Question

I’m working on a function that adds up all of the digits in a number, and to keep adding up digits until the number is reduced to a single digit. For example, the number 99999999999 should be evaluated like this:

99999999999
9+9+9+9+9+9+9+9+9+9+9
99
9+9
18
1+8
9

However, when I try running the number 99999999999 through my function, it returns 7, even though it should return 9. I’ve gone over my code several times, and I can’t think of any reason why.

Here’s the code:

sumdr x
  | x <= 9 = x
  | otherwise = sumdr $ addupnums x

addupnums y = foldl (+) 0 $ map read1 $ show y

read1 :: Char -> Int
read1 '1' = 1
read1 '2' = 2
read1 '3' = 3
read1 '4' = 4
read1 '5' = 5
read1 '6' = 6
read1 '7' = 7
read1 '8' = 8
read1 '9' = 9
read1 '0' = 0
read1 x = error "needs to be a number"

Answer 1

This works fine for me on a 64-bit system. If you’re on a 32 bit system, 99999999999 doesn’t fit into an Int, so it will overflow and give you wrong results.

If you change the type to Integer, it should work.