lua - How to get decimals to round down on the half

Question

I'm using this function to round currently:

function round(val, decimal)
  if (decimal) then
    return math.floor( (val * 10^decimal) + 0.5) / (10^decimal)
  else
    return math.floor(val+0.5)
  end
end

That function works perfectly except that when a number lands on .5 like 5.5 or 1000.5 or 7.5 I need it to round down instead of up. What changes would I have to make to the function to make that happen?

score 7 · Accepted Answer · answered Apr 16 '13 at 12:16

7

function round(val, decimal)
  local exp = decimal and 10^decimal or 1
  return math.ceil(val * exp - 0.5) / exp
end

answered Apr 16 '13 at 12:16

Egor Skriptunoff

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1

Ok, that looks cleaner than mine and runs slightly faster as well. – W.B. Apr 16 '13 at 12:19
Both work, but this one is much cleaner and a lot less to type (and of course, it works). So I'll accept this one. – Mark Kramer Apr 16 '13 at 12:26
I'd actually be really interested in an explanation of how this works because I have no idea. – Mark Kramer Mar 28 '14 at 22:30

W.B. · Answer 2 · 2013-04-16T12:13:50.080

function round(val, decimal)
  local rndval = math.floor(val)
  local decval = val - rndval
  if decimal and decimal ~= 0 then
    decval = decval * 10 ^ decimal
    return rndval + (decval % 1 > 0.5 and math.ceil(decval) or math.floor(decval)) / 10 ^ decimal
  else
    return decval > 0.5 and rndval + 1 or rndval 
  end
end

My lazy take. Don't have access to the interpreter to test it, but it should be fine. Don't know, if performance is ok though...

EDIT: On LuaJIT one billion calls to the function takes about half a second. On plain Lua it's one million per half a second (Core i7/3610 mobile).

lua - How to get decimals to round down on the half

2 Answers2