September 2017 Update

Task/Parsing-Shunting-yard-algorithm/C-sharp/parsing-shunting-yard-algorithm.cs

@@ -0,0 +1,68 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+public class Program
+{
+    public static void Main() {
+        string infix = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3";
+        Console.WriteLine(infix.ToPostfix());
+    }
+}
+
+public static class ShuntingYard
+{
+    private static readonly Dictionary<string, (string symbol, int precedence, bool rightAssociative)> operators
+        = new (string symbol, int precedence, bool rightAssociative) [] {
+            ("^", 4, true),
+            ("*", 3, false),
+            ("/", 3, false),
+            ("+", 2, false),
+            ("-", 2, false)
+    }.ToDictionary(op => op.symbol);
+
+    public static string ToPostfix(this string infix) {
+        string[] tokens = infix.Split(' ');
+        var stack = new Stack<string>();
+        var output = new List<string>();
+        foreach (string token in tokens) {
+            if (int.TryParse(token, out _)) {
+                output.Add(token);
+                Print(token);
+            } else if (operators.TryGetValue(token, out var op1)) {
+                while (stack.Count > 0 && operators.TryGetValue(stack.Peek(), out var op2)) {
+                    int c = op1.precedence.CompareTo(op2.precedence);
+                    if (c < 0 || !op1.rightAssociative && c <= 0) {
+                        output.Add(stack.Pop());
+                    } else {
+                        break;
+                    }
+                }
+                stack.Push(token);
+                Print(token);
+            } else if (token == "(") {
+                stack.Push(token);
+                Print(token);
+            } else if (token == ")") {
+                string top = "";
+                while (stack.Count > 0 && (top = stack.Pop()) != "(") {
+                    output.Add(top);
+                }
+                if (top != "(") throw new ArgumentException("No matching left parenthesis.");
+                Print(token);
+            }
+        }
+        while (stack.Count > 0) {
+            var top = stack.Pop();
+            if (!operators.ContainsKey(top)) throw new ArgumentException("No matching right parenthesis.");
+            output.Add(top);
+        }
+        Print("pop");
+        return string.Join(" ", output);
+
+        //Yikes!
+        void Print(string action) => Console.WriteLine($"{action + ":",-4} {$"stack[ {string.Join(" ", stack.Reverse())} ]",-18} {$"out[ {string.Join(" ", output)} ]"}");
+        //A little more readable?
+        void Print(string action) => Console.WriteLine("{0,-4} {1,-18} {2}", action + ":", $"stack[ {string.Join(" ", stack.Reverse())} ]", $"out[ {string.Join(" ", output)} ]");
+    }
+}