18 December 2005

I use the Python programming language, and came across a mathematics problem in an oldish (July 2005) copy of News Scientist.

The problem (number 1343 in their regular enigma column) was this: Find the largest whole number, whose digits are different, and do not include 0, that is divisible by all of its individual digits.

After playing with it, and reading around a little, I came up with a small python script which could solve the problem in 10 to 15 seconds on my machine - and that is not on an unused machine.

Some notes on my assumptions

As any even number x 5 make a 10, which would not be allowable because of the 0 rule, the number probably does not include a 5. This assumption, which reduces the possibilities vastly is used throughout the code.

The code uses a string form of the number quite extensively since the digits are so important here. The skipnumberatindex function is not as clean as I would like - but it does what it is supposed to well.

So - here is my code:

def has_repeating_number(numstr):
  for digit in numstr:
    first = numstr.find(digit)
    next = numstr.find(digit,first+1)
    if(next != -1):
      return (True, next)
  return (False, -1)

def can_be_divided_by_digits(number,numstr):
  for digstr in numstr:
    if number % int(digstr) > 0:
      return False;
  return True;

#This base number saves going through the repeat and 
#other tests when you have subtracted one from an inner
#number. An earlier version set the digits after to 9's

basenumberstr = "98764321"

def skipnumberatindex(index, number, numstr):
    #skip the complex stuff if it is merely enough to subtract 1
    if index == len(numstr) - 1:
            return number - 1

    digit = numstr[index]
    while digit=='0':
            index -= 1
            digit = numstr[index]

    digitminusone = str(int(digit)-1)

    rindex = (len(numstr) - index) - 1

    outnumstr = (numstr[:index],
    outnumstr = ''.join(outnumstr)
    return int(outnumstr)

#I have started at the following number for some good 
#reasons.  Since we can only have one of each digit, and
#that none of them are 5, or 0, then we can only have 8
#digits. Since we are going for the largest number,
#then the digits are present in descending order

currentnumber = 98764321

while currentnumber > 0:
    currentnumber -= 1
    numstr = str(currentnumber)
    #Skip zeros
    if (currentnumber % 10 == 0) or (currentnumber % 5 == 0):
            currentnumber -= 1;
    elif '0' in numstr:
            #Get the place of zero in the number, from the right
            currentnumber = skipnumberatindex(numstr.find('0'), currentnumber, numstr)
    elif '5' in numstr:
            #Get the place of a five in the number
            currentnumber = skipnumberatindex(numstr.find('5'), currentnumber, numstr)
            (hasrepeat, where) = has_repeating_number(numstr)
            if hasrepeat:
                    #Okay - reverse the where a little
                    currentnumber = skipnumberatindex(where, currentnumber, numstr)
            elif not can_be_divided_by_digits(currentnumber,numstr):
                    currentnumber -= 1

print "Number should be %s\n" % (str(currentnumber))

If you have any comments or further optimisations, please let me know!

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