 'm^2')
A =
2.4384e+00 square meters

Let's check: 1 ft = 0.3048 meter, so
2 ft * 4 m = 2*0.3048 * 4 = 2.4384 square meters. Cool!

Try the following:

>> A = Area(L*L*R, 'm^2')
Error using Area (line 13)
Can't create 'Area' (dimensions [L]²) from a quantity with dimensions [L]³.

>> A = Area(L*R, 'm^3')
Error using Area (line 13)
Dimensions implied by the given string ([L]³) are incompatible with those of an Area ([L]²).

>> tan( Area(L*R, 'm^2') )
Can't compute the tangent() of an Area.

Angles are dimensionless and are treated as special in this framework:

>> theta = Angle(-1.2, 'deg')
theta =
-1.2 degrees

>> cos(theta)
ans =
9.997806834748455e-01 % <- indeed the same as cosd(-1.2)

>> phi = Angle(0.5, 'rad')
phi =
500 milliradians

# Overview

In physics, a quantity's *unit* is intricately connected to the quantity itself. Typically, quantities with different units cannot be used in the same operation, without some sort of conversion between them - you can't "add apples to oranges".

This is not unlike data types in most programming languages. What do you expect to get when you divide two integers? Add a boolean to a character? You can't just do that without some sort of conversion.

Therefore, it makes sense to create a data type that takes physical units into consideration for all operations. Preferably, this data type is also super-easy to use, and produces intelligible error messages when it's used in a physically meaningless way.

And that is exactly what this tool set aims to provide.

# How to use

PysicalQuantities are constructed from scratch like this:

Q = <quantityname>(<value>, <units>)

Take a look in the PhysicalQuantity directory for an overview of currently
supported <quantities>. The <value> can be any numeric value (including
sparse, complex, etc.). The <units> should be a string specifying any of
the supported long/short unit names (see below), possibly prefixed with
an SI-multiplier (for metric units). Multiple units can be combined by the
following operations:

'*': multiply
'/': divide
'^': exponentiate

Example:

F = Force(300, 'Newton')
r = Density(2, 'kg/m^3')

To get a list of available units:

>> L = Length();
>> L.listUnits()
Length supports the following units of measurement:
- Chinese mile (li)
- Parsec (pc)
- astronomical unit (AU)
- foot (ft)
- furlong (fur)
- inch (in)
- lightyear (ly)
- meter (m)
- mile (mi)
- nautical mile (n.mi)
- smoot (smt)
- statute mile (st.mi)
- yard (yd)
- Ångström (Å)

Conversions between compatible units are seamless:

>> L = Length(20, 'yards');
>> L('meters')
ans =
1.8288e+01 meters

>> P = Length(3, 'inch');
>> Q = Length(18, 'foot');
>> Length( L*P/Q, 'meter' )
ans =
254 millimeters

Operations work as expected:

>> L = Length(1, 'inch');
>> t = Duration(32, 'seconds');
>> M = Mass(18, 'lb');
>> F = Force( M*L/t/t, 'Newton' )
ans =
2.025219058242187e+02 milliNewtons

# Note

If you observe something odd, please give me a heads up.
Preferably, raise an issue on GitHub :) Otherwise, plain ol'
email will do.