Menu Close

Etiqueta: L.length_append

Pruebas de length (xs ++ ys) = length xs + length ys

José A. Alonso-

En Lean están definidas las funciones

   length : list α → nat
   (++)   : list α → list α → list α

tales que

  • (length xs) es la longitud de xs. Por ejemplo,
     length [2,3,5,3] = 4
  • (xs ++ ys) es la lista obtenida concatenando xs e ys. Por ejemplo.
     [1,2] ++ [2,3,5,3] = [1,2,2,3,5,3]

Dichas funciones están caracterizadas por los siguientes lemas:

   length_nil  : length [] = 0
   length_cons : length (x :: xs) = length xs + 1
   nil_append  : [] ++ ys = ys
   cons_append : (x :: xs) ++ y = x :: (xs ++ ys)

Demostrar que

   length (xs ++ ys) = length xs + length ys

Para ello, completar la siguiente teoría de Lean:

import tactic
open list
 
variable  {α : Type}
variable  (x : α)
variables (xs ys zs : list α)
 
lemma length_nil  : length ([] : list α) = 0 := rfl
 
example :
  length (xs ++ ys) = length xs + length ys :=
sorry

[expand title=»Soluciones con Lean»]

import tactic
open list
 
variable  {α : Type}
variable  (x : α)
variables (xs ys zs : list α)
 
lemma length_nil  : length ([] : list α) = 0 := rfl
 
-- 1ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
begin
  induction xs with a as HI,
  { rw nil_append,
    rw length_nil,
    rw zero_add, },
  { rw cons_append,
    rw length_cons,
    rw HI,
    rw length_cons,
    rw add_assoc,
    rw add_comm (length ys),
    rw add_assoc, },
end
 
-- 2ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
begin
  induction xs with a as HI,
  { rw nil_append,
    rw length_nil,
    rw zero_add, },
  { rw cons_append,
    rw length_cons,
    rw HI,
    rw length_cons,
    -- library_search,
    exact add_right_comm (length as) (length ys) 1 },
end
 
-- 3ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
begin
  induction xs with a as HI,
  { rw nil_append,
    rw length_nil,
    rw zero_add, },
  { rw cons_append,
    rw length_cons,
    rw HI,
    rw length_cons,
    -- by hint,
    linarith, },
end
 
-- 4ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
begin
  induction xs with a as HI,
  { simp, },
  { simp [HI],
    linarith, },
end
 
-- 5ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
begin
  induction xs with a as HI,
  { simp, },
  { finish [HI],},
end
 
-- 6ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
by induction xs ; finish [*]
 
-- 7ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
begin
  induction xs with a as HI,
  { calc length ([] ++ ys)
         = length ys                    : congr_arg length (nil_append ys)
     ... = 0 + length ys                : (zero_add (length ys)).symm
     ... = length [] + length ys        : congr_arg2 (+) length_nil.symm rfl, },
  { calc length ((a :: as) ++ ys)
         = length (a :: (as ++ ys))     : congr_arg length (cons_append a as ys)
     ... = length (as ++ ys) + 1        : length_cons a (as ++ ys)
     ... = (length as + length ys) + 1  : congr_arg2 (+) HI rfl
     ... = (length as + 1) + length ys  : add_right_comm (length as) (length ys) 1
     ... = length (a :: as) + length ys : congr_arg2 (+) (length_cons a as).symm rfl, },
end
 
-- 8ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
begin
  induction xs with a as HI,
  { calc length ([] ++ ys)
         = length ys                    : by rw nil_append
     ... = 0 + length ys                : (zero_add (length ys)).symm
     ... = length [] + length ys        : by rw length_nil },
  { calc length ((a :: as) ++ ys)
         = length (a :: (as ++ ys))     : by rw cons_append
     ... = length (as ++ ys) + 1        : by rw length_cons
     ... = (length as + length ys) + 1  : by rw HI
     ... = (length as + 1) + length ys  : add_right_comm (length as) (length ys) 1
     ... = length (a :: as) + length ys : by rw length_cons, },
end
 
-- 9ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
list.rec_on xs
  ( show length ([] ++ ys) = length [] + length ys, from
      calc length ([] ++ ys)
           = length ys                    : by rw nil_append
       ... = 0 + length ys                : by exact (zero_add (length ys)).symm
       ... = length [] + length ys        : by rw length_nil )
  ( assume a as,
    assume HI : length (as ++ ys) = length as + length ys,
    show length ((a :: as) ++ ys) = length (a :: as) + length ys, from
      calc length ((a :: as) ++ ys)
           = length (a :: (as ++ ys))     : by rw cons_append
       ... = length (as ++ ys) + 1        : by rw length_cons
       ... = (length as + length ys) + 1  : by rw HI
       ... = (length as + 1) + length ys  : by exact add_right_comm (length as) (length ys) 1
       ... = length (a :: as) + length ys : by rw length_cons)
 
-- 10ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
list.rec_on xs
  ( by simp)
  ( λ a as HI, by simp [HI, add_right_comm])
 
-- 11ª demostración
lemma longitud_conc_1 :
  ∀ xs, length (xs ++ ys) = length xs + length ys
| [] := by calc
    length ([] ++ ys)
        = length ys                    : by rw nil_append
    ... = 0 + length ys                : by rw zero_add
    ... = length [] + length ys        : by rw length_nil
| (a :: as) := by calc
    length ((a :: as) ++ ys)
        = length (a :: (as ++ ys))     : by rw cons_append
    ... = length (as ++ ys) + 1        : by rw length_cons
    ... = (length as + length ys) + 1  : by rw longitud_conc_1
    ... = (length as + 1) + length ys  : by exact add_right_comm (length as) (length ys) 1
    ... = length (a :: as) + length ys : by rw length_cons
 
-- 12ª demostración
lemma longitud_conc_2 :
  ∀ xs, length (xs ++ ys) = length xs + length ys
| []        := by simp
| (a :: as) := by simp [longitud_conc_2 as, add_right_comm]
 
-- 13ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
-- by library_search
length_append xs ys
 
-- 14ª demostración
example :
  length (xs ++ ys) = length xs + length ys :=
by simp

Se puede interactuar con la prueba anterior en esta sesión con Lean.

En los comentarios se pueden escribir otras soluciones, escribiendo el código entre una línea con <pre lang="lean"> y otra con </pre>
[/expand]

[expand title=»Soluciones con Isabelle/HOL»]

theory "Pruebas_de_length(xs_++_ys)_Ig_length_xs+length_ys"
imports Main
begin
 
(* 1ª demostración *)
lemma "length (xs @ ys) = length xs + length ys"
proof (induct xs)
  have "length ([] @ ys) = length ys"
    by (simp only: append_Nil)
  also have "… = 0 + length ys"
    by (rule add_0 [symmetric])
  also have "… = length [] + length ys"
    by (simp only: list.size(3))
  finally show "length ([] @ ys) = length [] + length ys"
    by this
next
  fix x xs
  assume HI : "length (xs @ ys) = length xs + length ys"
  have "length ((x # xs) @ ys) =
        length (x # (xs @ ys))"
    by (simp only: append_Cons)
  also have "… = length (xs @ ys) + 1"
    by (simp only: list.size(4))
  also have "… = (length xs + length ys) + 1"
    by (simp only: HI)
  also have "… = (length xs + 1) + length ys"
    by (simp only: add.assoc add.commute)
  also have "… = length (x # xs) + length ys"
    by (simp only: list.size(4))
  then show "length ((x # xs) @ ys) = length (x # xs) + length ys"
    by simp
qed
 
(* 2ª demostración *)
lemma "length (xs @ ys) = length xs + length ys"
proof (induct xs)
  show "length ([] @ ys) = length [] + length ys"
    by simp
next
  fix x xs
  assume "length (xs @ ys) = length xs + length ys"
  then show "length ((x # xs) @ ys) = length (x # xs) + length ys"
    by simp
qed
 
(* 3ª demostración *)
lemma "length (xs @ ys) = length xs + length ys"
proof (induct xs)
  case Nil
  then show ?case by simp
next
  case (Cons a xs)
  then show ?case by simp
qed
 
(* 4ª demostración *)
lemma "length (xs @ ys) = length xs + length ys"
by (induct xs) simp_all
 
(* 5ª demostración *)
lemma "length (xs @ ys) = length xs + length ys"
by (fact length_append)
 
end

En los comentarios se pueden escribir otras soluciones, escribiendo el código entre una línea con <pre lang="isar"> y otra con </pre>
[/expand]

Soluciones →