Index: trunk/sources/HeuristicLab.Problems.DataAnalysis.Symbolic/3.4/Formatters/InfixExpressionFormatter.cs
===================================================================
--- trunk/sources/HeuristicLab.Problems.DataAnalysis.Symbolic/3.4/Formatters/InfixExpressionFormatter.cs	(revision 14024)
+++ trunk/sources/HeuristicLab.Problems.DataAnalysis.Symbolic/3.4/Formatters/InfixExpressionFormatter.cs	(revision 14024)
@@ -0,0 +1,135 @@
+#region License Information
+/* HeuristicLab
+ * Copyright (C) 2002-2016 Heuristic and Evolutionary Algorithms Laboratory (HEAL)
+ *
+ * This file is part of HeuristicLab.
+ *
+ * HeuristicLab is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * HeuristicLab is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with HeuristicLab. If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using HeuristicLab.Common;
+using HeuristicLab.Core;
+using HeuristicLab.Encodings.SymbolicExpressionTreeEncoding;
+using HeuristicLab.Persistence.Default.CompositeSerializers.Storable;
+
+namespace HeuristicLab.Problems.DataAnalysis.Symbolic {
+  /// <summary>
+  /// Formats mathematical expressions in infix form. E.g. x1 * (3.0 * x2 + x3)
+  /// </summary>
+  [StorableClass]
+  [Item("Infix Symbolic Expression Tree Formatter", "A string formatter that converts symbolic expression trees to infix expressions.")]
+
+  public class InfixExpressionFormatter : NamedItem, ISymbolicExpressionTreeStringFormatter {
+
+
+    [StorableConstructor]
+    private InfixExpressionFormatter(bool deserializing) : base(deserializing) { }
+    private InfixExpressionFormatter(InfixExpressionFormatter original, Cloner cloner) : base(original, cloner) { }
+    public InfixExpressionFormatter()
+      : base() {
+      Name = ItemName;
+      Description = ItemDescription;
+    }
+    public override IDeepCloneable Clone(Cloner cloner) {
+      return new InfixExpressionFormatter(this, cloner);
+    }
+
+    public string Format(ISymbolicExpressionTree symbolicExpressionTree) {
+      // skip root and start symbols
+      StringBuilder strBuilder = new StringBuilder();
+      FormatRecursively(symbolicExpressionTree.Root.GetSubtree(0).GetSubtree(0), strBuilder);
+      return strBuilder.ToString();
+    }
+
+    private void FormatRecursively(ISymbolicExpressionTreeNode node, StringBuilder strBuilder) {
+      if (node.SubtreeCount > 1) {
+        var token = GetToken(node.Symbol);
+        if (token == "+" || token == "-" || token == "OR" || token == "XOR") {
+          strBuilder.Append("(");
+          FormatRecursively(node.Subtrees.First(), strBuilder);
+
+          foreach (var subtree in node.Subtrees.Skip(1)) {
+            strBuilder.Append(" ").Append(token).Append(" ");
+            FormatRecursively(subtree, strBuilder);
+          }
+          strBuilder.Append(")");
+
+        } else if (token == "*" || token == "/" || token == "AND") {
+          strBuilder.Append("(");
+          FormatRecursively(node.Subtrees.First(), strBuilder);
+
+          foreach (var subtree in node.Subtrees.Skip(1)) {
+            strBuilder.Append(" ").Append(token).Append(" ");
+            FormatRecursively(subtree, strBuilder);
+          }
+          strBuilder.Append(")");
+        }
+      } else if (node.SubtreeCount == 1) {
+        var token = GetToken(node.Symbol);
+        if (token == "-" || token == "NOT") {
+          strBuilder.Append("(").Append(token).Append("(");
+          FormatRecursively(node.GetSubtree(0), strBuilder);
+          strBuilder.Append("))");
+        } else if (token == "/") {
+          strBuilder.Append("1/");
+          FormatRecursively(node.GetSubtree(0), strBuilder);
+        } else if (token == "+" || token == "*") {
+          FormatRecursively(node.GetSubtree(0), strBuilder);
+        } else {
+          // function
+          strBuilder.Append(token).Append("(");
+          FormatRecursively(node.GetSubtree(0), strBuilder);
+          strBuilder.Append(")");
+        }
+      } else {
+        // no subtrees
+        if (node.Symbol is Variable) {
+          var varNode = node as VariableTreeNode;
+          if (!varNode.Weight.IsAlmost(1.0)) {
+            strBuilder.Append("(");
+            strBuilder.AppendFormat(CultureInfo.InvariantCulture, "{0}", varNode.Weight);
+            strBuilder.Append("*");
+          }
+          if (varNode.VariableName.Contains("'")) {
+            strBuilder.AppendFormat("\"{0}\"", varNode.VariableName);
+          } else {
+            strBuilder.AppendFormat("'{0}'", varNode.VariableName);
+          }
+          if (!varNode.Weight.IsAlmost(1.0)) {
+            strBuilder.Append(")");
+          }
+        } else if (node.Symbol is Constant) {
+          var constNode = node as ConstantTreeNode;
+          if (constNode.Value >= 0.0)
+            strBuilder.AppendFormat(CultureInfo.InvariantCulture, "{0}", constNode.Value);
+          else
+            strBuilder.AppendFormat(CultureInfo.InvariantCulture, "({0})", constNode.Value);     // (-1)
+        }
+      }
+    }
+
+    private string GetToken(ISymbol symbol) {
+      var tok = InfixExpressionParser.knownSymbols.GetBySecond(symbol).SingleOrDefault();
+      if (tok == null)
+        throw new ArgumentException(string.Format("Unknown symbol {0} found.", symbol.Name));
+      return tok;
+    }
+  }
+}
