José A. AlonsoSea (b(i) | i ≥ 1) una sucesión infinita de números enteros mayores que 1. Entonces todo entero x mayor que cero se puede escribir de forma única como
x = x(0) + x(1)b(1) +x(2)b(1)b(2) + ... + x(n)b(1)b(2)...b(n) |
donde cada x(i) satisface la condición 0 ≤ x(i) < b(i+1). Se dice que [x(n),x(n-1),…,x(2),x(1),x(0)] es la representación de x en la base (b(i)). Por ejemplo, la representación de 377 en la base (2, 6, 8, …) es [7,5,0,1] ya que
377 = 1 + 0*2 + 5*2*4 + 7*2*4*6 |
y, además, 0 ≤ 1 < 2, 0 ≤ 0 < 4, 0 ≤ 5 < 6 y 0 ≤ 7 < 8.
Definir las funciones
decimalAmultiple :: [Integer] -> Integer -> [Integer] multipleAdecimal :: [Integer] -> [Integer] -> Integer |
tales que
- (decimalAmultiple bs x) es la representación del número x en la base bs. Por ejemplo,
decimalAmultiple [2,4..] 377 == [7,5,0,1]
decimalAmultiple [2,5..] 377 == [4,5,3,1]
decimalAmultiple [2^n | n < - [1..]] 2015 == [1,15,3,3,1]
decimalAmultiple (repeat 10) 2015 == [2,0,1,5] |
- (multipleAdecimal bs cs) es el número decimal cuya representación en la base bs es cs. Por ejemplo,
multipleAdecimal [2,4..] [7,5,0,1] == 377
multipleAdecimal [2,5..] [4,5,3,1] == 377
multipleAdecimal [2^n | n < - [1..]] [1,15,3,3,1] == 2015
multipleAdecimal (repeat 10) [2,0,1,5] == 2015 |
Comprobar con QuickCheck que se verifican las siguientes propiedades
- Para cualquier base bs y cualquier entero positivo n,
multipleAdecimal bs (decimalAmultiple bs x) == x |
- Para cualquier base bs y cualquier entero positivo n, el coefiente i-ésimo de la representación múltiple de n en la base bs es un entero no negativo menos que el i-ésimo elemento de bs.
Soluciones
import Test.QuickCheck import Data.List (unfoldr) -- 1ª solución de decimalAmultiple -- =============================== decimalAmultiple1 :: [Integer] -> Integer -> [Integer] decimalAmultiple1 bs n = reverse (aux bs n) where aux _ 0 = [] aux (d:ds) m = r : aux ds q where (q,r) = quotRem m d -- 2ª solución de decimalAmultiple -- =============================== decimalAmultiple2 :: [Integer] -> Integer -> [Integer] decimalAmultiple2 bs n = aux bs n [] where aux _ 0 xs = xs aux (d:ds) m xs = aux ds q (r:xs) where (q,r) = quotRem m d -- 3ª solución de decimalAmultiple -- =============================== decimalAmultiple3 :: [Integer] -> Integer -> [Integer] decimalAmultiple3 xs n = reverse (unfoldr f (xs,n)) where f (_ ,0) = Nothing f ((y:ys),m) = Just (r,(ys,q)) where (q,r) = quotRem m y -- Comprobación de equivalencia -- ============================ -- La propiedad es prop_decimalAmultiple :: InfiniteList (Positive Integer) -> Positive Integer -> Bool prop_decimalAmultiple (InfiniteList xs _) (Positive n) = all (== decimalAmultiple1 xs' n) [decimalAmultiple2 xs' n, decimalAmultiple3 xs' n] where xs' = map getPositive xs -- Comparación de eficiencia de decimalAmultiple -- ============================================= -- La comparación es -- λ> length (decimalAmultiple1 [2,7..] (10^(10^5))) -- 21731 -- (0.45 secs, 486,085,256 bytes) -- λ> length (decimalAmultiple2 [2,7..] (10^(10^5))) -- 21731 -- (0.32 secs, 485,563,664 bytes) -- λ> length (decimalAmultiple3 [2,7..] (10^(10^5))) -- 21731 -- (0.44 secs, 487,649,768 bytes) -- 1ª solución de multipleAdecimal -- =============================== multipleAdecimal1 :: [Integer] -> [Integer] -> Integer multipleAdecimal1 xs ns = aux xs (reverse ns) where aux (y:ys) (m:ms) = m + y * (aux ys ms) aux _ _ = 0 -- 2ª solución de multipleAdecimal -- =============================== multipleAdecimal2 :: [Integer] -> [Integer] -> Integer multipleAdecimal2 bs xs = sum (zipWith (*) (reverse xs) (1 : scanl1 (*) bs)) -- Comprobación de equivalencia de multipleAdecimal -- ================================================ -- La propiedad es prop_multipleAdecimal :: InfiniteList (Positive Integer) -> [Positive Integer] -> Bool prop_multipleAdecimal (InfiniteList xs _) ys = multipleAdecimal1 xs' ys' == multipleAdecimal2 xs' ys' where xs' = map getPositive xs ys' = map getPositive ys -- La comprobación es -- λ> quickCheck prop_multipleAdecimal -- +++ OK, passed 100 tests. -- Comparación de eficiencia de multipleAdecimal -- ============================================= -- La comparación es -- λ> length (show (multipleAdecimal1 [2,3..] [1..10^4])) -- 35660 -- (0.14 secs, 179,522,152 bytes) -- λ> length (show (multipleAdecimal2 [2,3..] [1..10^4])) -- 35660 -- (0.22 secs, 243,368,664 bytes) -- Comprobación de las propiedades -- =============================== -- La primera propiedad es prop_inversas :: InfiniteList (Positive Integer) -> Positive Integer -> Bool prop_inversas (InfiniteList xs _) (Positive n) = multipleAdecimal1 xs' (decimalAmultiple1 xs' n) == n where xs' = map getPositive xs -- Su comprobación es -- λ> quickCheck prop_inversas -- +++ OK, passed 100 tests. -- la 2ª propiedad es prop_coeficientes :: InfiniteList (Positive Integer) -> Positive Integer -> Bool prop_coeficientes (InfiniteList xs _) (Positive n) = and [0 <= c && c < b | (c,b) <- zip cs xs'] where xs' = map getPositive xs cs = reverse (decimalAmultiple1 xs' n) -- Su comprobación es -- λ> quickCheck prop_coeficientes -- +++ OK, passed 100 tests. |
El código se encuentra en GitHub.