each block create a Nil class - ruby-on-rails

Question

I want calculate components of an average
Controller
def set_average_course
#course = #group.courses.find(params[:course_id])
#evaluations = #course.evaluations
#evaluations.each do |evaluation|
#numerator += evaluation.average * evaluation.coefficient
#denumerator += evaluation.coefficient
end
#average = #numerator / #denumerator
#course.update(average: average)
end
Logs
NoMethodError (undefined method `+' for nil:NilClass)
But
#numerator += evaluation.average * evaluation.coefficient
is well defined outside the .each block

Answer 1

It would be better and more efficient to compute the values using SQL:
The code will look like this:
def set_average_course
#course = #group.courses.find(params[:course_id])
res = Evaluation.select("SUM(Evaluations.average * Evaluations.coefficient) AS numerator, SUM(Evaluations.coefficient) AS denumerator").
where(course_id: #course.id)
average = res.first['numerator'] / res.first['denominator'] # Raise an error if 0
#course.update(average: average)
end
Hope this leads you to the right direction

Answer 2

When you enter the each loop for the first time, #numerator and #denumerator are not initialized yet. Instance variables are different from local variables in that they evaluate to nil if not initialized yet, whereas a reference to an uninitialized local variable will thrown an error. The solution is to initialize the variables to the neutral element of addition, which is 0, before entering the loop:
def set_average_course
#course = #group.courses.find(params[:course_id])
#evaluations = #course.evaluations
#numerator = 0
#denumerator = 0
#evaluations.each do |evaluation|
#numerator += evaluation.average * evaluation.coefficient
#denumerator += evaluation.coefficient
end
#average = #numerator / #denumerator
#course.update(average: average)
end
Although, as a side note, I think you mean "denominator" instead of "denumerator". You will also have to account for the case where #denumerator is equal to 0, for example, when #evaluations is empty:
def set_average_course
#course = #group.courses.find(params[:course_id])
#evaluations = #course.evaluations
return if #evaluations.blank? # or raise an error
#numerator = 0
#denumerator = 0
#evaluations.each do |evaluation|
#numerator += evaluation.average * evaluation.coefficient
#denumerator += evaluation.coefficient
end
#average = #numerator / #denumerator
#course.update(average: average)
end

Answer 3

Try to initialize numerator and denumerator to number like #p11y's answer.
My suggestion is to calculate average more ruby way like this:
#numerator = #evaluations.map { |e| e.average * e.coefficient }.reduce(&:+)
#denumerator = #evaluations.map { |e| e.coefficient }.reduce(&:+)
#average = #numerator / #denumerator