# Temperature converter that seems to violate SRP

I've written this class for two purposes (I think only this fact shows that this code violates SRP): convert weight from one unit to another and to represent them accurate or not accurate. So class methods can be used for calculation and the present method rounds the value for presentation. I don't like this code - can you propose better solution?

class Temperature

UNITS = %w(C F)
DEFAULT_UNITS = "C"

def initialize(value, units = DEFAULT_UNITS)
@value = value
@units = units.to_s
end

def as(units, accurate)
return unless UNITS.include?(units.to_s)
value = units.to_s == @units ? @value : self.class.try("#{@units.downcase}_to_#{units.downcase}", @value)
accurate ? value : self.class.new(value, units).present
end

def default(accurate = false)
as(DEFAULT_UNITS, accurate)
end

def present
value.round(1)
end

def self.c_to_f(val)
val * 9 / 5 + 32
end

def self.f_to_c(val)
(val - 32) * 5 / 9
end

end

class Numeric

def fahrenheit_to_celsius
(self - 32) * 5 / 9
end

def celsius_to_fahrenheit
self * 9 / 5 + 32
end

end

p -7.5.celsius_to_fahrenheit #=> 18.5

Classes are open for a reason in ruby. A temperature is just a number, converting it is just a method. The accuracy thing disappears by itself - an integer results in an integer, a float will give a float.

I would separated logic for Celsius and Fahrenheit into separate classes. Here is a code to demonstrate the general approach:

class Temperature

def initialize value # or you can go further and dont
@value = value
end
end

class Celsius < Temperature
def to_fahrenheit
Fahrenheit.new value * 9 / 5 + 32
end
end

class Fahrenheit < Temperature
def to_celsius
Celsius.new (value - 32) * 5 / 9
end
end

With this approach you have good abstractions for different types of temperature and you don't mess different logic in the single class. Probably you will not need 'unit' property because you will have class names holding it (you can add it easily in both classes if you want).

Then you can write factory methods in Numeric class to write more clear code with temperature classes:

class Numeric
def fahrenheit
Fahrenheit.new(self)
end

def celsius
Celsius.new(self)
end
end

cels = -5.celsius
cels.to_fahrenheit.value # => 23

Regarding accurate property - you can accept it in the initializer or you can accept it in conversion methods (to_celsius, to_fahrenheit).

UPDATE

You can also implement accurate as a mixin (kind of decorator):

module Inaccurate
def value
@value.round(1)
end
end

class Numeric
def fahrenheit accurate = true
Fahrenheit.new(self).tap { |t| t.extend(Inaccurate) unless accurate }
end

def celsius accurate = true
Celsius.new(self).tap { |t| t.extend(Inaccurate) unless accurate }
end
end

puts -5.45.celsius.value         # => -5.45
puts -5.45.celsius(false).value  # => -5.5

UPDATE

Temperature is abstraction of real-world temperature. Your as conversion method is specific to your application's data format ('C' and 'F' strings in user profile). Taking SRP principle into account, it is better to extract this conversion logic from Temperature class. You can do this with module, something like this:

module UserTemperatureConverter
def as value, t_type, accurate = true
case t_type
when 'C'
value.celsius(accurate)
when 'F'
value.fahrenheit(accurate)
end
end

extend self # make 'as' a class method of UserTemperatureConverter
end
Temperature.extend UserTemperatureConverter # make 'as' a class method of Temperature

UserTemperatureConverter.as(17, 'C') # => Celsius
# or
Temperature.as(18, 'F') # => Celsius

# or make temperature conversion instance method and include it into Temperature as Temperature#as

Also you may find useful in your application to define basic self-conversion methods for Celsius and Fahrenteit:

class Celsius
def to_celsius; self; end
end

class Fahrenheit
def to_fahrenheit; self; end
end

17.celsius.to_celsius # Celsius
• Cool approach! But I need method as() instead of named methods because I get units from the current_user profile to display values in desired units. Feb 6 '12 at 9:16
• @Antiarchitect, added some thoughts on how you may implement temperature conversion/construction logic with my approach. Feb 6 '12 at 11:55

First of all, don't use plural identifiers if the variable or constant doesn't hold multiple values (e.g. rename DEFAULT_UNITS to DEFAULT_UNIT and units to unit) as it can be very confusing.

Why don't you just always store the temperature in ℃? Having only one possible data representation in a class can significantly simplify the methods in that class. Convert lazily to ℉ lazily, when you need to.

class Temperature
...

def initialize(value, unit = DEFAULT_UNIT)
raise SomeError unless UNITS.include?(unit)
if unit == "C"
@value = value
else
@value = (value - 32) * 5 / 9
end
end

...
end

Also, in your Temperature#as method you might want to change the ternary operator to an if-else statement to make it more readable.

• Measurements::Temperature stores the temperature values only in ℃. I cannot predict in what format temperature will be created: it has a value field and units accessor: Feb 1 '12 at 9:01

I would avoid to maintain different classes for different temperature scales. I mae an example based on Kelvin.

With Temperature.new( value[, unit]) you can define a temperature, the unit is optional.

Later you have access to the values with Temperature#celcius, Temperature#kelvin od Temperature#fahrenheit. If you need other scales (e.g. Réaumur) you could add them without any problem.

The as is also implemented, it has two parameters: The unit and a rounding factor.

My code:

class Temperature

class Too_Cold < ArgumentError; end
UNITS = %w(C F K)
DEFAULT_UNIT = "K"

def initialize(value, unit = DEFAULT_UNIT)
case unit
when 'K'
@kelvin = value
when 'C'
@kelvin       = 273.15 + value
when 'F'
@kelvin = (value + 459.67) * 5/9
else
raise ArgumentError, "Unknown Unit #{unit}"
end
raise Too_Cold if @kelvin < 0

@celcius    =   @kelvin - 273.15
@fahrenheit = @kelvin * 1.8 - 459.67
end

def ==(other)
@kelvin == other.kelvin
end

def as(unit, round = nil)
return unless UNITS.include?(unit.to_s)
value = case unit
when 'C'; @celcius
when 'F'; @fahrenheit
when 'K'; @kelvin
end
round ? value.round(round) : value
end

def default(accurate = false)
as(DEFAULT_UNITS, accurate)
end

def to_s(unit=DEFAULT_UNIT)
"#{as(unit,2)}#{unit}"
end
end

I calculated once the values for all units. You could also re-implement the class and calculate the value only if you need it.

And a corresponding test code. Please see the assert_in_delta-checks. I used float operations, so test on equality may fail.

require 'test/unit'

class TempTest < Test::Unit::TestCase
def test_unit
assert_raise(ArgumentError){ Temperature.new(0,'X')}
assert_nothing_raised{ Temperature.new(0,'K')}
assert_nothing_raised{ Temperature.new(0,'C')}
assert_nothing_raised{ Temperature.new(0,'F')}
end
def test_0K
assert_nothing_raised{ Temperature.new(0,'K') }
assert_nothing_raised{ Temperature.new(-273.15,'C')}
assert_nothing_raised{ Temperature.new(-459.67,'F')}
assert_equal(0, Temperature.new(0,'K').kelvin)
assert_equal(-273.15, Temperature.new(0,'K').celcius)
assert_equal(-459.67, Temperature.new(0,'K').fahrenheit)
assert_equal(Temperature.new(0,'K'), Temperature.new(-273.15,'C'))
assert_equal(Temperature.new(0,'K'), Temperature.new(-459.67,'F'))
end
def test_too_cold
assert_raise(Temperature::Too_Cold){ Temperature.new(-1,'K') }
assert_raise(Temperature::Too_Cold){ Temperature.new(-273.16,'C')}
assert_raise(Temperature::Too_Cold){ Temperature.new(-459.68,'F')}
end
def test_similar
-200.upto(300){|i|
assert_in_delta(i, Temperature.new(i,'K').kelvin) if i >= 0
assert_in_delta(i, Temperature.new(i,'C').celcius)
assert_in_delta(i, Temperature.new(i,'F').fahrenheit)
}
end

def test_as_kelvin
assert_equal(0, Temperature.new(0,'K').as('K'))
end
def test_as_celcius
assert_in_delta(10, Temperature.new(10,'C').as('C'))
assert_in_delta(10.2, Temperature.new(10.2,'C').as('C'))
assert_in_delta(10, Temperature.new(10.2,'C').as('C',0))

assert_equal(10, Temperature.new(10.2,'C').as('C',0))
assert_equal(10.2, Temperature.new(10.2,'C').as('C',2))
end
def test_as_fahrenheit
assert_in_delta(10, Temperature.new(10,'F').as('F'))
assert_in_delta(10.2, Temperature.new(10.2,'F').as('F'))
assert_in_delta(10, Temperature.new(10.2,'F').as('F',0))

assert_equal(10, Temperature.new(10.2,'F').as('F',0))
assert_equal(10.2, Temperature.new(10.2,'F').as('F',2))
end
def test_to_s
assert_equal("283.15K", Temperature.new(10,'C').to_s)
assert_equal("10.0C", Temperature.new(10,'C').to_s('C'))
end
end

One last remark: See also the SO-question Units of measurement conversion gem. The ruby-units-gem supports also temperatures.