I’m quite new to object-orientated programming and have a problem with a
class I am trying to design. At the moment I have
#!/usr/local/bin/ruby
require ‘mathn’
class MySignal
def initialize(input) @signal = input
end
def mean()
sum = 0; @signal.each {|i| sum+=i}
mean = sum/@signal.length
end
def variance()
m = @signal.mean
var = @signal.inject(0) { |var, x| var += (x - m) ** 2 }
var = var/(@signal.length-1)
end
end
I can create a new “signal” with :
a = MySignal.new([1,2,3])
and calculate the mean with :
puts a.mean
But if I try to calculate the variance:
puts a.variance
./signal.rb:18:in variance': undefined methodmean’ for [1, 2,
3]:Array (NoMethodError)
Which makes sense, as @signal is an object with the Array class. I don’t
want to add methods to Ruby’s array class as later my methods may be too
specific to my problem.
How do I write a class such that methods defined within it can “refer”
to each other, for example “variance” can call and use “mean” ? I am
sure this question arises from my lack of familiarity with
object-orientated programming. In addition to suggestions for the
specific problem above, any pointers to references that could help me
learn more general OO-design would also be appreciated.
Methods called without an explicit receiver called using self as
receiver.
Since you’re inside the definition of an instance method of class
MySignal,
self is an instance of class MySignal, and has a method called mean.
@signal = input
var = @signal.inject(0) { |var, x| var += (x - m) ** 2 }
puts a.mean
specific to my problem.
How do I write a class such that methods defined within it can “refer”
to each other, for example “variance” can call and use “mean” ? I am
sure this question arises from my lack of familiarity with
object-orientated programming. In addition to suggestions for the
specific problem above, any pointers to references that could help me
learn more general OO-design would also be appreciated.
You just need to change the line to
m = mean
or - if you want to be more verbose -
m = self.mean
i.e. you just picked the wrong receiver for the call.
you could use inject here too @signal.inject{|s,i|s+i}
you do not need a start value, try this to see why
%w{Hello Brave Gnu World}.inject{|a,b| p [a,b]}
and
%w{Hello Brave Gnu World}.inject{|a,b| p [a,b]; b}
I just learned that recently from Ruby Q. 113 http://www.rubyquiz.com/quiz113.html
but be aware that inject is slower
mean = sum/@signal.length
end
def variance()
m = @signal.mean
m = self.mean
(which is equivalent to m = mean unless a local variable exists)
var = @signal.inject(0) { |var, x| var += (x - m) ** 2 }
@signal.inject(0){|v,x| v + (x-m) ** 2 } # the start value is needed
here but assignment is an unnecessary step
again #each might be faster albeit less elegant, if you run into
performance issues
var = var/(@signal.length-1)
maybe you want to rescue ZeroDevision here?
begin …
rescue ZeroDevisionError
end
You can, and I probably would, but you will then be calling the method
“mean” a lot. In fact, for a signal of size n, you will find the mean n
times, which makes your complexity scale n**2.
However, you can make the method “mean” cache, or memoize the value it
calulates:
def mean @mean ||= calculate_mean
end
def calculate_mean @signal.inject {|s,x| s+x} / @signal.size
end
Thank you all for your help, knowing exactly what “self” refers to in
each context seems to be the stumbling block for me at the moment. I’ll
have to read about this some more, along with the documentation for some
of the Enumerables .
I suppose that you variance method should read something like
def variance
m = mean @signal.inject(0) { |sum, x| sum + (x-m)**2 } / @signal.length
end
Note that I prefer not using meaningless assignment in the inject block.
And to that end, using the suggestions above, I suppose I could equally
well write :
def variance() @signal.inject(0) { |var, x| var += (x - mean) ** 2 } /
(@signal.length-1)
end
And to that end, using the suggestions above, I suppose I could equally
well write :
def variance() @signal.inject(0) { |var, x| var += (x - mean) ** 2 } /
(@signal.length-1)
end
removing the assignment “m = mean” ?
I wrote about removing the “m=mean” in another post…
I don’t think the above will work, and even if it does, I would advise
very strongly against putting an assignment in to your inject block. It
breaks the semantics of what people (well, me anyway) expect inject to
do. What you’re trying to do with inject is make a more “functional”
program; you’re trying to avoid assigning to things; you’re closer to
saying what you want done, rather than how you want it done (declarative
verses imperative style). If you start making assignments, you break
away from that style.
What you’re trying to do with inject is make a more “functional”
program; you’re trying to avoid assigning to things; you’re closer to
saying what you want done, rather than how you want it done (declarative
verses imperative style). If you start making assignments, you break
away from that style.
I think I understand what you mean, but I am so used to programming in
languages where assignments are common place (eg. Matlab) that I am
struggling to adjust to this new way of thinking. For example I have a
method :
def diff()
result = [] @signal.each_index do |i|
result << @signal[i+1] - @signal[i] unless i == (@signal.length -
1)
end
result
end
which takes a @signal object and returns an array of the differences
between each successive element of signal. But now I cannot write
a = MySignal.new([1,2,3])
a.diff.variance
because diff has returned an object of class “array” and the variance
method is not defined . How do I write “diff” in someway that it gives
me a new “signal” instead of an array ?
method is not defined . How do I write “diff” in someway that it gives
me a new “signal” instead of an array ?
Thank you all for your help !
Chris
You want to return a new Signal so… just do it !
def diff()
result = [] @signal.each_index do |i|
result << @signal[i+1] - @signal[i] unless i == (@signal.length-1)
end
return MySignal.new(result)
end
Maybe I missed something in your question, or maybe you would like your
class
MySignal to act more like an Array. If so, there are many solutions (in
order
of preference) :
Use the Forwardable module to give access to specific methods from
Array
Use the Delegate lib to delegate all unknown methods to Array
Make your class a subclass of Array (but delegation is almost always
better)
Reopen Array and add extra features (but this should be reserved for
some
specific needs, not in your case)
method :
which takes a @signal object and returns an array of the differences
between each successive element of signal. But now I cannot write
a = MySignal.new([1,2,3])
a.diff.variance
because diff has returned an object of class “array” and the variance
method is not defined . How do I write “diff” in someway that it gives
me a new “signal” instead of an array ?
Get it to wrap the array in to a signal, and return that.
So, at the end of the method you’ll do something like:
Thanks to all who gave the solution, it seems obvious now I’ve had it
pointed out to me ! I guess the question for me now is whether
signal.diff is still a signal or not, and if not, perhaps I need a
different kind of class to represent these objects.
Can you recommend books/tutorials for any (computer) language that deal
with modelling a non-trivial system using objects ? A lot of the
examples I have seen up to now are quite simple .
Chris
This forum is not affiliated to the Ruby language, Ruby on Rails framework, nor any Ruby applications discussed here.
