diff --git a/.gitignore b/.gitignore
index 478d91f..0021882 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,6 @@
 *.cmo
 *.cmi
 *.zip
+*.native
+*/_build/*
 
diff --git a/lecture05/cat.ml b/lecture05/cat.ml
new file mode 100644
index 0000000..614d4c9
--- /dev/null
+++ b/lecture05/cat.ml
@@ -0,0 +1,26 @@
+(* display content of file read via stdin *)
+let rec cat () =
+  try
+    let line = read_line () in
+    print_endline line;
+    cat()
+  with
+  | _ -> ();; (* EOF return unit *)
+
+(* similar to main function *)
+let () =
+  let rec aux () =
+    try
+      print_endline @@ read_line ();
+      aux ()
+      with
+      | _ -> ()
+  in
+  aux()
+  (* cat() *)
+
+(**** build and test ****)
+(*
+  ocamlbuild cat.native
+  ./cat.native < filename
+*)
diff --git a/lecture05/data.txt b/lecture05/data.txt
new file mode 100644
index 0000000..9f57138
--- /dev/null
+++ b/lecture05/data.txt
@@ -0,0 +1,4 @@
+123 456
+-123 -456
+abc
+25
diff --git a/lecture05/echo.ml b/lecture05/echo.ml
new file mode 100644
index 0000000..b2774fd
--- /dev/null
+++ b/lecture05/echo.ml
@@ -0,0 +1,6 @@
+let () =
+  let len = Array.length Sys.argv - 1 in
+  for i = 1 to len do
+    Printf.printf (if i <> len then "%s " else "%s\n") Sys.argv.(i)
+  done;;
+
diff --git a/lecture05/notes.ml b/lecture05/notes.ml
new file mode 100644
index 0000000..bdf4153
--- /dev/null
+++ b/lecture05/notes.ml
@@ -0,0 +1,161 @@
+(**** RECORDS ****)
+type student = {id: string; name: string; gpa: float};;
+
+let s1 = {id = "a12345678"; name = "homer simpson"; gpa = 25.5};;
+let make_student id name gpa = { id = id; name = name; gpa = gpa };;
+let s2 = make_student "a000000" "monty burns" 99.9;;
+
+(* defining a function to return the name field of a record *)
+let name {id = id; name = name; gpa = gpa} = name;;
+let name {id; name; gpa} = name;;
+let name {name} = name;;
+let name s = s.name;;
+
+name s2;;
+
+type instructor = {id: string; name: string; salary: float};;
+let i1 = {id = "a777777"; name = "lisa simpson"; salary = 100000.};;
+
+(* name cannot be applied to constructor because name was defined before instructor
+   and it deferred from student *)
+(* name i1;; *)
+
+let id x = x.id;;
+
+(* id s1;; <-- this doesnt work because id infers from the latest defined, such as instructor *)
+id i1;;
+
+type isntr = Instructor of {id: string; name: string; salary: float};;
+let i1 = Instructor {id = "a777777"; name = "lisa simpson"; salary = 100000.};;
+
+let s1 = make_student "a123456678" "homer simpson" 25.5;;
+let s2 = s1;; (* s2 is its own student it just shares the same data *)
+let s2 = {s1 with id = "a22222222"; name = "bart simpson"} (* this allows us to copy data and modify selected data *)
+
+(**** MUTABILITY - you can mark fields as mutable ****)
+type student = {id: string; name: string; mutable gpa: float};;
+let make_student id name gpa = { id = id; name = name; gpa = gpa };;
+let s1 = make_student "a123456678" "homer simpson" 25.5;;
+let s2 = s1;; (* refer to the same thing *)
+
+s2;;
+
+(* update field for s1 *)
+s1.gpa <- 15.5;
+
+(* shows changes applied to s1 since s2 refers to the same data *)
+s2;;
+
+(**** REFERENCES & DEREFEREMCES ****)
+let x = ref 1;;
+!x;;     (* deref *)
+x := 2;; (* update val of references *)
+x;;
+!x;;
+
+let y = x;;
+y;;
+x;;
+incr x;; (* y is the same as x (ref to int) so updating x also shows in y *)
+x;;
+y;;
+
+let x = ref (Some 1);;
+let y = ref None;;
+(* val y : '_wek1 option ref = {contents = None} *)
+(* curremtly the contents of y are weakly polymorphic
+  because it can be any type. the type inference cannot
+  deduce the value of the optional *)
+y := Some 2;;
+y;;
+(* - : int option ref = {contents = Some 1} *)
+
+(**** ARRAYS - arrays are mutable ****)
+[|1;2;3;3;4;5|];; (* arrays have vertical bars unlike lists *)
+let a = [|1;2;3|];;
+a.(0);; (* get first element *);;
+a.(1);;
+a.(2);;
+(* a.(3);; (* Invalid_argument excetption *) *)
+
+a.(0) <- -1;;
+a;;
+
+(**** FOR / WHILE LOOPS ****)
+(* for loops on arrays... yikes *)
+Array.length a;;
+for i = 0 to Array.length a - 1 do
+  a.(i) <- 2 * a.(i)
+done;;
+
+a;;
+
+(* you can go in reverse also using downto *)
+for i = Array.length a - 1 downto 0 do
+  a.(i) <- 2 * a.(i)
+done;;
+
+(* while loops also... *)
+let i = ref 0;;
+while !i < Array.length a do
+  (* adding semi-colon to end of expression means
+     this is a garbage value, throw it away so we
+     dont return unit() type early *)
+  a.(!i) <- - !i;
+  incr i
+done;;
+
+a;;
+
+(**** IGNORE ****)
+(* x will be 2 and the expression 1;
+   is ignored because of the semi-colon *)
+let x = ignore 1; 2;;
+
+(**** EXCEPTIONS ****)
+(* defining exceptions and rasing *)
+(* exception is a variant type and the
+   number of variants can be extended *)
+(* exception Hell;; *)
+(* raise Hell;;
+Failure "hell";; *)
+
+(* exceptions can have parameters *)
+exception Hell of int;;
+Hell 1;;
+Hell 2;;
+
+(* List.hd [];; (* raises an exception *) *)
+
+(* you can pattern match exceptions with try-with *)
+try
+  List.hd []
+with
+| Failure _ -> -1;;
+
+try
+  List.hd []
+with
+| _ -> -1;;
+
+
+(**** IO ****)
+print_string "hello world\n";;
+print_endline "hello world";;
+print_int 123;;
+print_float 123.4;;
+print_newline ();;
+Printf.printf "%5d\n" 123;;
+Printf.printf "%05d %s\n" 123 "hello";;
+Printf.eprintf "%05d %s\n" 123 "hello";; (* print to standard error channel *)
+
+let x = read_line ();;
+
+(* read in int - add space at the front, it matches any amount of leading whitespace *)
+Scanf.scanf " %d" (fun x -> x)
+
+(* open file *)
+(* open_in "filename";; *)
+let ic = open_in "output";;
+input_line ic;;
+
diff --git a/lecture05/number_file_lines.ml b/lecture05/number_file_lines.ml
new file mode 100644
index 0000000..477043c
--- /dev/null
+++ b/lecture05/number_file_lines.ml
@@ -0,0 +1,16 @@
+let rec number acc ic =
+  try
+    Printf.printf "%5d| %s\n" acc @@ input_line ic;
+    number (acc + 1) ic
+  with
+  | _ -> if ic <> stdin then close_in ic else ()
+
+let () =
+  if Array.length Sys.argv = 1 then
+    number 1 stdin
+  else
+    try
+      number 1 @@ open_in Sys.argv.(1)
+    with
+    | Sys_error s -> Printf.eprintf "%s\n" s;;
+
diff --git a/lecture05/number_lines.ml b/lecture05/number_lines.ml
new file mode 100644
index 0000000..4028e41
--- /dev/null
+++ b/lecture05/number_lines.ml
@@ -0,0 +1,9 @@
+let () =
+  let rec aux acc =
+    try
+      Printf.printf "%5d| %s\n" acc @@ read_line ();
+      aux (acc + 1)
+    with
+    | _ -> ()
+  in
+  aux 1;;
diff --git a/lecture05/output b/lecture05/output
new file mode 100644
index 0000000..3b18e51
--- /dev/null
+++ b/lecture05/output
@@ -0,0 +1 @@
+hello world
diff --git a/lecture05/ref.ml b/lecture05/ref.ml
new file mode 100644
index 0000000..d3b62cf
--- /dev/null
+++ b/lecture05/ref.ml
@@ -0,0 +1,12 @@
+(* we can impliment our own ref type *)
+type 'a ref = { mutable contents: 'a };;
+
+(* reference and dereference implimentation *)
+let ref x = { contents = x };;
+let (!) r = r.contents;;
+
+(* assign to a mutable field *)
+let (:=) r x = r.contents <- x;;
+
+let incr r = r.contents <- r.contents + 1;;
+let decr r = r.contents <- r.contents - 1;;
diff --git a/lecture05/sum_integers.ml b/lecture05/sum_integers.ml
new file mode 100644
index 0000000..d9b5fac
--- /dev/null
+++ b/lecture05/sum_integers.ml
@@ -0,0 +1,12 @@
+let () =
+  let rec aux sum =
+    try
+      aux @@ Scanf.scanf " %d" (fun x -> sum + x)
+    with
+    | Scanf.Scan_failure _ ->
+      Scanf.scanf " %s" (fun _ -> ());
+      aux sum
+    | End_of_file -> sum
+  in
+  Printf.printf "%d\n" @@ aux 0;;
+