LÖVE

davisdude wrote:Also, if you don't mind, how would I decrease the amount of time taken? I didn't look up a formula, I just sort of did what made sense to me.

function mlib.math.prime( n )
     if n < 2 then return end
     for i = 2, math.sqrt(n) do
         if n % i == 0 then
             return false
         end
     end
     return true
end

• I return nil for things that are neither prime nor composite number. Since nil is falsy, this works out just as you would want in an if-statement.
• You only need to check up to sqrt(n), because you already checked the numbers larger than that (for example, we don't need to check if 10 % 5 == 0, because we already know that 10 % 2 == 0).
• if math.floor( math.sqrt( num ) ) ~= math.sqrt( num ) then this just checks if num isn't a square. It already gets checked in the for-loop, so it is unneeded.
• For programs that need to check for primes a lot, you'd probably want another solution.
• You can trivially make it twice as fast:

  Code: Select all

  function mlib.math.prime( n )
       if n < 2 then return end
       if n ~= 2 and n % 2 == 0 then return false end
       for i = 3, math.sqrt(n), 2 do
           if n % i == 0 then
               return false
           end
       end
       return true
  end

  This makes the code a bit elegant and only speeds it up with a constant factor, so I usually wouldn't do that.

davisdude wrote:I don't really know of a case where you would check for multiple numbers, but just thought it would be a nice added functionality.

Robin wrote:Generally, it's a bad idea to make functions do different things when given different arguments, because it leads to confusion and bugs.