Installation

Features

1. Significantly improved speed
2. Will use the ‘Chronic’ gem to interpret times/dates if loaded
3. some nice time helpers.
   ‘1 day’.from ‘now’ #=> Wed Oct 04 18:27:13 EDT 2006
   ‘1 day’.ago #=> Mon Oct 02 18:38:56 EDT 2006
   ‘7 days’.before ‘1/1/2007′ #=> Mon Dec 25 00:00:00 EST 2006
   ‘7 days’.after ‘12/25/2006′ #=> Mon Jan 01 00:00:00 EST 2007
   ‘days’.since ‘1/1/2006′ #=> 275.436 d
   ‘days’.until ‘1/1/2007′ #=> 89.7683 d

   (works for sec, min, hours, days, weeks, fortnights, years, decades, centuries….
   but not months).

   and some others..
   these helpers will automagically give you a DateTime object
   if a Time object won’t work.

   ‘years’.since ‘1/1/1900′ #=> 106.756 y
   ‘50 years’.from ‘now’ #=> 2056-10-02T18:46:55Z
4. units can now identify themselves (sort of..)
   ‘60 mph’.unit.kind #=> :speed
   ‘9.8 m/s^2′.unit.kind #=> :acceleration
5. some new constructors
   Unit(10,’m') #=> 10 m
6. Now throws an exception if it donsn’t understand a unit (during creation)
7. Temperature handling improved
   “0 tempC”.unit #=> 273.15 degK
   “310.15 degK”.to(’tempC’) #=> 37 degC
8. Trig functions can accept angular units
   Math.sin(’90 deg’.unit) #=> 1.0
   Math.sin(’100 grad’.unit) #=> 1.0

Ruby has a few libraries (notably, Facets) available that handle unit conversions in one form or another. However, most of these systems don’t quite meet my somewhat demanding and arbitrary requirements, so I set out to write a new one.

Installation

You can get ‘ruby-units’ using

Usage

To use this gem, you need to do the standard

Creating a Unit object

Because I use a lot of units in my work, and I’m lazy, there are several different ways to create a new ‘Unit’ object.

1. unit = Unit.new(”1 mm”)
2. unit = Unit(”1 mm”)
3. unit = U(”1 mm”)
4. unit = “1 mm”.to_unit
5. unit = “1 mm”.unit
6. unit = “1 mm”.u
7. unit = “mm”.unit (yields ‘1 mm’)

Of these, I use #5 the most since it is reasonably terse without becoming obscure.
You can also create Unit objects from Numerics, Arrays, and Time objects. Numerics just become unitless Unit objects, Arrays are interpreted as [scalar, numerator, denominator], and Time objects are converted into a number of seconds.

Unit objects can be serialized through YAML using ‘unit.to_yaml’, which facilitates them being stored in database columns using Rails’ :serialize functionality.

Unit strings generally consist of text in the following format:

Since Units are constructed from strings, it is easy to have a user specify them in input fields. Because the format is fairly standard, there should not be too many problems with misinterpreted units.

A couple of points:

1. only one ‘/’ per unit. No ‘m/s/s’
2. everything after the ‘/’ is in the denominator
3. you can use negative exponents (’9.8 m*s^-2′)
4. use either a space or a ‘*’ to separate units (’9.8 ms^-2′ == ‘9.8 1/ms^2′)
5. ruby-units understands all SI prefixes, and a few others as well

Unit Compatibility

Sometimes it is helpful to know if two units are ‘compatible’. Compatible units can be converted into each other, so ‘feet’ and ‘meters’ are compatbile, but ‘feet’ and ‘liters’ are not.

Unit Conversion

Units can be converted to each other so long as they are ‘compatible’. Ruby-units will throw an exception if units need to be compatible, but aren’t, so be sure to trap those exceptions when malformed input is possible.

Explicit Conversion

Explicit unit conversion occurs when the user asks for it. There are a couple of ways to force conversion to a particular unit.

Implicit Conversion

There are times when unit conversions are implied or assumed for simplicity. For example, when adding two units, the gem will convert the second unit to the same base units as the first prior to doing the math.

Unit Math

Unit objects come in really handy when doing complex math calculations:

You can add, multiply, subtract, and divide units as you can with any Numeric class. You can also exponentiate or root them, although I restrict roots to integer values, since ‘m^0.5′ doesn’t really make any sense.

As an added bonus, all Trig functions (sin, cos, tan, sinh, …) can accept units that are compatible with radians. The values will be converted to radians before returning the result. So you can do this..

Temperatures

Note that we specified the temperature in this equation as a ‘tempC’. Specifying a temperature will convert that unit into the equivalent number of degrees kelvin. So in this case, “0 tempC”.unit => 273.15 degK.

This is necessary to make the calculations work out right. Most times unit conversions are done by just adjusting the size of the units, but temperatures require you to also specify the zero point on the scale.

You can convert ‘degree’ units (like ‘degC’, ‘degF’, ‘degR’, or ‘degK’) to a temperature this way…

This really only makes sense when the value represents the degrees between absolute zero and the point of interest. Only the user knows when that is the case, so use it carefully.

Time and Date Math

Ruby-units understands a wide range of time units, and can convert back and forth between them with ease. Nothing new there.

However, ruby-units can add and subtract time units from Date, DateTime, and Time objects. Throw in a few rails-esque helpers and you can write code like this…

Formatting Output

You can format the output of a Unit object by using the “.to_s” function and string format specifiers…

Miscellaneous Stuff

You can create ranges of units, so long as they have scalar integers.

If a unit is ‘unitless?’ then it can be converted back to other types of Numerics.

Note that ‘Unit’ objects are pretty slow compared to more common ones like Integers and Floats, so if performance is a big issue, you might want to consider using these sparingly.

Defining new units

You can define custom units by adding a code block like this..

The format for the array is [%w{primary_name synonyms}, conversion_factor, unit_type, %w{}]

Useful References

Wikipedia: Units of Measurement