In today’s Programming Praxis exercise our task is to write two functions to convert numbers to and from Excel’s letter-based column names. Let’s get started, shall we?
A quick import:
import Data.Char
The easiest way to covert numbers to Excel notation is recursively. I also wrote a version that uses an unfold to mirror the single fold below, but this version is more concise.
toExcel :: Int -> String toExcel 0 = "" toExcel n = toExcel d ++ [chr $ m + 65] where (d,m) = divMod (n - 1) 26
Converting back to numbers is even easier and can be done in a single fold.
fromExcel :: String -> Int fromExcel = foldl (\a x -> 26 * a + ord x - 64) 0
Some test cases:
main :: IO () main = do print $ toExcel 1 == "A" print $ toExcel 26 == "Z" print $ toExcel 27 == "AA" print $ toExcel 256 == "IV" print $ fromExcel "A" == 1 print $ fromExcel "Z" == 26 print $ fromExcel "AA" == 27 print $ fromExcel "IV" == 256 print $ all (\n -> n == (fromExcel . toExcel) n) [1..2^16]
Everything seems to be working correctly.
Tags: bonsai, code, column, columns, excel, Haskell, kata, praxis, programming
