How to solve Euler Problem 16: Python

I really enjoy doing programming problems every now and then. Euler Problems range in difficulty and they are all math related. They are a great exercise for someone who knows math/logic and is trying to dabble with a new programming language. I’ve completed Euler problems 1-15 in C# and was working on tackling problem 16 today. For those too lazy to click the link, the problem is rather simple:

215 = 32768 and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26. What is the sum of the digits of the number 21000?

So the main problem with doing this in C# is that 21000 is a pretty big number and there aren’t any simple data types in C# that can hold a number that large. So the next place I went was Python, where you don’t have to worry about overflow – it just takes care of things for you auto-magically.

The algorithm is pretty straightforward:

  1. Calculate/store 21000
  2. Convert it to a string
  3. Print it, just to see how big it is
  4. Instantiate a new variable for the sum of digits
  5. Loop over each character in the string
  6. Add the converted-to-int character to the rolling sum
  7. Print the sum for checking

Dang, Python is fast! You can even use an online interpreter (I didn’t feel like downloading it).

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3 comments on “How to solve Euler Problem 16: Python
  1. puremcc says:

    Here is how I solved it in Python back when I was working through these problems:

    def solve(n):
    return sum([int(s) for s in str(2**n)])

    assert(solve(15) == 26)

    print solve(1000)

    • brhlavinka says:

      A bit more elegant than my solution :) … I love one-liners!

      • Mike Duffy says:

        Yeah, Python lends itself well to some really nice one-liners, with things like: lambda functions, list-comprehensions, and the trusty ol’ map() function. When I was doing the Euler problems, I made it a point to solve each one in a Pythonic way whenever possible. I also put together a little framework with an ‘EulerProblem’ class and a test harness, using Python’s ‘unittest’ library, that goes through each EulerProblem and tests it’s solve() method using the examples from each problem.

        One of my favorites was probably problem #13 (

        def solve(fileName):
        return int(str(sum([int(line) for line in open(fileName).readlines()]))[:10])

        where filename is just a txt file containing the one hundred 50-digit numbers.

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