diff -r e48ea8300b2d -r 9471c3b7ea02 core_solution3/postfix.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/core_solution3/postfix.scala	Mon Nov 08 01:03:09 2021 +0000
@@ -0,0 +1,103 @@
+// Shunting Yard Algorithm
+// by Edsger Dijkstra
+// ========================
+
+object C3a {
+
+type Toks = List[String]
+
+// the operations in the simple version
+val ops = List("+", "-", "*", "/")
+
+// the precedences of the operators
+val precs = Map("+" -> 1,
+		"-" -> 1,
+		"*" -> 2,
+		"/" -> 2)
+
+// helper function for splitting strings into tokens
+def split(s: String) : Toks = s.split(" ").toList
+
+// (6) Implement below the shunting yard algorithm. The most
+// convenient way to this in Scala is to implement a recursive 
+// 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 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 additional helper function you need. I found 
+// it helpful to implement two auxiliary functions for the pattern matching:  
+// 
+ 
+def is_op(op: String) : Boolean = ops.contains(op)
+
+def prec(op1: String, op2: String) : Boolean = precs(op1) <= precs(op2)
+
+
+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
+  }  
+} 
+
+
+// test cases
+//syard(split("3 + 4 * ( 2 - 1 )"))  // 3 4 2 1 - * +
+//syard(split("10 + 12 * 33"))       // 10 12 33 * +
+//syard(split("( 5 + 7 ) * 2"))      // 5 7 + 2 *
+//syard(split("5 + 7 / 2"))          // 5 7 2 / +
+//syard(split("5 * 7 / 2"))          // 5 7 * 2 /
+//syard(split("9 + 24 / ( 7 - 3 )")) // 9 24 7 3 - / +
+
+//syard(split("3 + 4 + 5"))           // 3 4 + 5 +
+//syard(split("( ( 3 + 4 ) + 5 )"))    // 3 4 + 5 +
+//syard(split("( 3 + ( 4 + 5 ) )"))    // 3 4 5 + +
+//syard(split("( ( ( 3 ) ) + ( ( 4 + ( 5 ) ) ) )")) // 3 4 5 + +
+
+// (7) Implement a compute function that evaluates an input list
+// 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 = (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
+// compute(syard(split("10 + 12 * 33")))       // 406
+// compute(syard(split("( 5 + 7 ) * 2")))      // 24
+// compute(syard(split("5 + 7 / 2")))          // 8
+// compute(syard(split("5 * 7 / 2")))          // 17
+// compute(syard(split("9 + 24 / ( 7 - 3 )"))) // 15
+
+}
+
+