I have started looking at the project Euler site as a way to learn Haskell, and improve my Python and Ruby. I think the Haskell and Python versions are ok, but I'm sure there must be a cleaner way for Ruby.
This is not about how can I make one language look like another one.
This is Problem 1:
Q: Add all the natural numbers below one thousand that are multiples of 3 or 5.
Haskell:
sum [ x | x <- [1..999], mod x 3 == 0 || mod x 5 == 0 ]
Python:
sum ( [ x for x in range(1,1000) if x % 3 == 0 or x % 5 == 0 ] )
Ruby:
(1..999) . map {|x| x if x % 3 == 0 || x % 5 == 0 } . compact . inject(:+)
They all give the same answer.
OK, so Python can become:
sum ( x for x in range(1,1000) if x % 3 == 0 or x % 5 == 0 )
it is now a generator (a good thing as we are not storing the list)
but even more fun is:
sum( set(range(0,1000,3)) | set(range(0,1000,5)) )
For some reason I was looking at this again and tried a summation approach which should be constant time. In Python 3:
def step_sum(mn,mx,step):
amax = mx - (mx - mn) % step
return (mn + amax) * ((1 + ((amax - mn) / step)) / 2)
step_sum(3,999,3) + step_sum(5,999,5) - step_sum(15,999,15)
Ruby can become:
(1..999) . select {|x| x % 3 == 0 || x % 5 == 0} . inject(:+)
or
(1..999) . select {|x| x % 3 == 0 or x % 5 == 0} . reduce(:+)
I am presuming as unlike map, select doesn't produce 'nul' and therefore there is no need to call compact. nice.
Haskell can also be:
let ƒ n = sum [0,n..9