Problem 13 – Too Easy?

I’m a little confused about problem 13. I wanted to come up with some nice way to find the first (leftmost) 10 digits of the sum. The only way I came up with was to actually do the sum though. I thought maybe that coding this up would point out something I hadn’t thought about that made the problem more difficult. Like maybe the 50 digit numbers were too big to store as integer values, or the sum was, or something. So I just coded up the naive solution to see what would happen:

from __future__ import with_statement

import sys, time

def readin(filename):
    """ Input a file, reading into a list of integers """
    ret = []
    with open(filename) as file:
        for line in file:
            ret.append(int(line))
    return ret

def solve(nums):
    """ The leading 10 digits of the sum of the numbers in the list nums """
    return str(sum(nums))[0:10] # loving this syntax

if __name__ == "__main__":
    t = time.time()
    ans = solve(readin(sys.argv[1]))
    t = time.time() - t
    print solve(readin(sys.argv[1])), t

I ran that, and it spit out an answer in about 0.0004 seconds.

So, am I missing something from this exercise, or is it that easy? Is there a more clever way to do it anyway?

Problem 14 – A Class

I was thinking a little bit more about my solutions to problem 14, and thinking that passing around the “mem” dictionary wasn’t particularly efficient. I didn’t want to make it a global variable, because I have a sort of stigma against those. I think, maybe, in Python it’s a stigma I should/could get over, because the file named prob14_v3.py that contains my script is actually a module (Python does that bit for me, no extra work on my part) that gets its own namespace without any extra work from me. But I could be wrong about that.

So, anyway, I thought maybe I’d try wrapping up my code into a class, and making the dictionary an attribute of that class. Here’s what I coded up:

class ProblemSolver:
    def __init__(self, bound = 1000000):
        self.mem = {1:1}
        self.bound = bound

    def lenseq(self, n):
        if(n in self.mem):
            return self.mem[n]
        suc = n%2 and 3*n+1 or n//2
        self.lenseq(suc)
        self.mem[n] = self.mem[suc] + 1
        return self.mem[n]

    def solve(self):
        retn,retl = 0,0
        for n in xrange(1, self.bound):
            l = self.lenseq(n)
            if(l > retl):
                retn,retl = n,l
        return (retn,retl)

if __name__ == "__main__":
    solver = ProblemSolver(int(sys.argv[1]))
    print solver.solve()

I’ve not done much in the way of rigorous performance testing, comparing this script with my other solution from yesterday, or with one that just uses a global variable to store the dictionary. A few quick tests indicate that they take approximately the same time to run.