Programming Languages, Part A, Week 2, hw1

hw1.sml

(* 1 *)
fun is_older(d1: (int*int*int), d2: (int*int*int)) =
    if (#1 d1) < (#1 d2)
    then true
    else if (#1 d1) > (#1 d2)
    then false
    else if (#2 d1) < (#2 d2)
    then true
    else if (#2 d1) > (#2 d2)
    then false
    else if (#3 d1) < (#3 d2)
    then true
    else false

(* 2 *)
fun number_in_month(ds: (int*int*int) list, m: int) =
    if null ds
    then 0
    else
	let
	    val d = hd ds
	in
	    if (#2 d) = m
	    then 1+number_in_month(tl ds, m)
	    else number_in_month(tl ds, m)
	end

(* 3 *)
fun number_in_months(ds: (int*int*int) list, ms: int list) =
    if null ms
    then 0
    else
	let
	    val m = hd ms
	in
	    number_in_month(ds, m) + number_in_months(ds, tl ms)
	end

(* 4 *)
fun dates_in_month(ds: (int*int*int) list, m: int) =
    if null ds
    then []
    else
	if #2 (hd ds) = m
	then
	    (hd ds)::dates_in_month((tl ds),m)
	else
	    dates_in_month((tl ds),m)
(* 5 *)
fun dates_in_months(ds: (int*int*int) list, ms: int list) =
    if null ms
    then []
    else
	dates_in_month(ds, (hd ms)) @ dates_in_months(ds, (tl ms))

(* 6 *)
fun get_nth(ss: string list, n: int) =
    if null ss
    then ""
    else if n = 1
    then hd ss
    else get_nth((tl ss),n-1)
    
(* 7 *)
fun date_to_string(d:(int*int*int)) =

    let
	val months = ["January", "February", "March", "April",
		      "May", "June", "July", "August", "September", "October", "November", "December"]
    in
	get_nth(months, (#2 d)) ^ " " ^ Int.toString (#3 d) ^ ", " ^ Int.toString (#1 d)
    end

(* 8 *)
fun number_before_reaching_sum(sum: int, ns:int list) =
    if sum <= 0
    then ~1
    else
	1+number_before_reaching_sum(sum-(hd ns), (tl ns))
	
fun number_before_reaching_sum(sum: int, ns:int list) =
    if sum <= 0
    then ~1
    else
	1+number_before_reaching_sum(sum-(hd ns), (tl ns))

fun what_month(d: int) =
    let
	val mnts = [31,28,31,30,31,30,31,31,30,31,30,31]
    in
	1+number_before_reaching_sum(d, mnts)
    end
fun month_range(day1:int, day2:int) =
    if day1 > day2
    then []
    else
	what_month(day1)::month_range(day1+1, day2)

fun oldest(ds: (int*int*int) list) =
    if null ds
    then NONE
    else
	let
	    val d1 = hd ds
	    val d2 = oldest((tl ds))
	in
	    if isSome d2 andalso is_older((valOf d2), d1)
	    then d2
	    else SOME(d1)
	end