5 ms·

I am not a Haskell programmer, but is this correct? (Poly a) + (Poly b) = Poly $ addup a b where addup [] b = b addup a [] = a add

by jlebar 8y ago

I am not a Haskell programmer, but is this correct? (Poly a) + (Poly b) = Poly $ addup a b where addup [] b = b addup a [] = a addup (a:as) (b:bs) = (a+b):(addup as bs) Imagine a simple example, adding `x+2` and `10`. In OP's representation, these would be represented as the lists [1, 2] and [10]. That is, the first element is the coefficient of the term of highest degree. But doesn't this implementation add list elements left-to-right, so we'd end up with the result [11, 2] instead of [1, 12]?

bojo 8y ago
module Main where newtype Poly a = Poly [a] deriving (Eq, Show) instance Num a => Num (Poly a) where Poly a + Poly b = Poly $ addup a b where addup [] b = b addup a [] = a addup (a:as) (b:bs) = (a+b):(addup as bs) main :: IO () main = print (Poly [1, 2] + Poly [10]) *Main> main Poly [11,2] Yep, you are correct.
ajcodez 8y ago
Yeah it’s the cons operator so it matches the first element (head) of the list and next elements (tail). It would need a reverse and reverse back to work correctly. reverse (addup (reverse a) (reverse b))
desdiv 8y ago
You're misreading OP's representation: > The polynomial x^3 −3x +1 is represented as Poly [1, -3, 0, 1] It starts with the 0th coefficient and goes up. So adding `x+2` and `10` would be zip-adding lists [2,1] and [10,0].
- jlebar 8y ago
  >> The polynomial x^3 −3x +1 is represented as Poly [1, -3, 0, 1] > It starts with the 0th coefficient and goes up. Is a list in Haskell canonically written in the opposite order of what I expect? I expect that the 0'th element of the list [a, b, c] is `a`. In Haskell, is it `c`? Assuming `a` is the 0th element, then the coefficient for the highest-degree term is the 0th element of the Poly. And since the degree of the two polynomials doesn't necessarily match, matching up the two highest-degree coefficients and adding them is obviously wrong. Or am I going crazy here?

by jlebar 8y ago

bojo 8y ago

module Main where newtype Poly a = Poly [a] deriving (Eq, Show) instance Num a => Num (Poly a) where Poly a + Poly b = Poly $ addup a b where addup [] b = b addup a [] = a addup (a:as) (b:bs) = (a+b):(addup as bs) main :: IO () main = print (Poly [1, 2] + Poly [10]) *Main> main Poly [11,2] Yep, you are correct.

ajcodez 8y ago

Yeah it’s the cons operator so it matches the first element (head) of the list and next elements (tail). It would need a reverse and reverse back to work correctly. reverse (addup (reverse a) (reverse b))

desdiv 8y ago

You're misreading OP's representation: > The polynomial x^3 −3x +1 is represented as Poly [1, -3, 0, 1] It starts with the 0th coefficient and goes up. So adding `x+2` and `10` would be zip-adding lists [2,1] and [10,0].

jlebar 8y ago
>> The polynomial x^3 −3x +1 is represented as Poly [1, -3, 0, 1] > It starts with the 0th coefficient and goes up. Is a list in Haskell canonically written in the opposite order of what I expect? I expect that the 0'th element of the list [a, b, c] is `a`. In Haskell, is it `c`? Assuming `a` is the 0th element, then the coefficient for the highest-degree term is the 0th element of the Poly. And since the degree of the two polynomials doesn't necessarily match, matching up the two highest-degree coefficients and adding them is obviously wrong. Or am I going crazy here?