A command parser for building logical querying of array contents
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A system that allows me to define the logic for comparing arrays of objects.

One prerequisite for the comparison is that the objects have an id method that returns a unique (within the set of objects) integer.

The logic for an active record model Answer, looks like this:

a1 = Answer.find(1)
a2 = Answer.find(2)
a5 = Answer.find(5)

rule_one = ArrayLogic::Rule.new "(a1 and a2) or (a3 and a4)"

rule_two = ArrayLogic::Rule.new "a1 and not a2"

rule_three = ArrayLogic::Rule.new "2 in a1 a2 a3"

rule_four = ArrayLogic::Rule.new "(2 in a1 a2 a3) and (1 in a4 a5)"

              rule_one      rule_two      rule_three      rule_four
[a1, a2]      true          false         true            false
[a3, a4]      true          false         false           false
[a1, a3, a5]  false         true          true            true

The match and matches methods allow arrays to be tested against these rules:

rule_two.match([a1, a2])            --> false
rule_two.matches([a1, a2], [a1])    --> [[a1]]

You can also test for arrays that do not match the rule by using block and blockers:

rule_two.block([a1, a2])            --> true
rule_two.blockers([a1, a2], [a1])    --> [[a1, a2]]

See test/array_logic/rule_test for more examples


Version 0.2 introduces the concept of functions to ArrayLogic. The function syntax is:

<function>(<object_method as symbol>) <operator> <number>



One of the functions listed below


A method that can be called on all of the objects in the array


one of: <, <=, >, >=, ==, !=


a number to compare with the result

Using this array as an example:

answers = [Answer.find(1), Answer.find(5), Answer.find(6)]


Sums the values returned by the object_method and compares them with the number

rule = ArrayLogic::Rule.new 'sum(:id) == 12'
rule.match(answers)   --> true

rule = ArrayLogic::Rule.new 'sum(:id) > 12'
rule.match(answers)   --> false

rule = ArrayLogic::Rule.new 'sum(:id) >= 12'
rule.match(answers)   --> true


Averages the values returned by the object_method and compares them with the number

rule = ArrayLogic::Rule.new 'average(:id) == 4'
rule.match(answers)   --> true

rule = ArrayLogic::Rule.new 'average(:id) < 4'
rule.match(answers)   --> false

rule = ArrayLogic::Rule.new 'average(:id) <= 4'
rule.match(answers)   --> true


Counts the number of items not returning nil.

rule = ArrayLogic::Rule.new 'count(:id) == 3'
rule.match(answers)   --> true

If answer has a method :is_odd? that returned nil if the :id is even:

rule = ArrayLogic::Rule.new 'count(:is_odd?) == 2'
rule.match(answers)   --> true

Combining functions with other rules

functions can be combined with other rules:

rule = ArrayLogic::Rule.new 'sum(:id) == 12 and a6'
rule.match(answers)   --> true

rule = ArrayLogic::Rule.new '(sum(:id) == 12) and not a6'
rule.match(answers)   --> false

Combinations that match

Two methods allow you to determine sample combinations that match the current rule.

rule = ArrayLogic::Rule.new 'a1 and a2'

rule.matching_combinations   -->  [[1,2]]
rule.blocking_combinations   -->  [[1],[2]]

To limit the number of samples presented, both only use ids used within the rule. For the example above, an array that includes [1,2] would match, and so would [1,2,3]. However, arrays that only contain 1 or 2 would not match (for example [1,3])

Combinations and functions

Combinations are determined by analysing arrays of integers matching the ids found in the rule. This works for most rules but not those that contain functions.

Therefore, when either matching_combinations, or blocking_combinations are called on a rule that contains a function, an exception is raised.

rule = ArrayLogic::Rule.new 'sum(:id) > 5'
rule.matching_combinations   -->  raises ArrayLogic::UnableToDetermineCombinationsError

Thereby handling this issue is passed back to the host application

Run example.rb to see some more examples

ruby /lib/example.rb