Question

What might be a simple Ruby way to round numbers using probability, i.e., based on how close the value is to one boundary or the other (floor or ceiling)?

For example, given a current price value of 28.33, I need to add 0.014.

Equivalent to starting with 28.34 and needing to add 0.004, but the final value must be rounded to two decimal places(which can be provided as parameter, or fixed for now).

The final value should therefore be:

• 28.34 with 60% chance, since it is that much closer, OR
• 28.35 with 40% random chance

The reason it occured to me this could serve best is that the application is stateless and independent across runs, but still needs to approximate the net effect of accumulating the less significant digits normally rounded into oblivion (eg. micropenny values that do have an impact over time). For example, reducing a stop-loss by some variable increment every day (subtraction like -0.014 above instead).

It would be useful to extend this method to the Float class directly.

Answer 1

How about:

rand(lower..upper) < current ? lower.round(2) : upper.round(2)

EDIT:

The above will only work if you use Ruby 1.9.3 (due to earlier versions not supporting rand in a range).

Else

random_number = rand * (upper-lower) + lower
random_number < current ? lower.round(2) : upper.round(2)