Suma de elementos en posiciones dadas

José A. Alonso

17-12-2015

Definir la función

sumaEnPosicion :: [Int] -> [Int] -> Int

tal que (sumaEnPosicion xs ys) es la suma de todos los elementos de xs cuyas posiciones se indican en ys. Por ejemplo,

sumaEnPosicion [1,2,3] [0,2]  ==  4
sumaEnPosicion [4,6,2] [1,3]  ==  6
sumaEnPosicion [3,5,1] [0,1]  ==  8

Soluciones

-- 1ª solución
sumaEnPosicion1 :: [Int] -> [Int] -> Int
sumaEnPosicion1 xs ys = sum [xs !! y | y <- ys, 0 <= y, y < n]
    where n = length xs

-- 2ª solución
sumaEnPosicion2 :: [Int] -> [Int] -> Int
sumaEnPosicion2 xs ys = aux xs [y | y <- ys, 0 <= y, y < n] 0
    where n = length xs
          aux _  []     r = r
          aux xs (y:ys) r = aux xs ys (r + xs!!y)