The first problem of Project Euler is pretty simple. It reads:
If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
Find the sum of all the multiples of 3 or 5 below 1000.
Well, the naive way to solve this problem is simply to generate a list, and then to find the sum of its elements:
sum(filter(lambda x: x%3==0 or x%5==0, range(1,1000)))
and the answer is 233168.
Although I appreciate the elegance of functional programming (lambda function, filter, sum), mathematically the above code is not a good solution. The sum of all numbers that are either dividable by 3 or 5 in fact equals the sum of all numbers that ar dividable by 3, plus the sum of all numbers that are dividable by 5, minus the sum of all numbers that are dividable by 15, therefore
rg = range(1,1000)
sum(filter(lambda x: x%3==0, rg)) + sum(filter(lambda x: x%5==0, rg)) - sum(filter(lambda x: x%15==0, rg))
And again, the answer is 233168.
Tags: functional programming, programming, project euler, Python
