Problem 33 – Cancelling Fractions

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49/98 = 4/8 if you cancel the 9s. This happens for few others fractions n/d where 0<n<d<100, and we’ll also ignore (n'\cdot 10)/(d'\cdot 10) for digits n',d'. Now take all these fractions that cancel like this, consider their product as a reduced fraction, and find the denominator. This is the challenge of problem 33.

Here’s what I came up with:

def gcd(a,b):
    if b == 0:
        return a
    return gcd(b,a%b)

def solve():
    fracs = [ ]
    conv = lambda t:int("%s%s" % t)
    for bothdig in xrange(1,10): # the digit that cancels
        for numdig in xrange(1,10): # digit left in numerator
            for dendig in xrange(1,10): # digit left in denom
            nums = map(conv, [(numdig,bothdig),(bothdig,numdig)]) # consider bother orderings
            dens = map(conv, [(dendig,bothdig),(bothdig,dendig)])
            for v in [(n,d) for n in nums for d in dens if n<d]:
                if v[0]*dendig == v[1]*numdig: # v[0]/v[1] = numdig/dendig
                    fracs += [ v ]
    p = reduce(lambda l,r:(l[0]*r[0],l[1]*r[1]),fracs) # multiply together solutions
    return p[1] / gcd(*p) # reduce the fraction, return denom

Certainly room for improvement, but it works.

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