(* 学生ひとり分のデータを表す型 *)
type gakusei_t = {
  namae : string;   (* 名前 *)
  tensuu : int;     (* 点数 *)
  seiseki : string; (* 成績 *)
}

(* 学生データの例 *)
let gakusei1  = {namae="asai"; tensuu=70; seiseki="?"}
let gakusei1' = {namae="asai"; tensuu=70; seiseki="B"}
let gakusei2  = {namae="tanaka"; tensuu=90; seiseki="?"}
let gakusei2' = {namae="tanaka"; tensuu=90; seiseki="A"}
let gakusei3  = {namae="yamada"; tensuu=60; seiseki="?"}
let gakusei3' = {namae="yamada"; tensuu=60; seiseki="C"}
let gakusei4  = {namae="yoshida"; tensuu=80; seiseki="?"}
let gakusei4' = {namae="yoshida"; tensuu=80; seiseki="A"}
let gakusei5  = {namae="kimura"; tensuu=85; seiseki="?"}
let gakusei5' = {namae="kimura"; tensuu=85; seiseki="A"}
let gakusei6  = {namae="kudo"; tensuu=65; seiseki="?"}
let gakusei6' = {namae="kudo"; tensuu=65; seiseki="C"}
let gakusei7  = {namae="ito"; tensuu=75; seiseki="?"}
let gakusei7' = {namae="ito"; tensuu=75; seiseki="B"}

(* 学生リストの例 *)
let gakusei_list  = [gakusei1; gakusei2; gakusei3; gakusei4;
		     gakusei5; gakusei6; gakusei7] (* 成績未定 *)
let gakusei_list' = [gakusei1'; gakusei2'; gakusei3'; gakusei4';
		     gakusei5'; gakusei6'; gakusei7'] (* 成績確定 *)

(* 目的：学生データのリストに成績を入れる *)
(* hyouka_list : gakusei_t list -> gakusei_t list *)
let hyouka_list lst =
  (* 目的：学生データ(ひとり)に成績を入れる *)
  (* hyouka : gakusei_t -> gakusei_t *)
  let hyouka gakusei = match gakusei with
    {namae=n; tensuu=t; seiseki=s} ->
      {namae=n; tensuu=t;
       seiseki = if t >= 80 then "A" else
                 if t >= 70 then "B" else
                 if t >= 60 then "C" else "D"}
  in List.map hyouka lst

(* テスト *)
let test1 = hyouka_list [] = []
let test2 = hyouka_list gakusei_list = gakusei_list'


(* 目的：成績が A の人の数を返す *)
(* count_A : gakusei_t list -> int *)
let count_A lst =
  (* 目的：学生(ひとり)の成績が A かを調べる *)
  (* seiseki_A : gakusei_t -> bool *)
  let seiseki_A gakusei = match gakusei with
    {namae=n; tensuu=t; seiseki=s} ->
      s = "A"
  in List.length (List.filter seiseki_A lst)

(* テスト *)
let test3 = count_A [] = 0
let test4 = count_A gakusei_list' = 3
let test5 = count_A [gakusei2'; gakusei3'; gakusei4'] = 2


(* 目的：学生リストの平均点を出す *)
(* heikin : gakusei_t list -> int *)
let heikin lst =
  (* 目的：学生(ひとり)の点数を goukei に足す *)
  (* add : gakusei_t -> int -> int *)
  let add gakusei goukei = match gakusei with
    {namae=n; tensuu=t; seiseki=s} ->
      t + goukei
  in
  (* 目的：学生の点数の合計を出す *)
  (* add_list : gakusei_t -> int *)
  let add_list lst = List.fold_right add lst 0
  in add_list lst / List.length lst

(* テスト *)
let test6 = heikin gakusei_list' = 75
let test7 = heikin [gakusei2'; gakusei3'; gakusei4'] = 76
let test8 = heikin [gakusei2'; gakusei4'; gakusei5'] = 85


(* 目的：成績が A の人の平均点を出す *)
(* heikin_A : gakusei_t list -> int *)
let heikin_A lst =
  (* 目的：学生(ひとり)の成績が A かを調べる *)
  (* seiseki_A : gakusei_t -> bool *)
  let seiseki_A gakusei = match gakusei with
    {namae=n; tensuu=t; seiseki=s} ->
      s = "A"
  in
  (* 目的：学生(ひとり)の点数を goukei に足す *)
  (* add : gakusei_t -> int -> int *)
  let add gakusei goukei = match gakusei with
    {namae=n; tensuu=t; seiseki=s} ->
      t + goukei
  in
  let lst_A = List.filter seiseki_A lst in
  let goukei = List.fold_right add lst_A 0 in
  let ninzuu = List.length lst_A in
  goukei / ninzuu

(* テスト *)
let test6 = heikin_A gakusei_list' = 85
let test7 = heikin_A [gakusei2'; gakusei3'; gakusei4'] = 85
let test8 = heikin_A [gakusei2'; gakusei4'; gakusei5'] = 85