Round to nearest odd digit

Question

I want to round some numbers to the nearest tenth, using odd tenths only. e.g.

91.15 -> 91.1
91.21 -> 91.3

What's a simple and generic equation for doing this?

Answer 1

This is not rounding to the nearest tenth, but rather rounding to the nearest fifth (two-tenth) with a one-tenth offset. With that in mind, the general equation is:

# Any language where 'round' rounds to the nearest integer
radioStation = round( (original-offset)/interval ) * interval + offset

In Lua:

-- number:   the original value to round
-- interval: the distance between desired values
-- offset:   an optional shifting of the values
function roundToNearest( number, interval, offset )
  offset   = offset   or 0  -- default value
  interval = interval or 1  -- default value
  return math.floor( (number-offset)/interval + 0.5 ) * interval + offset
end

for n=1, 2, 0.09 do
  local result = roundToNearest(n, 0.2, 0.1)
  print(string.format("%.2f : %g", n, result))
end
--> 1.00 : 1.1
--> 1.09 : 1.1
--> 1.18 : 1.1
--> 1.27 : 1.3
--> 1.36 : 1.3
--> 1.45 : 1.5
--> 1.54 : 1.5
--> 1.63 : 1.7
--> 1.72 : 1.7
--> 1.81 : 1.9
--> 1.90 : 1.9
--> 1.99 : 1.9