excel-columns.factor

! A simple program that generates Excel spreadsheet column id corresponding to given column number.
! For example:
!   14 - N
!   26 - Z
!   27 - AA
!   29 - AC
USING: kernel math math.ranges sequences strings ;
IN: excel-columns

! Use the following after 'alphabet' definition to make it a compile time constant.
!   << \ alphabet alphabet define-constant >>
: alphabet    (   -- seq  ) CHAR: A CHAR: Z [a,b] ;
: nth-char    ( n -- char ) 25 + 26 mod alphabet nth 1string ;
: break-26    ( m -- n o  ) 26 /mod ;
: column-id   ( i -- str  ) dup 26 <= [ nth-char ] [ break-26 [ column-id ] bi@ append ] if ;

: test-run    (   --      ) 1000 [1,b] [ dup column-id 2array . ] each ;

MAIN: test-run

ExcelColumns.hs

import Control.Applicative
import Control.Arrow
import Control.Monad
import Control.Monad.Instances
import qualified Data.Vector as V

-- Y U NO IN STANDARD LIBRARY????!!!!
if' :: Bool -> a -> a -> a
if' cond x y = if cond then x else y

alphabet :: Vector Char
alphabet = V.enumFromTo 'A' 'Z'
 
nthChar :: Int -> String
nthChar = (:[]) . (alphabet V.!) . (`mod` 26) . (+ 25)
 
break26 :: Int -> (Int, Int)
break26 = (`divMod` 26)
 
columnId :: Int -> String
columnId = if' <$> (<= 26) <*> nthChar <*> uncurry (++) . join (***) columnId . break26
               
main :: IO ()
main = forM_ [1..100] (print . (id &&& columnId))

There is a tool for it. :D

daniel@Macbook ~ % pointfree -v "\x y -> (y \`div\` x, y \`mod\` x)"
Transformed to pointfree style:
ap (ap . ((,) .) . flip div) (flip mod)

let break26' = (ap (ap . ((,) .) . flip div) (flip mod)) 26

@DanielOberg: Yes, I know about that tool. (It's the same thing as lambdabot's @pl, right?) I meant, can you write something that can be "read"? Sections of (.) don't count (as I said here: http://missingfaktor.blogspot.com/2011/12/sections-of.html). I wrote the columnId definition without any help from lambdabot, and I find it readable. The data flow is easy to visualize. Can we do something like that for break26?

We can simplify it a bit without much effort:

(ap ((&&&) . flip div) (flip mod)) 26

@DanielOberg: That one looks cool. Thanks!

To write break26 while only using 26 once, why not write

let break26 = (`divMod` 26)

@chris-taylor: Thanks. I wasn't aware of that function. Turns out Factor has /mod as well. Will update the snippets accordingly.