Kyung-Goo Doh Hanyang University - ERICAComputer Science & Engineering Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Fall List Processing Version 2.0

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Predefined Polymorphic Lists # [] ;; (+ Nil +) - : 'a list = [] # 1::[] ;; (+ Cons +) - : int list = [1] # true::[];; - : bool list = [true] # 1::2::3::4::5::[] ;; - : int list = [1; 2; 3; 4; 5] # [1;2;3;4;5] ;; - : int list = [1; 2; 3; 4; 5] # 1::2::[3;4;5] ;; - : int list = [1; 2; 3; 4; 5] # List.hd [1;2;3;4;5] ;; (+ head +) - : int = 1 # List.tl [1;2;3;4;5] ;; (+ tail +) - : int list = [2; 3; 4; 5] # ;; (+ append +) - : int list = [1; 2; 3; 4; 5]

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Standard Library: List

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Definition: Lists in OCaml Structural Inductive Definition of Lists of type ‘a 1. (basis) An empty list, [], is a list of type ‘a. 2. (induction) If xs is a list of values of type ‘a and x is a value of type ‘a, then so is x::xs. 3. Nothing else is a list. Structural Induction 1. is P([]) true? 2. Assume P(xs) is true, is P(x::xs) true? 3. If we can say “yes” to these two questions, then we can assure that P(xs) is true for all lists xs.

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Computing List Length # let rec length xs = match xs with | [] -> 0 | _::xs -> 1 + length xs ;; val length : 'a list -> int = # length [3;3;3;3;3;3;3;3;3;3;3;3;3;3;3;3;3;3] ;; - : int = 18 # length [] ;; - : int = 0 # let length xs = let rec loop xs n = match xs with | [] -> n | _::xs -> loop xs (n+1) in loop xs 0 ;; val length : 'a list -> int = Primitive Tail

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Computation Trace (Primitive list length) length [1;2;3;4;5]  1 + length [2;3;4;5]  1 + (1 + length [3;4;5])  1 + (1 + (1 + length [4;5]))  1 + (1 + (1 + (1 + length [5])))  1 + (1 + (1 + (1 + (1 + length []))))  1 + (1 + (1 + (1 + (1 + 0))))  1 + (1 + (1 + (1 + 1)))  1 + (1 + (1 + 2))  1 + (1 + 3)   5 Complexity - time: θ(n) - space: θ(n) let rec length xs = match xs with | [] -> 0 | _::xs -> 1 + length xs

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Computation Trace (Tail list length) length [1;2;3;4;5]  loop [1;2;3;4;5] 0  loop [2;3;4;5] 1  loop [3;4;5] 2  loop [4;5] 3  loop [5] 4  loop [] 5  5 Complexity - time: θ(n) - space: θ(1) let length xs = let rec loop xs n = match xs with | [] -> n | _::xs -> loop xs (n+1) in loop xs 0

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Verification (Primitive list length) Theorem : The primitive recursive function call length xs evaluates to the length of xs, |xs|, for all lists xs. Proof: We prove by structural induction on xs. (Basis) xs = [], length [] = 0 by definition of program ‘length’ = |[]| by inspection (Induction hypothesis) For some list xs of length ≥ 0, length xs evaluates to |xs| (Induction step) It suffices to show that length (x::xs) evaluates to |x::xs| length (x::xs) = 1 + length xs by definition of program length = 1 + |xs| by induction hypothesis = |x::xs| by inspection let rec length xs = match xs with | [] -> 0 | _::xs -> 1 + length xs

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Verification (Tail list length) Theorem : The tail recursive function call length xs evaluates to the length of xs, |xs|, for all lists xs. Proof: We first show that loop xs n evaluates to n+|xs| by structural induction on xs. (Basis) xs = [], loop [] n = n by definition of program ‘loop’ = n + 0 by algebra = n + |[]| by inspection (Induction hypothesis) For some list xs of length ≥ 0, loop xs n evaluates to n+|xs| (Induction step) It suffices to show that loop (x::xs) n evaluates to n+|x::xs| loop (x::xs) n = loop xs (n+1) by definition of program loop = n+1+|xs| by induction hypothesis = n+|x::xs| by inspection Thus length xs = loop xs 0 evaluates to 0+|xs| = |xs|. let length xs = let rec loop xs n = match xs with | [] -> n | _::xs -> loop xs (n+1) in loop xs 0

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Computing List Append (* Concatenate two lists next to each other *) # let rec append xs ys = match xs with | [] -> ys | x::xs -> x::append xs ys ;; val append : 'a list -> 'a list -> 'a list = Primitive append [1;2;3] [4;5]  1 :: append [2;3] [4;5]  1 :: 2 :: append [3] [4;5]  1 :: 2 :: 3 :: append [] [4;5]  1 :: 2 :: 3 :: [4;5]  1 :: 2 :: [3;4;5]  1 :: [2;3;4;5]  [1;2;3;4;5] Execution Trace Complexity - time: θ(n) - space: is the infix version of this append function

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Verification (Primitive list append) Theorem : The primitive recursive function call append xs ys evaluates to the concatenation of lists xs and ys, for all lists xs and ys. Proof: We prove by structural induction on xs. (Basis) xs = [], append [] ys = ys by definition of program ‘append’ = by property of concatenation (Induction hypothesis) For some list xs of length ≥ 0, append xs ys evaluates to (Induction step) It suffices to show that append (x::xs) ys evaluates to append (x::xs) ys = x :: append xs ys by definition of program ‘append’ = x :: by induction hypothesis = by property of :: and concatenation let rec append xs ys = match xs with | [] -> ys | x::xs -> x::append xs ys

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Computing List Reverse (* Rearrange a list in reverse order*) # let rec rev xs = match xs with | [] -> [] | x::xs -> rev [x] ;; val rev : 'a list -> 'a list = Primitive Complexity - time: θ(n 2 ) - space: θ(n) rev [1;2;3;4]  rev [1]  rev [1]  rev [1]  [1]  [1]  [1]  [1]  [1]  [4;3;2;1] Execution Trace

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Verification (Primitive list reverse) Theorem : The primitive recursive function call reverse xs evaluates to the reverse of xs, ~xs, for all lists xs. Proof: We prove by structural induction on xs. (Basis) xs = [], rev [] = [] by definition of program ‘rev’ = ~[] by inspection (Induction hypothesis) For some list xs of length ≥ 0, rev xs evaluates to ~xs (Induction step) It suffices to show that rev (x::xs) evaluates to ~(x::xs) rev (x::xs) = rev [x] by definition of program ‘rev’ = [x] by induction hypothesis = ~(x::xs) by inspection let rec rev xs = match xs with | [] -> [] | x::xs -> rev [x]

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Computing List Reverse (* Rearrange a list in reverse order*) # let rev xs = let rec loop xs ys = match xs with | [] -> ys | x::xs -> loop xs (x::ys) in loop xs [] ;; val rev : 'a list -> 'a list = Tail Complexity - time: θ(n) - space: θ(1) rev [1;2;3;4]  loop [1;2;3;4] []  loop [2;3;4] [1]  loop [3;4] [2;1]  loop [4] [3;2;1]  loop [] [4;3;2;1]  [4;3;2;1] Execution Trace

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Verification (Tail list reverse) Theorem : The tail recursive function call rev xs evaluates to the reverse of xs, ~xs, for all lists xs. Proof: We first show that loop xs ys evaluates to by structural induction on xs. (Basis) xs = [], loop [] ys = ys by definition of program ‘loop’ = by property = by inspection (Induction hypothesis) For some list xs of length ≥ 0, loop xs ys evaluates to (Induction step) It suffices to show that loop (x::xs) ys evaluates to loop (x::xs) ys = loop xs (x::ys) by definition of program ‘loop’ = by induction hypothesis = = by inspection Thus rev xs = loop xs [] evaluates to = ~xs let rev xs = let rec loop xs ys = match xs with | [] -> ys | x::xs -> loop xs (x::ys) in loop xs []

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Computing List Append # let append xs ys = let rec loop xs ys = match xs with | [] -> ys | x::xs -> loop xs (x::ys) in loop (rev xs) ys ;; val append : 'a list -> 'a list -> 'a list = Tail append [1;2;3] [4;5]  loop (rev [1;2;3]) [4;5]  loop (loop [1;2;3] []) [4;5]  loop (loop [2;3] [1]) [4;5]  loop (loop [3] [2;1]) [4;5]  loop (loop [] [3;2;1]) [4;5]  loop [3;2;1] [4;5]  loop [2;1] [3;4;5]  loop [1] [2;3;4;5]  loop [] [1;2;3;4;5]  [1;2;3;4;5] Execution Trace Complexity - time: θ(n) - space: θ(1)

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Verification (Tail list append) Theorem : The tail recursive function call append xs ys evaluates to the concatenation of xs and ys, for all lists xs and ys. Proof: The ‘loop’ function here is identical to the ‘loop’ function in the tail recursive version of ‘rev’. Thus we already proved that loop xs ys evaluates to [1]. append xs ys = loop (rev xs) ys by definition of ‘append’ = loop ~xs ys by previous theorem = by [1] = by property of reverse let append xs ys = let rec loop xs ys = match xs with | [] -> ys | x::xs -> loop xs (x::ys) in loop (rev xs) ys

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Sorting sort : ‘a list -> ‘a list input: the list of values of type  output: the rearranged list of values in ascending order.

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Selection Sort Algorithm 1. Initialization unsorted list = input list sorted list = empty 2. Find the minimum value in the unsorted list. 3. Move it to the back of the sorted list. 4. Repeat the steps 2~3 until the unsorted list is empty.

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Animation Selection Sort

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Program Selection Sort (* find_min : ‘a list -> ‘a * ‘a list *) let rec find_min xs = match xs with | y::[] -> (y,[]) | y::ys -> let (m,zs) = find_min ys in if y<m then (y,ys) else (m,y::zs) | [] -> failwith "This message will never be displayed" (* selection_sort : 'a list -> 'a list *) let rec selection_sort xs = match xs with | [] -> [] | _ -> let (m,ys) = find_min xs in m :: selection_sort ys

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Insertion Sort Algorithm 1. Initialization unsorted list = input list sorted list = empty 2. Remove a value from the unsorted list. 3. Insert it at the correct position in the sorted list. 4. Repeat the steps 2~3 until the unsorted list is empty.

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Animation Insertion Sort

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Program Insertion Sort (* insert : 'a list -> 'a -> 'a list *) let rec insert xs x = match xs with | [] -> [x] | y::ys -> if y<x then y :: insert ys x else x :: xs (* insertion_sort : 'a list -> 'a list *) let insertion_sort xs = let rec loop sorted unsorted = match unsorted with | [] -> sorted | y::ys -> loop (insert sorted y) ys in loop [] xs

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Merge Sort Algorithm 1. If the list is of length 0 or 1, then it is already sorted. 2. Otherwise: Split the unsorted list into two sublists of about half the size. Sort each sublist recursively by re-applying merge sort. Merge the two sorted sublist back into one sorted list.

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Animation Merge Sort

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Program Merge Sort ~ (* split : 'a list -> 'a list * 'a list *) let rec split xs = match xs with | [] -> ([],[]) | [x] -> ([x],[]) | x::xs' -> let (ls,rs) = split xs' in if List.length ls <= List.length rs then (x::ls,rs) else (ls,x::rs) (* merge : 'a list -> 'a list -> 'a list *) let rec merge xs ys = match (xs,ys) with | ([],_) -> ys | (_,[]) -> xs | (x::xs',y::ys') -> if x<y then x :: merge xs' ys else y :: merge xs ys'

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Program Merge Sort (* merge_sort : 'a list -> 'a list *) let rec merge_sort xs = match xs with | [] -> [] | [x] -> [x] | _ -> let (ls,rs) = split xs in merge (merge_sort ls) (merge_sort rs)

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Quicksort Algorithm 1. Pick an element, called a pivot, from the list. 2. [Partition] Reorder the list  so that all elements which are less than the pivot come before the pivot and  so that all elements greater than the pivot come after it 3. Recursively sort the sublist of lesser elements and the sublist of greater elements.

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Animation Quicksort

Hanyang University - ERICA Functional Programming / Imperative Programming CSE215 Fundamentals of Program Design Computer Science & Engineering CSE215 Fundamentals of Program Design 06 List Processing Fall 2009 Kyung-Goo Doh Program Quicksort (* quicksort : 'a list -> 'a list *) let rec quicksort xs = match xs with | [] -> [] | pivot::xs' -> let (ls,rs) = partition pivot xs' in (quicksort (quicksort rs) (* partition : 'a -> 'a list -> 'a list * 'a list *) let partition pivot xs = (List.filter (fun x -> x < pivot) xs, List.filter (fun x -> pivot <= x) xs)