CODEWITHSUNDEEP
haskell
flamusdiu
flamusdiu

Reputation: 1742

Project Euler: #1 in Haskell implementation

For Project Euler Question #1, the shortest one-liner in GHCI is:

sum [3,6..9999] + sum [5,10..9999] - sum [15,30..9999]

I found this after I solved the problem is a much bloody way. However, since I am new to Haskell, I decide to see if I could take this and put it together as a set of function returning similar answer to any value of x (step value, i.e. '3' or '5' above] and y (length of the list).

I have the first function done here:

sumList :: (Enum a, Num a) => a -> a -> a
sumList a b = sum[a,a+a..b]

Next I was trying to take this function and do something such as sumListTotals [3,5] 1000 for example from the question. This would all sumList for each item in the list then one subtract the duplicate numbers (i.e. [15,30..1000] using the example.

I am not looking for someone to actually solve it but to help me in pointing me in the proper direction.

I was attempting to use the map function something like below:

sumListTotals list = map f list
    where f = sumlist a b

But, I am unsure how to pull out the stuff from the list or if I do something like sumListTotals ([3,5],1000) or am I completely on the wrong track here?

Update per @user5402:

module Project1 where

import Data.List (union)

sumListTotals :: (Enum a, Eq a, Num a) => a -> a -> a -> a
sumListTotals a b c = sum $ union [a,(*2)a..c] [b,(*2)b..c]

Upvotes: 2

Views: 416

Answers (1)

ErikR
ErikR

Reputation: 52057

You're on the right track:

sumListTotals a b c = sum [a,a+a..c] + sum [b,b+b..c] - sum[m,m+m..c]
  where m = ...???...

I'll leave you to figure out the definition of m since that's really a number theory problem and not a Haskell programming problem.

Clearly for a = 3, b = 5 m should be 15. But what should m be for a = 3, b = 3?

Upvotes: 1

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