Question

I have to calculate returns between each elements of an array

Please see a sample of it:

nav = ["100.00", "86.21", "84.65", "82.46", "86.94"]

and here’s the loop I used:

0.upto(nav.count - 2) do |i|
  perf = (nav[i + 1].to_f / nav[i].to_f - 1) * 100
  p perf
end

It works, but my point is all about the last element of the array.

First I wanted to use something like:

nav.length.times do 
  ... 
end

Then, I used:

0.upto(nav.count - 2) 

in order to avoid the calculation of the (last value + 1) which would be nil.

However, I would be pleased to know if there is a better way to do it.

Thanks for your suggestion.

Answer 1

nav.map(&:to_f).each_cons(2){|a,b| p (b/a-1)*100}
#=> -13.790000000000003
#=> -1.809534856745143
#=> -2.587123449497941
#=> 5.432937181663844
#=>  => nil

and if you need to return an array of values:

nav.map(&:to_f).each_cons(2).inject([]){|ar,(a,b)| ar << (b/a-1)*100}
#=> [-13.790000000000003, -1.809534856745143, -2.587123449497941, 5.432937181663844]

On the second example: this inject construction is better known as “map” 😉 – @tokland

nav.map(&:to_f).each_cons(2).map{|a,b| (b/a-1)*100}