Quantify

A gem to support physical quantities and unit conversions in Ruby

Author: Andrew Berkeley (andrew.berkeley.is@googlemail.com)

Homepage: https://github.com/spatchcock/quantify

Quick introduction

# Operating on quantities

12.feet + 12.feet                   #=> <Quantify::Quantity:0xb7332bbc ... >
_.to_s                              #=> "24.0 feet"

6.m ** 2                            #=> <Quantify::Quantity:0xb7332bbc ... >
_.to_s                              #=> "36.0 m²"

100.km / 2.h                        #=> <Quantify::Quantity:0xb7332bbc ... >
_.to_s                              #=> "50 kilometers per hour"

5000.L.to_bbl                       #=> <Quantify::Quantity:0xb7332bbc ... >
_.to_s                              #=> "31.4490528488754 barrels"

1.5.lb.to_si.round(2)               #=> <Quantify::Quantity:0xb7332bbc ... >
_.to_s                              #=> "0.68 kg"

1.kg < 1.g                          #=> false

Unit.ratio(:kg, :ton)               #=> <Quantify::Quantity:0xb7332bbc ... >
_.to_s                              #=> "1016.047 kilograms per long ton"

Quantity.new(nil)                   #=> <Quantify::Quantity:0xb7332bbc ... >
_.value                             #=> nil
_.unit                              #=> <Quantify::Unit::Base:0x007ff622bd9320 ...>
_.unit.name                         #=> "unity" #== 'unitless' unit

Quantity.new(100)                   #=> <Quantify::Quantity:0xb7332bbc ... >
_.value                             #=> 100.0
_.unit                              #=> <Quantify::Unit::Base:0x007ff622bd9320 ...>
_.unit.name                         #=> "unity" #== 'unitless' unit
_.to_si.value                       #=> 100.0 #identity transformation for unitless quantities

Quantity.new(100)                   #=> <Quantify::Quantity:0xb7332bbc ... >
_.value = 'invalid value'           #=> raises ArgumentError

Quantity.new(100)                   #=> <Quantify::Quantity:0xb7332bbc ... >
_.value = nil                       #=> nil

Quantity.new(100)                   #=> <Quantify::Quantity:0xb7332bbc ... >
_.unit = nil                        #=> "unity" #== 'unitless' unit

General introduction

Quantify represents physical quantities using the Quantify::Quantity class.

A Quantity object holds both a value (Numeric) and a unit (of the class Quantify::Unit::Base), for example a Quantity object might represent 12 kgs (value, 12; unit, kilogram).

Quantities can be manipulated and operated on, in all of the ways that might be required for real physical quantities. Operations include, addition, subtraction, multiplying and dividing by scalar values or other quantities, raising to powers, converting into alternative unit representations (e.g. kgs to lbs, miles per hour to metres per second), and rounding of values. Quantify handles the converting of both values and units so that the result is a an accurate representation of the operation. For example, multiplying 10 metres by 10 metres will result in a quanity of square metres, whereas dividing, say, 10 metres by 2 seconds will result in a quantity in metres per second.

In all cases the results of operations are a changes to the Quantity instance or a new instance of a Quantity object. The new value or unit can be accessed using the #value and #unit attributes, or the #to_s method which renders the quantity in string form.

There are several ways to initialize a quantity object

long_text = "I travelled 220 miles driving my car and using 0.13 UK gallons per mile of diesel"

mass = Quantity.new(100,:lb)          #=> <Quantify::Quantity:0xb7332bbc ... >
mass = Quantity.new(100,'pound')      #=> <Quantify::Quantity:0xb7332bbc ... >
mass = 100.lb                         #=> <Quantify::Quantity:0xb7332bbc ... >
mass = 100.pound                      #=> <Quantify::Quantity:0xb7332bbc ... >
mass = Quantity.parse "100 lb"        #=> [<Quantify::Quantity:0xb7332bbc ... >]
mass = Quantity.parse long_text       #=> [<Quantify::Quantity:0xb7332bbc ... >, <Quantify::Quantity:0xc564321c ... >]
mass = "100 lb".to_q                  #=> [<Quantify::Quantity:0xb7332bbc ... >]

Quantity object can be interrogated for a range of attributes

mass = 100.pound                      #=> <Quantify::Quantity:0xb7332bbc ... >

mass.value                            #=> 100.0
mass.unit                             #=> <Quantify::Unit::NonSI:0xb7332b08 ... >
mass.unit.name                        #=> "pound"
mass.unit.symbol                      #=> "lb"

mass.unit.label                       #=> :lb  # unique identifier
mas.unit,j_science                    #=> 'lb' # Unit reference for integration with JScience
mass.unit.pluralized_name             #=> "pounds"
mass.to_s                             #=> "100 lb"
mass.to_s(:name)                      #=> "100 pounds"

mass.represents                       #=> "mass" # Describe the physical quantity represented by the quantity
mass.unit.measures                    #=> "mass" # Describe the physical quantity described by the unit

mass.unit.alternatives(:name)         #=> [ "kilogram",
                                      #     "gram",
                                      #     "carat",
                                      #     "electron mass",
                                      #     "grain",
                                      #     "hundredweight long",
                                      #     "hundredweight short",
                                      #     "ounce",
                                      #     "pennyweight",
                                      #     "pound",
                                      #     "short ton",
                                      #     "stone",
                                      #     "tonne",
                                      #     "unified atomic mass" ]
 
mass.unit.si_unit                     #=> 'kg'

mass.unit.dimensions                  #=> <Quantify::Dimensions:0xb75467c8 ... >
mass.unit.dimensions.describe         #=> "mass"
mass.unit.dimensions.mass             #=> 1  # index of base dimension 'mass'

Convert a quantity to a different unit

energy = 100.kWh                      #=> <Quantify::Quantity:0xb7332bbc ... >
energy.to_s                           #=> "100 kilowatt hours"
  
new_energy = energy.to_megajoules     #=> <Quantify::Quantity:0xb7332bbc ... >
new_energy.to_s                       #=> "360.0 MJ"
  
new_energy = energy.to_MJ             #=> <Quantify::Quantity:0xb7332bbc ... >
new_energy.to_s                       #=> "360.0 MJ"
  
new_energy = energy.to(:MJ)           #=> <Quantify::Quantity:0xb7332bbc ... >
new_energy.to_s                       #=> "360.0 MJ"

# Initialize a unit object and pass as conversion argument
unit = Unit.MJ                        #=> <Quantify::Unit::SI:0xb75c9718 ... >
new_energy = energy.to(unit)          #=> <Quantify::Quantity:0xb7332bbc ... >
new_energy.to_s                       #=> "360.0 MJ"

Convert the units of a quantity with a compound unit

speed = 70.mi/1.h                     #=> <Quantify::Quantity:0xb7332bbc ... >
speed.to_s                            #=> "70.0 mi/h"

speed_in_kms = speed.to_km            #=> <Quantify::Quantity:0xb7332bbc ... >
speed_in_kms.to_s                     #=> "112.65408 km/h"

speed_in_mins = speed_in_kms.to_min   #=> <Quantify::Quantity:0xb7332bbc ... >
speed_in_mins.to_s                    #=> "1.877568 km/min"

Convert a quantity to the corresponding SI unit

energy = 100.kWh                      #=> <Quantify::Quantity:0xb7332bbc ... >
energy.to_s                           #=> "100 kWh"

si = quantity.to_si                   #=> <Quantify::Quantity:0xb7332bbc ... >
si.to_s                               #=> "360000000.0 J"

Operate on a quantity

mass = 10.kg * 3                      #=> <Quantify::Quantity:0xb7332bbc ... >
mass.to_s                             #=> "30.0 kg"

distance = 100.light_years / 20       #=> <Quantify::Quantity:0xb7332bbc ... >
distance.to_s                         #=> "5.0 ly"

area = 10.m * 10.m                    #=> <Quantify::Quantity:0xb7332bbc ... >
area.to_s                             #=> "100.0 square metres"

speed = 250.mi / 3.h                  #=> <Quantify::Quantity:0xb7332bbc ... >
speed.to_s                            #=> "83.3333333333333 miles per hour"

speed = 70.mi/1.h                     #=> <Quantify::Quantity:0xb7332bbc ... >
time = 0.5.h                          #=> <Quantify::Quantity:3xf3472hjc ... >
distance = speed * time               #=> <Quantify::Quantity:7d7f8g9d5g ... >
distance.to_s                         #=> "35.0 mi"

Compare quantities

1.kg < 1.g                          #=> false
1.kg > 1.g                          #=> true

# Comparisons only valid if quantities are of same dimension
1.kg < 1.m                          #=> <Quantify::Exceptions::InvalidArgumentError ... >

Additional operations

The result of quantity operations is commonly a new quantity with a compound unit. Unless the result is equivalent to one of the base SI units (m, kg, s, K, etc.) or one of the following, square metre, cubic metre, joule, watt, newton or pascal, then the compound unit represents appropriate combination of the units involved, albeit with any like-units within the numerator and denominator grouped under a single power/index.

Units are not automatically cancelled or rationalized (made consistent). This is because it cannot be assumed that that is the desire of the user. For example, a quantity with units mass per mass is technically dimensionless, but the user might prefer to explicitly represent the units rather than reduce to a dimensionless quantity. In addition, this quantity might be expressed in terms of grams per tonne, and the user may not necessarily prefer a conversion into consistent mass units (e.g. g/g or t/t). Therefore, the following methods are available...

Where units representing the same physical quantity appear together, they can be made consistent by simply converting to the unit which is desired:

area = 12.yd * 36.ft                  #=> <Quantify::Quantity:0xb7332bbc ... >
area.to_s                             #=> "432.0 yd ft"

area = area.to_yd                     #=> <Quantify::Quantity:0xb7332bbc ... >
area.to_s                             #=> "144.0 yd²"

Alternatively, all units within the numerator and denominator respectively can be standardized:

quantity = (12.ft*8.mi)/(1.s*8.min)   #=> <Quantify::Quantity:0xb7332bbc ... >
quantity.to_s                         #=> "12.0 ft mi/s min"
quantity.rationalize_units!
quantity.to_s                         #=> "1056.0 ft²/s²"

A quantity with arbitrary cancelable units can be cancelled manually:

quantity = (12.m**6) / 2.m**2
quantity.to_s                         #=> "746496.0 m^6/m²"
quantity.cancel_base_units! :m
quantity.to_s                         #=> "746496.0 m^4"

Note: there are more comprehensive and flexible methods for manupulating compound units available as part of of the class Unit::Compound. These can be used to convert a conpound unit into the precise form required. If such an approach is used, any quantity object can be converted to the new form by simply passing the new unit object into the Quantity#to method.

Contributing

If you find a bug or think that you improve on the code, feel free to contribute.

You can:

Send the author a message (andrew.berkeley.is@googlemail.com)
Create an issue
Fork the project and submit a pull request.

License

Licensed under the MIT license (See COPYING file for details)

quantify