Kaprekar's constant

gistfile1.coffee

[i, o, n, e] = [0, '', prompt('Enter a number between 1 and 9998 whose digits are not all identical'), '']
while n!=6174
  n = e + n
  q = (n='0'+n if !n[3]) for x in [0..2]
  n ?= q
  o+=n+"\n" if i
  a = n.split(e).sort().join(e)
  n = a.split(e).reverse().join(e)
  o += n + ' - ' + a + ' = '
  n-=a
  i++
alert(o + "6174\nIt took " + i + " iterations to reach Kaprekar's constant.")

Test run here http://alicebobandmallory.com/kaprekars_constant.html

Kaprekar showed that 6174 is reached in the limit as one repeatedly subtracts the highest and lowest numbers that can be constructed from a set of four digits that are not all identical. - http://en.wikipedia.org/wiki/D._R._Kaprekar#Kaprekar_constant