diff -r 44161f2c3226 -r 3020f8c76baa templates4/postfix.scala
--- a/templates4/postfix.scala	Wed Nov 28 17:13:40 2018 +0000
+++ b/templates4/postfix.scala	Wed Nov 28 23:26:47 2018 +0000
@@ -3,10 +3,10 @@
 // ========================
 
 
-
+// type of tokens
 type Toks = List[String]
 
-// the operations in the simple version
+// the operations in the basic version of the algorithm
 val ops = List("+", "-", "*", "/")
 
 // the precedences of the operators
@@ -21,39 +21,25 @@
 
 // (6) Implement below the shunting yard algorithm. The most
 // convenient way to this in Scala is to implement a recursive 
-// function using pattern matching. The function takes some input 
-// tokens as first argument. The second and third arguments represent 
-// the stack and the output or the shunting yard algorithm.
+// function and to heavily use pattern matching. The function syard 
+// takes some input tokens as first argument. The second and third 
+// arguments represent the stack and the output of the shunting yard 
+// algorithm.
 //
 // In the marking, you can assume the function is called only with 
-// an empty stack and empty output list. You can also assume the
-// input are only properly formated (infix) arithmetic expressions
-// (for example all parentheses are well-nested, the input only contains 
+// an empty stack and an empty output list. You can also assume the
+// input os  only properly formatted (infix) arithmetic expressions
+// (all parentheses will be well-nested, the input only contains 
 // operators and numbers).
 
-// You can implement any helper function you need. I found it helpful 
-// to implement auxiliary functions:  
- 
-def is_op(op: String) : Boolean = ops.contains(op)
-
-def prec(op1: String, op2: String) : Boolean = precs(op1) <= precs(op2)
+// You can implement any additional helper function you need. I found 
+// it helpful to implement two auxiliary functions for the pattern matching:  
+// 
+// def is_op(op: String) : Boolean = ...
+// def prec(op1: String, op2: String) : Boolean = ...
 
 
-def syard(toks: Toks, st: Toks = Nil, out: Toks = Nil) : Toks = (toks, st, out) match {
-  case (Nil, _, _) => out.reverse ::: st
-  case (num::in, st, out) if (num.forall(_.isDigit)) => 
-    syard(in, st, num :: out)
-  case (op1::in, op2::st, out)  if (is_op(op1) && is_op(op2) && prec(op1, op2)) =>
-    syard(op1::in, st, op2 :: out) 
-  case (op1::in, st, out) if (is_op(op1)) => syard(in, op1::st, out)
-  case ("("::in, st, out) => syard(in, "("::st, out)
-  case (")"::in, op2::st, out) =>
-    if (op2 == "(") syard(in, st, out) else syard(")"::in, st, op2 :: out)
-  case (in, st, out) => {
-    println(s"in: ${in}   st: ${st}   out: ${out.reverse}")
-    Nil
-  }  
-} 
+// def syard(toks: Toks, st: Toks = Nil, out: Toks = Nil) : Toks = ...
 
 
 // test cases
@@ -71,23 +57,14 @@
 
  
 // (7) Implement a compute function that evaluates an input list
-// in postfix notation. This function takes an input list of tokens
-// and a stack as argument. The function should produce the 
-// result in form of an integer using the stack. You can assume 
-// this function will be only called with proper postfix expressions.    
+// in postfix notation. This function takes a list of tokens
+// and a stack as argumenta. The function should produce the 
+// result as an integer using the stack. You can assume 
+// this function will be only called with proper postfix 
+// expressions.    
 
-def op_comp(s: String, n1: Int, n2: Int) = s match {
-  case "+" => n2 + n1
-  case "-" => n2 - n1
-  case "*" => n2 * n1
-  case "/" => n2 / n1
-} 
+// def compute(toks: Toks, st: List[Int] = Nil) : Int = ...
 
-def compute(toks: Toks, st: List[Int] = Nil) : Int = (toks, st) match {
-  case (Nil, st) => st.head
-  case (op::in, n1::n2::st) if (is_op(op)) => compute(in, op_comp(op, n1, n2)::st)
-  case (num::in, st) => compute(in, num.toInt::st)  
-}
 
 // test cases
 // compute(syard(split("3 + 4 * ( 2 - 1 )")))  // 7