Ruby Floating Point Math - Issue with Precision in Sum Calc

I'm having some issues with floating point math, and have gotten totally lost in ".to_f"'s, "*100"'s and ".0"'s!

I was hoping someone could help me with my specific problem, and also explain exactly why their solution works so that I understand this for next time.

My program needs to do two things:

1. Sum a list of decimals, determine if they sum to exactly 1.0
2. Determine a difference between 1.0 and a sum of numbers - set the value of a variable to the exact difference to make the sum equal 1.0.

For example:

1. [0.28, 0.55, 0.17] -> should sum to 1.0, however I keep getting 1.xxxxxx. I am implementing the sum in the following fashion:

   sum = array.inject(0.0){|sum,x| sum+ (x*100)} / 100

2. The reason I need this functionality is that I'm reading in a set of decimals that come from excel. They are not 100% precise (they are lacking some decimal points) so the sum usually comes out of 0.999999xxxxx or 1.000xxxxx. For example, I will get values like the following:

   0.568887955,0.070564759,0.360547286

To fix this, I am ok taking the sum of the first n-1 numbers, and then changing the final number slightly so that all of the numbers together sum to 1.0 (must meet validation using the equation above, or whatever I end up with). I'm currently implementing this as follows:

          sum = 0.0          array.each do |item|            sum += item * 100.0          end          array[i] = (100 - sum.round)/100.0

I know I could do this with inject, but was trying to play with it to see what works. I think this is generally working (from inspecting the output), but it doesn't always meet the validation sum above. So if need be I can adjust this one as well. Note that I only need two decimal precision in these numbers - i.e. 0.56 not 0.5623225. I can either round them down at time of presentation, or during this calculation... It doesn't matter to me.

Thank you VERY MUCH for your help!