Guidance on how to implement certain problems in Ruby - ruby

Question

For the class NumberSet, I have to define the [] method so that it takes one argument, which it uses as a filter to select members of the set and returns a new set containing those elements.
For example, I have
Filter.new { |number| number.even? }
and
SignFilter.new(:non_negative)
which are classes that I must construct.
I also have to define the & and | operators so that they work with the filters. Something along the lines of
numbers[SignFilter.new(:non_negative) & Filter.new { |number| number.even? }]
The class so far is:
class NumberSet
include Enumerable
def initialize
#arr=[]
end
def each (&block)
#arr.each do |member|
block.call (member)
end
end
def << number
#arr<<number unless #arr.include?(number)
end
end
For Filter I think something like works:
class Filter
def initialize
yield
end
end
My biggest problem is the [], $and | parts, which I have no idea how to do.
What I want to accomplish is :
numbers = NumberSet.new
[-3, -2, -1, 0, 1, 2, 3, 4, 5].each do |number|
numbers << number
end
numbers[SignFilter.new(:non_negative) & Filter.new { |number| number.even?}].to_a
#=>[0, 2, 4]

Answer 1

I think this should do it.
Code
SignFilter class
class SignFilter
def initialize(sign)
#sign = sign
end
def filter
case #sign
when :negative
-> i { i < 0 }
when :positive
-> i { i > 0 }
when :non_negative
-> i { i >= 0 }
when :non_positive
-> i { i <= 0 }
when :zero
-> i { i.zero? }
end
end
end
Filter class
class Filter
attr_reader :filter
def initialize &filter
#filter = filter
end
end
NumberSet class
class NumberSet
def initialize arr
#arr = arr
end
def process(op, sf, f)
sign_filter = sf.filter
filter = f.filter
#arr.send(op).each { |e| sign_filter.call(e) }
.send(op).each { |e| filter.call(e) }
end
end
Examples
sf = SignFilter.new :negative
f = Filter.new { |number| number.even? }
ns = NumberSet.new [-3, -2, -1, 0, 1, 2, 3, 4, 5]
ns.process(:reject, sf, f)
#=> [1, 3, 5]
sf = SignFilter.new :non_negative
f = Filter.new { |number| number % 3 == 0 }
ns = NumberSet.new [-3, -2, -1, 0, 1, 2, 3, 4, 5, 6]
ns.process(:select, sf, f)
#=> [0, 3, 6]
Explanation
Blocks are not objects, so you can't pass them around. They can be converted to procs, however, which are objects (instances of the class Proc), and therefore can be passed. You will need to understand procs (and lambdas, which are a type of proc). Here is a good explanation. (Aside: one thing that's a little confusing about procs is the multitude of ways they can be invoked. For example, if p -> i { 3*i }, p.call(2), p.yield(2), p[2] and p.(2) all return the value 6.)
Here is what is happening:
An instance sf of the SignFilter class is created by passing the symbol denoting the signs of the elements of the array that are to be selected or rejected (:negative, :non_negative and so on). The symbol is saved in an instance variable #sign.
The SignFilter class has a method filter which returns a lambda based on the value of #sign.
An instance of the Filter class is created by passing a block which is received as a proc and saved into the instance variable #filter, which has a read accessor.
An instance of the NumberSet class is created by passing the array, which is saved into the instance variable #arr.
NumberSet#process is passed three arguments: the Enumerable method :reject or :select (saved into op), the SignFilter instance (sf) and the Filter instance (f).
Within NumberSet#process, #arr is sent op, which creates an enumerator (arr.reject or arr.select).
The SignFilter lambda is called on each element of the enumerator. An array comprised of a subcollection of #arr is returned.
The returned array is sent op, creating another reject or select enumerator.
The Filter proc is called on the each element of the new enumerator, causing the desired array to be returned.
Considering that both reject and select are not necessary (either would do), it would have been simpler to just replace .send(op) in NumberSet#process with, say, reject, but I chose to do it this way just to illustrate the generalization.
Notice that it would be easy to generalize NumberSet#process to process(sf,f,o), where, as now, sf and f provide filters and o would be an instance of an Operation class that specifies a proc to send to the filtered subcollection of #arr to invoke an Enumerable method with a block (map, group_by and so on).

Answer 2

Since this appears to be a homework question, I'm not going to give you the full implementation, but I'll see you on your way. The big thing you need to realize here is that methods such as [] and | can be defined as normal methods in Ruby. In Ruby, you simply send messages (such as []) to a "receiver". How you handle those methods are defined via def instructions on your class definition. So, for example:
class Filter
def |(other_filter)
# Do some work to build a new filter which is a union of this filter and other_filter
end
end
class SignFilter < Filter
# SignFilter inherits the & method from Filter, and you can then go on to implement SignFilter-specific functionality
# ...
end
class NumberSet
def [](filter)
# Given a filter, apply each of the numbers of this NumberSet to the filter, and return those which pass the filter
end
end
To invoke those methods, would do something like:
filter = Filter.new {|num| num.even? }
sign_filter = SignFilter.new(:non_negative)
union_filter = filter | sign_filter
This third line is equivalent to calling:
union_filter = filter.|(sign_filter)
or:
union_filter = filter.send("|", sign_filter)
In all three cases, the "|" message is sent to the the filter instance, with sign_filter as an additional argument. You could also do something like:
numset = NumberSet.new(1,2,3,4,5)
filtered_numbers = numset[union_filter]
This second line is equivalent to:
numset.[](union_filter)
# or
numset.send("[]", union_filter)
The original syntax is just syntactic sugar for those method calls - it's just there because it makes Ruby code look nicer, and helps the programmer mentally map it to common conventions for those kinds of operations.