diff --git a/src/com/bytezone/diskbrowser/visicalc/Abs.java b/src/com/bytezone/diskbrowser/visicalc/Abs.java
index 6a1d407..9aa8a08 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Abs.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Abs.java
@@ -2,33 +2,14 @@ package com.bytezone.diskbrowser.visicalc;
 
 public class Abs extends Function
 {
-  private boolean hasChecked;
-  private double value = 0;
-
   Abs (Sheet parent, String text)
   {
     super (parent, text);
   }
 
   @Override
-  public boolean hasValue ()
+  public void calculate ()
   {
-    if (!hasChecked)
-      calculate ();
-    return hasValue;
-  }
-
-  @Override
-  public double getValue ()
-  {
-    return hasValue () ? value : 0;
-  }
-
-  private void calculate ()
-  {
-    hasChecked = true;
-    hasValue = true;
-
     Expression exp = new Expression (parent, functionText);
     value = Math.abs (exp.getValue ());
   }
diff --git a/src/com/bytezone/diskbrowser/visicalc/And.java b/src/com/bytezone/diskbrowser/visicalc/And.java
index 57d0d64..e95b21a 100644
--- a/src/com/bytezone/diskbrowser/visicalc/And.java
+++ b/src/com/bytezone/diskbrowser/visicalc/And.java
@@ -9,8 +9,8 @@ class And extends Function
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
-    return 0;
+    value = 0;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Cell.java b/src/com/bytezone/diskbrowser/visicalc/Cell.java
index e223ffe..78dea82 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Cell.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Cell.java
@@ -28,6 +28,7 @@ class Cell implements Comparable<Cell>, Value
   {
     this.parent = parent;
     this.address = address;
+    type = CellType.VALUE;            // default to VALUE, formatting may change it
   }
 
   public boolean isValue ()
@@ -110,8 +111,8 @@ class Cell implements Comparable<Cell>, Value
   String getText (int colWidth, char defaultFormat)
   {
     // cell may have been created when formatted but no type set
-    if (type == null)
-      return justify ("", colWidth);
+    //    if (type == null)
+    //      return justify ("", colWidth);
 
     switch (type)
     {
@@ -122,47 +123,43 @@ class Cell implements Comparable<Cell>, Value
         return justify (repeat, colWidth);
 
       case VALUE:
-        if (hasValue ())
-        {
-          Double value = getValue ();
-          if (Double.isNaN (value))
-            return justify ("", colWidth);
+        if (value.isError () || value.isNaN ())
+          return justify (value.getText (), colWidth);
 
-          char format = cellFormat != ' ' ? cellFormat : defaultFormat;
-          if (format == 'I')
-          {
-            String integerFormat = String.format ("%%%d.0f", colWidth);
-            return String.format (integerFormat, value);
-          }
-          else if (format == '$')
-          {
-            String currencyFormat = String.format ("%%%d.%ds", colWidth, colWidth);
-            return String.format (currencyFormat, nf.format (value));
-          }
-          else if (format == '*')
-          {
-            String graphFormat = String.format ("%%-%d.%ds", colWidth, colWidth);
-            // this is not finished
-            return String.format (graphFormat, "********************");
-          }
-          else
-          {
-            // this could be improved
-            String numberFormat = String.format ("%%%d.3f", colWidth + 4);
-            String val = String.format (numberFormat, value);
-            while (val.endsWith ("0"))
-              val = ' ' + val.substring (0, val.length () - 1);
-            if (val.endsWith ("."))
-              val = ' ' + val.substring (0, val.length () - 1);
-            if (val.length () > colWidth)
-              val = val.substring (val.length () - colWidth);
-            return val;
-          }
+        Double thisValue = value.getValue ();
+
+        char format = cellFormat != ' ' ? cellFormat : defaultFormat;
+        if (format == 'I')
+        {
+          String integerFormat = String.format ("%%%d.0f", colWidth);
+          return String.format (integerFormat, thisValue);
+        }
+        else if (format == '$')
+        {
+          String currencyFormat = String.format ("%%%d.%ds", colWidth, colWidth);
+          return String.format (currencyFormat, nf.format (thisValue));
+        }
+        else if (format == '*')
+        {
+          String graphFormat = String.format ("%%-%d.%ds", colWidth, colWidth);
+          // this is not finished
+          return String.format (graphFormat, "********************");
+        }
+        else
+        {
+          // this could be improved
+          String numberFormat = String.format ("%%%d.3f", colWidth + 4);
+          String val = String.format (numberFormat, thisValue);
+          while (val.endsWith ("0"))
+            val = ' ' + val.substring (0, val.length () - 1);
+          if (val.endsWith ("."))
+            val = ' ' + val.substring (0, val.length () - 1);
+          if (val.length () > colWidth)
+            val = val.substring (val.length () - colWidth);
+          return val;
         }
-        //        else
-        //          return justify ("", colWidth);
     }
-    return getError ();
+    return getText ();
   }
 
   private String justify (String text, int colWidth)
@@ -177,40 +174,48 @@ class Cell implements Comparable<Cell>, Value
     return (String.format (labelFormat, text));
   }
 
-  @Override
-  public boolean hasValue ()
-  {
-    if (type == CellType.VALUE)
-      return value.hasValue ();
-    return false;
-  }
-
-  // this should be called when doing calculations
   @Override
   public double getValue ()
   {
-    if (type != CellType.VALUE)
-      return 0;
-
+    assert type == CellType.VALUE;
     return value.getValue ();
   }
 
   @Override
-  public String getError ()
+  public String getText ()
   {
-    return value.getError ();
+    assert type == CellType.VALUE;
+    return value.getText ();
   }
 
-  void calculate ()
+  @Override
+  public boolean isError ()
   {
+    assert type == CellType.VALUE;
+    return value.isError ();
+  }
+
+  @Override
+  public boolean isNaN ()
+  {
+    assert type == CellType.VALUE;
+    return value.isNaN ();
+  }
+
+  @Override
+  public void calculate ()
+  {
+    assert type == CellType.VALUE;
     if (expressionText == null)
     {
       System.out.printf ("%s null expression text %n", address);
-      value = Function.getInstance (parent, "@ERROR()");
+      value = Function.getInstance (parent, "@ERROR");
     }
     else
-      // could use Number or Cell for simple Values
+      // should use Number or Cell for simple Values
       value = new Expression (parent, expressionText);
+
+    value.calculate ();
   }
 
   @Override
diff --git a/src/com/bytezone/diskbrowser/visicalc/Count.java b/src/com/bytezone/diskbrowser/visicalc/Count.java
index 405f711..43ff191 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Count.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Count.java
@@ -3,8 +3,6 @@ package com.bytezone.diskbrowser.visicalc;
 class Count extends Function
 {
   private final Range range;
-  private boolean hasChecked;
-  private double count = 0;
 
   public Count (Sheet parent, String text)
   {
@@ -13,33 +11,20 @@ class Count extends Function
   }
 
   @Override
-  public boolean hasValue ()
+  public void calculate ()
   {
-    if (!hasChecked)
-      calculate ();
-    return hasValue;
-  }
-
-  @Override
-  public double getValue ()
-  {
-    return hasValue () ? count : 0;
-  }
-
-  private void calculate ()
-  {
-    hasChecked = true;
-    hasValue = false;
-
+    value = 0;
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell != null && cell.hasValue ())
+      if (cell == null || cell.isError () || cell.isNaN ())
       {
-        hasValue = true;
-        if (cell.getValue () != 0.0)
-          count++;
+        isError = true;
+        break;
       }
+
+      if (cell.getValue () != 0.0)
+        value++;
     }
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Error.java b/src/com/bytezone/diskbrowser/visicalc/Error.java
index 922a794..de74c9d 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Error.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Error.java
@@ -8,15 +8,15 @@ class Error extends Function
   }
 
   @Override
-  public boolean hasValue ()
+  public boolean isError ()
   {
-    return false;
+    return true;
   }
 
   @Override
-  public String getError ()
+  public boolean isNaN ()
   {
-    return "@Error";
+    return true;
   }
 
   @Override
@@ -24,4 +24,9 @@ class Error extends Function
   {
     return 0;
   }
+
+  @Override
+  public void calculate ()
+  {
+  }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Expression.java b/src/com/bytezone/diskbrowser/visicalc/Expression.java
index 2b7f5e8..780e023 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Expression.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Expression.java
@@ -32,7 +32,8 @@ class Expression implements Value
   private final List<String> operators = new ArrayList<String> ();
   private final List<String> signs = new ArrayList<String> ();
 
-  private boolean hasValue;
+  protected boolean isError;
+  private double value;
 
   public Expression (Sheet parent, String text)
   {
@@ -112,14 +113,13 @@ class Expression implements Value
     }
 
     assert values.size () > 0;
-    hasValue = true;
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
     Value thisValue = values.get (0);
-    double value = thisValue == null ? 0 : values.get (0).getValue ();
+    value = thisValue == null ? 0 : values.get (0).getValue ();
 
     String sign = signs.get (0);
     if (sign.equals ("(-)"))
@@ -146,21 +146,36 @@ class Expression implements Value
       else if (operator.equals ("^"))
         value = Math.pow (value, nextValue);
     }
+  }
+
+  @Override
+  public boolean isNaN ()
+  {
+    return Double.isNaN (value);
+  }
+
+  @Override
+  public String getText ()
+  {
+    if (isNaN ())
+      return "NaN";
+    if (isError ())
+      return "Error";
+    return "";
+  }
+
+  @Override
+  public boolean isError ()
+  {
+    return isError;
+  }
+
+  @Override
+  public double getValue ()
+  {
     return value;
   }
 
-  @Override
-  public boolean hasValue ()
-  {
-    return hasValue;
-  }
-
-  @Override
-  public String getError ()
-  {
-    return hasValue ? "" : "Error";
-  }
-
   private String checkBrackets (String input)
   {
     String line = input.trim ();
diff --git a/src/com/bytezone/diskbrowser/visicalc/Function.java b/src/com/bytezone/diskbrowser/visicalc/Function.java
index 6e676c0..e3ad8c0 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Function.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Function.java
@@ -32,7 +32,9 @@ abstract class Function implements Value
 
   protected final Sheet parent;
   protected String functionText;
-  protected boolean hasValue;
+
+  protected boolean isError;
+  protected double value;
 
   static Function getInstance (Sheet parent, String text)
   {
@@ -95,19 +97,37 @@ abstract class Function implements Value
     // get function's parameter string
     int pos = text.indexOf ('(');
     if (pos >= 0)
-      this.functionText = text.substring (pos + 1, text.length () - 1);
+      functionText = text.substring (pos + 1, text.length () - 1);
+    else
+      functionText = "";
   }
 
   @Override
-  public boolean hasValue ()
+  public boolean isError ()
   {
-    return hasValue;
+    return isError;
   }
 
   @Override
-  public String getError ()
+  public boolean isNaN ()
   {
-    return hasValue ? "" : "Error";
+    return Double.isNaN (value);
+  }
+
+  @Override
+  public double getValue ()
+  {
+    return value;
+  }
+
+  @Override
+  public String getText ()
+  {
+    if (isNaN ())
+      return "NaN";
+    if (isError ())
+      return "Error";
+    return "";
   }
 
   protected Range getRange (String text)
diff --git a/src/com/bytezone/diskbrowser/visicalc/If.java b/src/com/bytezone/diskbrowser/visicalc/If.java
index c43b4b8..6121900 100644
--- a/src/com/bytezone/diskbrowser/visicalc/If.java
+++ b/src/com/bytezone/diskbrowser/visicalc/If.java
@@ -15,25 +15,27 @@ class If extends Function
 
     int pos1 = functionText.indexOf (',');
     int pos2 = functionText.indexOf (',', pos1 + 1);
+
     condition = new Condition (parent, functionText.substring (0, pos1));
+
     textTrue = functionText.substring (pos1 + 1, pos2);
     textFalse = functionText.substring (pos2 + 1);
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
     if (condition.getResult ())
     {
       if (expTrue == null)
         expTrue = new Expression (parent, textTrue);
-      return expTrue.getValue ();
+      value = expTrue.getValue ();
     }
     else
     {
       if (expFalse == null)
         expFalse = new Expression (parent, textFalse);
-      return expFalse.getValue ();
+      value = expFalse.getValue ();
     }
   }
 
diff --git a/src/com/bytezone/diskbrowser/visicalc/Int.java b/src/com/bytezone/diskbrowser/visicalc/Int.java
index bb47ecb..3e5dcf0 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Int.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Int.java
@@ -9,8 +9,9 @@ public class Int extends Function
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
-    return 0;
+    Expression exp = new Expression (parent, functionText);
+    value = (int) exp.getValue ();
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/IsError.java b/src/com/bytezone/diskbrowser/visicalc/IsError.java
index 8077342..9f5ec12 100644
--- a/src/com/bytezone/diskbrowser/visicalc/IsError.java
+++ b/src/com/bytezone/diskbrowser/visicalc/IsError.java
@@ -2,22 +2,18 @@ package com.bytezone.diskbrowser.visicalc;
 
 class IsError extends Function
 {
-  boolean firstTime = true;
-  Cell cell;
+  Expression expression;
 
   public IsError (Sheet parent, String text)
   {
     super (parent, text);
+    expression = new Expression (parent, functionText);
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
-    if (firstTime)
-    {
-      firstTime = false;
-      cell = parent.getCell (new Address (functionText));
-    }
-    return cell == null ? 1 : 0;
+    value = expression.getValue ();
+    isError = expression.isError ();
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/IsNa.java b/src/com/bytezone/diskbrowser/visicalc/IsNa.java
index cff1910..30dc907 100644
--- a/src/com/bytezone/diskbrowser/visicalc/IsNa.java
+++ b/src/com/bytezone/diskbrowser/visicalc/IsNa.java
@@ -2,15 +2,18 @@ package com.bytezone.diskbrowser.visicalc;
 
 public class IsNa extends Function
 {
+  Expression expression;
 
   IsNa (Sheet parent, String text)
   {
     super (parent, text);
+    expression = new Expression (parent, functionText);
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
-    return 0;
+    value = expression.getValue ();
+    isError = expression.isError ();
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Lookup.java b/src/com/bytezone/diskbrowser/visicalc/Lookup.java
index 4b8bc90..0793fda 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Lookup.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Lookup.java
@@ -3,7 +3,7 @@ package com.bytezone.diskbrowser.visicalc;
 class Lookup extends Function
 {
   Range range;
-  boolean hasValue;
+  //  boolean hasValue;
   String sourceText;
   String rangeText;
   Expression source;
@@ -21,14 +21,13 @@ class Lookup extends Function
   }
 
   // need a mechanism to return NA and ERROR
-  @Override
-  public boolean hasValue ()
-  {
-    return hasValue;
-  }
+  //  @Override
+  //  public boolean hasValue ()
+  //  {
+  //    return hasValue;
+  //  }
 
-  @Override
-  public double getValue ()
+  public void calculate ()
   {
     double sourceValue = source.getValue ();
 
@@ -43,10 +42,10 @@ class Lookup extends Function
 
     if (target != null)
       if (range.isVertical ())
-        return parent.getCell (target.nextColumn ()).getValue ();
+        value = parent.getCell (target.nextColumn ()).getValue ();
       else
-        return parent.getCell (target.nextRow ()).getValue ();
+        value = parent.getCell (target.nextRow ()).getValue ();
 
-    return 0;
+    //    return 0;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Max.java b/src/com/bytezone/diskbrowser/visicalc/Max.java
index ce7a733..863f4b8 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Max.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Max.java
@@ -3,8 +3,6 @@ package com.bytezone.diskbrowser.visicalc;
 class Max extends Function
 {
   private final Range range;
-  private boolean hasChecked;
-  private double max = Double.MIN_VALUE;
 
   public Max (Sheet parent, String text)
   {
@@ -13,34 +11,31 @@ class Max extends Function
   }
 
   @Override
-  public boolean hasValue ()
+  public void calculate ()
   {
-    if (!hasChecked)
-      calculate ();
-    return hasValue;
-  }
-
-  @Override
-  public double getValue ()
-  {
-    return hasValue () ? max : 0;
-  }
-
-  private void calculate ()
-  {
-    hasChecked = true;
-    hasValue = false;
+    value = Double.MIN_VALUE;
+    isError = false;
+    int totalChecked = 0;
 
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell != null && cell.hasValue ())
+      if (cell == null)
+        continue;
+
+      if (cell.isError () || cell.isNaN ())
       {
-        hasValue = true;
-        double value = cell.getValue ();
-        if (value > max)
-          max = value;
+        isError = true;
+        break;
       }
+
+      double temp = cell.getValue ();
+      if (temp > value)
+        value = temp;
+      totalChecked++;
     }
+
+    if (totalChecked == 0)
+      isError = true;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Min.java b/src/com/bytezone/diskbrowser/visicalc/Min.java
index 5cd3dde..8763f58 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Min.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Min.java
@@ -3,8 +3,6 @@ package com.bytezone.diskbrowser.visicalc;
 class Min extends Function
 {
   private final Range range;
-  private boolean hasChecked;
-  private double min = Double.MAX_VALUE;
 
   public Min (Sheet parent, String text)
   {
@@ -13,34 +11,31 @@ class Min extends Function
   }
 
   @Override
-  public boolean hasValue ()
+  public void calculate ()
   {
-    if (!hasChecked)
-      calculate ();
-    return hasValue;
-  }
-
-  @Override
-  public double getValue ()
-  {
-    return hasValue () ? min : 0;
-  }
-
-  private void calculate ()
-  {
-    hasChecked = true;
-    hasValue = false;
+    value = Double.MAX_VALUE;
+    isError = false;
+    int totalChecked = 0;
 
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell != null && cell.hasValue ())
+      if (cell == null)
+        continue;
+
+      if (cell.isError () || cell.isNaN ())
       {
-        hasValue = true;
-        double value = cell.getValue ();
-        if (value < min)
-          min = value;
+        isError = true;
+        break;
       }
+
+      double temp = cell.getValue ();
+      if (temp < value)
+        value = temp;
+      totalChecked++;
     }
+
+    if (totalChecked == 0)
+      isError = true;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Na.java b/src/com/bytezone/diskbrowser/visicalc/Na.java
index c1a5237..055a9b0 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Na.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Na.java
@@ -8,15 +8,15 @@ public class Na extends Function
   }
 
   @Override
-  public boolean hasValue ()
+  public boolean isError ()
   {
-    return false;
+    return true;
   }
 
   @Override
-  public String getError ()
+  public boolean isNaN ()
   {
-    return "@NA";
+    return true;
   }
 
   @Override
@@ -24,4 +24,9 @@ public class Na extends Function
   {
     return 0;
   }
+
+  @Override
+  public void calculate ()
+  {
+  }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Npv.java b/src/com/bytezone/diskbrowser/visicalc/Npv.java
index f7ee6ed..95ede29 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Npv.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Npv.java
@@ -8,9 +8,6 @@ public class Npv extends Function
   private final Expression valueExp;
   private final Range range;
 
-  private boolean hasChecked;
-  private final double value = 0;
-
   Npv (Sheet parent, String text)
   {
     super (parent, text);
@@ -24,32 +21,23 @@ public class Npv extends Function
   }
 
   @Override
-  public boolean hasValue ()
+  public void calculate ()
   {
-    if (!hasChecked)
-      calculate ();
-    return hasValue;
-  }
-
-  @Override
-  public double getValue ()
-  {
-    return hasValue () ? value : 0;
-  }
-
-  private void calculate ()
-  {
-    hasChecked = true;
-    hasValue = false;
+    value = 0;
 
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell != null && cell.hasValue ())
+      if (cell == null)
+        continue;
+
+      if (cell.isError () || cell.isNaN ())
       {
-        hasValue = true;
-        // calculate something
+        isError = true;
+        break;
       }
+
+      double temp = cell.getValue ();
     }
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Number.java b/src/com/bytezone/diskbrowser/visicalc/Number.java
index 0d5a5d7..e30fdfb 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Number.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Number.java
@@ -2,26 +2,37 @@ package com.bytezone.diskbrowser.visicalc;
 
 class Number implements Value
 {
-  private double value;
-  private boolean hasValue;
+  double value;
+  boolean isError;
 
   public Number (String text)
   {
     try
     {
-      this.value = Double.parseDouble (text);
-      hasValue = true;
+      value = Double.parseDouble (text);
     }
     catch (NumberFormatException e)
     {
-      hasValue = false;
+      isError = true;
     }
   }
 
   @Override
-  public boolean hasValue ()
+  public boolean isError ()
   {
-    return hasValue;
+    return isError;
+  }
+
+  @Override
+  public boolean isNaN ()
+  {
+    return Double.isNaN (value);
+  }
+
+  @Override
+  public String toString ()
+  {
+    return String.format ("Number: %f", value);
   }
 
   @Override
@@ -31,14 +42,13 @@ class Number implements Value
   }
 
   @Override
-  public String getError ()
+  public String getText ()
   {
-    return hasValue ? "" : "@NA";
+    return null;
   }
 
   @Override
-  public String toString ()
+  public void calculate ()
   {
-    return String.format ("Number: %f", value);
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Or.java b/src/com/bytezone/diskbrowser/visicalc/Or.java
index 1d475a9..61751c2 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Or.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Or.java
@@ -9,8 +9,8 @@ class Or extends Function
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
-    return 0;
+    value = 0;
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Pi.java b/src/com/bytezone/diskbrowser/visicalc/Pi.java
index fc8a2ba..5739787 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Pi.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Pi.java
@@ -8,8 +8,8 @@ class Pi extends Function
   }
 
   @Override
-  public double getValue ()
+  public void calculate ()
   {
-    return 3.1415926536;
+    value = 3.1415926536;
   }
 }
diff --git a/src/com/bytezone/diskbrowser/visicalc/Sheet.java b/src/com/bytezone/diskbrowser/visicalc/Sheet.java
index b5fb32e..4fa4364 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Sheet.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Sheet.java
@@ -14,6 +14,7 @@ public class Sheet
   private static final Pattern addressPattern =
       Pattern.compile ("([AB]?[A-Z])([0-9]{1,3}):");
   private static final byte END_OF_LINE_TOKEN = (byte) 0x8D;
+  private static final int FORMAT_LENGTH = 3;
 
   private final Map<Integer, Cell> rowOrderCells = new TreeMap<Integer, Cell> ();
   private final Map<Integer, Cell> columnOrderCells = new TreeMap<Integer, Cell> ();
@@ -266,7 +267,8 @@ public class Sheet
     String format = "";
     while (line.startsWith ("/"))
     {
-      String fmt = line.substring (0, 3);
+      String fmt = line.substring (0, FORMAT_LENGTH);
+      line = line.substring (FORMAT_LENGTH);
 
       if (fmt.equals ("/TH") || fmt.equals ("/TV"))     // lock titles ??
       {
@@ -275,7 +277,6 @@ public class Sheet
       else
         currentCell.format (fmt);                       // formatting command
 
-      line = line.substring (3);
       format += fmt;
     }
 
diff --git a/src/com/bytezone/diskbrowser/visicalc/Sum.java b/src/com/bytezone/diskbrowser/visicalc/Sum.java
index 08f1643..99b1478 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Sum.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Sum.java
@@ -3,8 +3,6 @@ package com.bytezone.diskbrowser.visicalc;
 class Sum extends Function
 {
   private final Range range;
-  private boolean hasChecked;
-  private double sum = 0;
 
   public Sum (Sheet parent, String text)
   {
@@ -13,32 +11,21 @@ class Sum extends Function
   }
 
   @Override
-  public boolean hasValue ()
-  {
-    if (!hasChecked)
-      calculate ();
-    return hasValue;
-  }
-
-  @Override
-  public double getValue ()
-  {
-    return hasValue () ? sum : 0;
-  }
-
   public void calculate ()
   {
-    hasChecked = true;
-    hasValue = false;
-
+    value = 0;
     for (Address address : range)
     {
       Cell cell = parent.getCell (address);
-      if (cell != null && cell.hasValue ())
+      if (cell == null)
+        continue;
+
+      if (cell.isError () || cell.isNaN ())
       {
-        hasValue = true;
-        sum += cell.getValue ();
+        isError = true;
+        break;
       }
+      value += cell.getValue ();
     }
   }
 }
\ No newline at end of file
diff --git a/src/com/bytezone/diskbrowser/visicalc/Value.java b/src/com/bytezone/diskbrowser/visicalc/Value.java
index ca3ee74..06638c9 100644
--- a/src/com/bytezone/diskbrowser/visicalc/Value.java
+++ b/src/com/bytezone/diskbrowser/visicalc/Value.java
@@ -2,9 +2,15 @@ package com.bytezone.diskbrowser.visicalc;
 
 interface Value
 {
-  public boolean hasValue ();
+  //  public boolean hasValue ();
 
   public double getValue ();
 
-  public String getError ();
+  public String getText ();
+
+  public boolean isError ();
+
+  public boolean isNaN ();
+
+  public void calculate ();
 }
\ No newline at end of file