Question

I think that Ruby’s inject method is really neat for calculating sums. But if I have to calculate an average and use inject, I always end up with something like this:

total_ranked_query_results = @results.count { |result| result.class == RankedQueryResult }
@results.inject(0.0) { |sum, result| result.class == RankedResult ? sum + result.average_precision : sum } / total_ranked_query_results

So I basically have to iterate twice to find out how many RankedResults there are in order to calculate the average.

Is there any better (rubyesque) way of avoiding this?

Answer 1

Either do the count and sum in the same iteration:

tmp = @results.inject([0.0, 0]) { |sum, result|
    if RankedResult === result
        [sum.first + result.average_precision, sum.last+1]
    else
        sum
    end
}
tmp.first / tmp.last

Or first filter the collection:

tmp = @results.filter { |result| RankedResult === result }
tmp.inject(0.0) { |sum, result| sum+result.avergage_precision } / tmp.length