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U of I CS 421 - Lecture notes

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OutlineOption ExampleCS421 Lecture 5b: Option1Mark [email protected] of Illinois at Urbana-ChampaignJune 8, 20061Based on slides by Mattox Beckman, as updated by Vikram Adve, GulAgha, and Elsa GunterMark Hills CS421 Lecture 5b: OptionOutlineOption ExampleOption ExampleMark Hills CS421 Lecture 5b: OptionOutlineOption ExampleObjectivesOption is useful (really!), but maybe not obviously so. This lecturesupplement provides a short example of using the option type. Atthe end of this supplement, you should:◮better understand why you would use option◮know how to write functions which match over optioncorrectlyMark Hills CS421 Lecture 5b: OptionOutlineOption ExampleThe Example: Association ListsAssociation lists provide a means of associating a keyword with adata value. This example will illustrate creating association listsusing the option type.Mark Hills CS421 Lecture 5b: OptionOutlineOption ExampleAssociation List: Type DefinitionWe will use strings for keys, but keep the other type of data open.1 # type ’a alist = (string * ’a) list;;2 type ’a alist = (string * ’a) listMark Hills CS421 Lecture 5b: OptionOutlineOption ExampleConstructing Association ListsWe can build association lists by taking a list of keys and a list ofvalues and zipping them together.1 # let make_alist list1 (list2:(’a list)) : (’a alist) =2 zip list1 list2;;3 val make_alist : string list -> ’a list -> ’a alist = <fun>Mark Hills CS421 Lecture 5b: OptionOutlineOption ExampleSource ListsThese lists provide the source data...1 # let l1 = ["Jim";"Steve";"Anne";"James";"Rachel"];;2 val l1 : string list = ["Jim"; "Steve"; "Anne";3 "James"; "Rachel"]45 # let l2 = [21;32;22;19;20];;6 val l2 : int list = [21; 32; 22; 19; 20]78 # let l3 = ["English";"Math";"Computer Science";9 "Biology";"Physics"];;10 val l3 : string list =11 ["English"; "Math"; "Computer Science"; "Biology"; "Physics"]Mark Hills CS421 Lecture 5b: OptionOutlineOption ExampleSome Sample ALists...and these are the resulting association lists.1 # let a1 = make_alist l1 l2;;2 val a1 : int alist =3 [("Jim", 21); ("Steve", 32); ("Anne", 22); ("James", 19);4 ("Rachel", 20)]56 # let a2 = make_alist l1 l3;;7 val a2 : string alist =8 [("Jim", "English"); ("Steve", "Math");9 ("Anne", "Computer Science");10 ("James", "Biology"); ("Rachel", "Physics")]Mark Hills CS421 Lecture 5b: OptionOutlineOption ExampleLooking up AList ItemsTo look up an item in an association list, we should provide thekey. If the key isn’t there, though, what should we return? This iswhere option comes in handy...1 # let rec lookup_alist key list =2 match list with3 | [] -> None4 | (k,v)::vs ->5 if key = k then Some v else lookup_alist key vs;;6 val lookup_alist : ’a -> (’a * ’b) list -> ’b option = <fun>78 # lookup_alist "Mark" a1;;9 - : int option = None10 # lookup_alist "James" a1;;11 - : int option = Some 1912 # lookup_alist "Rachel" a2;;13 - : string option = Some "Physics"Mark Hills CS421 Lecture 5b: OptionOutlineOption ExampleUsing Option ValuesOnce we get option values back how do we use them? Patternmatching!1 # let print_major name alist =2 let result = lookup_alist name alist3 in match result with4 | None -> print_string "No record found!"5 | Some s -> print_string (name ^ "’s major is " ^ s);;6 val print_major :7 string -> (string * string) list -> unit = <fun>89 # print_major "Mark" a2;;10 No record found!- : unit = ()1112 # print_major "Anne" a2;;13 Anne’s major is Computer Science- : unit = ()Mark Hills CS421 Lecture 5b:


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U of I CS 421 - Lecture notes

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