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cs776 (Prasad)L5lists1 Programming with Lists

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cs776 (Prasad)L5lists2 Lists is a type list list is a type (* Homogeneous lists. *) –E.g., (true, [fn i:int => "i"]) : bool * (int -> string) list. –E.g., [1, 2, 3], 1::2::3::[] : int list; –E.g., (op ::) : a * a list ->a list; –List constructors [] and :: can be used in patterns.

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cs776 (Prasad)L5lists3 Built-in operations on lists hd : a list -> a tl : a list -> a list null: a list -> bool : a list * a list -> a list (* append operation; infix operator *) length : a list -> int (* sets vs lists -- multiplicity; ordering *)

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cs776 (Prasad)L5lists4 Catalog of List functions init [1,2,3] = [1,2] last [1,2,3] = 3 Specs : init [x]) = xs last [x]) = x Definitions: fun init (x::[]) = [] | init (x::xs) = x :: init xs; fun last (x::[]) = x | last (x::xs) = last xs;

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cs776 (Prasad)L5lists5 take 3 [1,2,3,4] = [1,2,3] drop 2 [1,2,3] = [3] Definition: fun take 0 xs = [] | take n [] = [] | take n (x::xs) = x::take (n-1) xs; fun drop 0 xs = xs | drop n [] = [] | drop n (x::xs) = drop (n-1) xs;

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cs776 (Prasad)L5lists6 takewhile even [2,4,1,6,2] = [2,4] dropwhile even [2,3,8] = [3,8] Definition: fun takewhile p [] = [] | takewhile p (x::xs) = if p x then x :: takewhile p xs else []; fun dropwhile p [] = [] | dropwhile p (x::xs) = if p x then dropwhile p xs else x::xs;

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cs776 (Prasad)L5lists7 Role of patterns –For testing type (discrimination) –For picking sub-expressions apart Signatures take, drop : int -> a list -> a list takewhile, dropwhile : (a -> bool) -> a list -> a list List. take, List.drop : a list * int -> a list

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cs776 (Prasad)L5lists8 Selectors #i (a1,…, ai, …, an) = ai nth ([a0,…,ai,…,an],i) = ai Type of #i cannot be described in ML. List.nth : a list * int -> a fun nth (x::xs, 0) = x | nth (x::xs, i) = nth (xs, i-1) (* Patterns not exhaustive. Exception raised for null list input. *)

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cs776 (Prasad)L5lists9 fun filter p [] = [] | filter p (x::xs) = if p x then x::filter p xs else filter p xs filter : ( a -> bool) -> a list -> a list

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cs776 (Prasad)L5lists10 fun exists p [] = false | exists p (x::xs) = orelse (p x) orelse (exists p xs) exists : (a -> bool) -> a list -> bool fun all p [] = true | all p (x::xs) = andalso (p x) andalso (all p xs) all : (a -> bool) -> a list -> bool

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cs776 (Prasad)L5lists11 fun pair [] ys = [] | pair (x::xs) [] = [] | pair (x::xs) (y::ys) = (x,y) :: pair xs ys ; pair: a list -> b list ->(a * b) list exception error; fun zip f (x::xs) (y::ys) = (f x y) :: zip f xs ys | zip f [] [] = [] | zip f xs ys = raise error; zip : (a -> b -> c ) -> a list -> b list -> c list

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cs776 (Prasad)L5lists12 Module List - open List; opening List datatype 'a list = :: of 'a * 'a list | nil exception Empty val null : 'a list -> bool val hd : 'a list -> 'a val tl : 'a list -> 'a list val last : 'a list -> 'a val getItem : 'a list -> ('a * 'a list) option val nth : 'a list * int -> 'a val take : 'a list * int -> 'a list val drop : 'a list * int -> 'a list val length : 'a list -> int val rev : 'a list -> 'a list …

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cs776 (Prasad)L5lists13 … : 'a list * 'a list -> 'a list val concat : 'a list list -> 'a list val revAppend : 'a list * 'a list -> 'a list val app : ('a -> unit) -> 'a list -> unit val map : ('a -> 'b) -> 'a list -> 'b list val mapPartial : ('a -> 'b option) -> 'a list -> 'b list val find : ('a -> bool) -> 'a list -> 'a option val filter : ('a -> bool) -> 'a list -> 'a list val partition : ('a -> bool) -> 'a list -> 'a list * 'a list val foldr : ('a * 'b -> 'b) -> 'b -> 'a list -> 'b val foldl : ('a * 'b -> 'b) -> 'b -> 'a list -> 'b val exists : ('a -> bool) -> 'a list -> bool val all : ('a -> bool) -> 'a list -> bool val tabulate : int * (int -> 'a) -> 'a list - …

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cs776 (Prasad)L5lists14 Properties of functions Semantic Equivalence –Efficiency Transformations –Formal verification ; Debugging tool map f (map g x) = map (f o g) x all p (filter p x) = true (map f) o (filter (p o f)) = (filter p) o (map f)

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cs776 (Prasad)L5lists15 Modular Designs using Lists Abstraction and Reuse Ref: Structure and Interpretation of Computer Programs (Abelson and Sussman)

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cs776 (Prasad)L5lists16 (define (sum-odd-squares tree) (cond ((null? tree) 0) ((pair? tree) (+ (sum-odd-squares (car tree)) (sum-odd-squares (cdr tree)) ) (else (if (odd? tree) (* tree tree) 0)) )) Takes a tree and computes the sum of the squares of the leaves that are odd.

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cs776 (Prasad)L5lists17 (define (even-fibs n) (define (next k) (if (> k n) ( ) (let ((f (fib k)) (if (even? f) (cons f (next (+ k 1))) (next (+ k 1)) )) )) (next 0)) Takes a number n and constructs a list of even numbers from among the first n Fibonacci numbers.

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cs776 (Prasad)L5lists18 Abstract Descriptions enumerates the leaves of a tree filters them, selecting the odd ones squares each of the selected ones accumulates the results using +, starting with 0 enumerates the integers from 0 to n computes the Fibonacci number for each integer filters them, selecting the even ones accumulates the results using cons, starting with ()

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cs776 (Prasad)L5lists19 (define (filter pred seq) (cond ((null? seq) ( )) ((pred (car seq)) (cons (car seq) (filter pred (cdr seq)))) (else (filter pred (cdr seq))) )) (define (accumulate op init seq) (if (null? seq) init (op (car seq) (accumulate op init (cdr seq))) ))

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cs776 (Prasad)L5lists20 (define (enum-interval low high) (if (> low high) ( ) (cons low (enum-interval (+ low 1) high)) )) (define (enum-tree tree) (if ((null? tree) ( )) ((pair? tree) (append (enum-tree (car tree)) (enum-tree (cdr tree)) )) (else (list tree))))

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cs776 (Prasad)L5lists21 (define (sum-odd-squares tree) (accumulate + 0 (map (lambda (x) (* x x)) (filter odd? (enum-tree tree))))) (define (even-fibs n) (accumulate cons nil (filter even? (map fib (enum-interval 0 n)))))

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cs776 (Prasad)L5lists22 Generality (define (list-fib-squares n) (map square (map fib (enum-interval 0 n) )) ) (define ( highest-salary-of-programmer records) (accumulate max 0 (map salary (filter programmer? records))))

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cs776 (Prasad)L5lists23 Inefficiencies Find the fifth prime in the interval 100 to 1000 (caddddr (filter prime? (enum-interval )) Sum all primes between x and y (define (sum-prime x y) (accumulate + 0 (filter prime? (enum-interval x y))))

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cs776 (Prasad)L5lists24 Rewrite (define (sum-prime x y) (define (iter count accum) (if (> count y) accum (if (prime? count) (iter (+ 1 count) (+ accum count)) (iter (+ 1 count) accum) ))) (iter x 0))

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