programming languanges (coursera) week-1 homework.

test.sml

(*------------------------  WEEK 1  ------------------------------------*)

(* 1 *)
fun is_older (fst: int*int*int, snd: int*int*int) =
    (#1 fst) * 10000 + (#2 fst) * 100 + #3 fst <
    (#1 snd) * 10000 + (#2 snd) * 100 + #3 snd

(* 2 *)
fun number_in_month (dates: (int*int*int) list, month: int) =
    if null dates then 0
    else if #2 (hd dates) = month then 1 + number_in_month (tl dates, month)
    else number_in_month (tl dates, month);

(* 3 *)
fun number_in_months (dates: (int*int*int) list, months: int list) =
    if null months then 0
    else number_in_month (dates, hd months) +
         number_in_months (dates, tl months);

(* 4 *)
fun dates_in_month (dates: (int*int*int) list, month) =
    if null dates then []
    else
        let
            val date = hd dates
        in
            if #2 date = month then date:: dates_in_month (tl dates, month)
            else dates_in_month (tl dates, month)
        end;

(* 5 *)
fun dates_in_months (dates: (int*int*int) list, months: int list) =
    if null months then []
    else dates_in_month (dates, hd months) @
         dates_in_months (dates, tl months);

(* 6 *)
fun get_nth (strings: string list, n: int) =
    if n = 1 then hd strings
    else get_nth (tl strings, n-1);

(* 7 *)
fun date_to_string (date: int*int*int) =
    let
        val months = ["January", "February", "March", "April", "May",
                      "June", "July", "August", "September", "October",
                      "November", "December"]
    in
        get_nth(months, #2 date) ^ " " ^
        Int.toString (#3 date) ^ ", " ^
        Int.toString (#1 date)
    end;

(* 8 *)
fun number_before_reaching_sum (sum: int, numbers: int list) =
    let
        fun helper (total: int, index: int, numbers: int list) =
            if total >= sum then index - 1
            else helper (total + (hd numbers), index + 1, tl numbers)
    in
        helper (hd numbers, 1, tl numbers)
    end;

(* 9 *)
fun what_month (day: int) =
    1 + number_before_reaching_sum (day, [31, 28, 31, 30, 31, 30,
                                          31, 31, 30, 31, 30, 31]);

(* 10 *)
fun month_range (day1: int, day2: int) =
    if day1 > day2 then []
    else what_month day1 :: month_range (day1+1, day2);

(* 11 *)
fun oldest (dates: (int*int*int) list) =
    if null dates then NONE
    else
        let
            fun helper (date: (int*int*int), dates: (int*int*int) list) =
                if null dates then date
                else if is_older (date, hd dates) then helper (date, tl dates)
                else helper (hd dates, tl dates);
        in
            SOME (helper (hd dates, tl dates))
        end;

(* 12 *)
fun number_in_months_challenge (dates: (int*int*int) list, months: int list) =
    if null months then 0
    else
        let
            fun is_in (item: int, alist: int list) =
                if null alist then false
                else if item = hd alist then true
                else is_in (item, alist);

            fun remove_duplicates (alist: int list) =
                if null alist then alist
                else if is_in (hd alist, tl alist) then
                    remove_duplicates (tl alist)
                else
                    hd alist :: remove_duplicates (tl alist)

            val months = remove_duplicates months
        in
            number_in_months (dates, months)
        end;