Added some methods to „Working With the Expression Tree“ section

MathPad/MPExpression.h

@@ -44,13 +44,17 @@ extern NSString *MPDivisionOperator;
 - (instancetype)initWithString:(NSString *)aString;
 - (instancetype)initWithFunction:(MPFunction *)aFunction;
 
-
 + (instancetype)expression;
 + (instancetype)expressionWithString:(NSString *)aString;
 + (instancetype)expressionWithFunction:(MPFunction *)aFunction;
 + (instancetype)expressionWithSymbols:(NSArray *)symbols;
 
-#pragma mark Working with Expressions
+#pragma mark Working With the Expression Tree
+
+- (NSUInteger)indexOfSymbol:(id)symbol;
+- (id)symbolAtIndexPath:(NSIndexPath *)indexPath; // May also return MPExpression
+
+#pragma mark Working With Expressions
 
 - (NSUInteger)length;
 

MathPad/MPExpression.m

@@ -11,6 +11,7 @@
 #import "MPException.h"
 
 #import "NSObject+MPStringTest.h"
+#import "NSIndexPath+MPRemoveFirstIndex.h"
 
 NSString *MPAdditionOperator = @"+";
 NSString *MPSubtractionOperator = @"-";
@@ -214,7 +215,29 @@ NSString *MPDivisionOperator = @"/";
     return [[self alloc] initWithSymbols:symbols];
 }
 
-#pragma mark Working with Expressions
+#pragma mark Working With the Expression Tree
+
+- (NSUInteger)indexOfSymbol:(id)symbol
+{
+    return [_symbols indexOfObject:symbol];
+}
+
+- (id)symbolAtIndexPath:(NSIndexPath *)indexPath
+{
+    if (indexPath.length == 0) {
+        return self;
+    }
+    id symbol = [self symbolAtIndex:[indexPath indexAtPosition:0]];
+    if (indexPath.length == 1) {
+        return symbol;
+    }
+    if ([symbol isKindOfClass:[MPFunction class]]) {
+        return [symbol symbolAtIndexPath:[indexPath indexPathByRemovingFirstIndex]];
+    }
+    return nil;
+}
+
+#pragma mark Working With Expressions
 
 - (NSUInteger)length
 {

MathPad/MPFunction.h

@@ -35,10 +35,13 @@
 
 @interface MPFunction (MPFunctionExtensionMethods)
 
-#pragma mark Children
+#pragma mark Working With the Expression Tree
 
 // May be overridden for performance improvements
 - (NSArray *)children;
+- (NSUInteger)indexOfChild:(MPExpression *)child;
+
+- (id)symbolAtIndexPath:(NSIndexPath *)indexPath;
 
 #pragma mark Evaluating Functions
 

MathPad/MPFunction.m

@@ -9,6 +9,8 @@
 #import "MPFunction.h"
 #import "MPExpression.h"
 
+#import "NSIndexPath+MPRemoveFirstIndex.h"
+
 @implementation MPFunction
 
 #pragma mark Creation Methods
@@ -90,7 +92,7 @@
 
 @implementation MPFunction (MPFunctionExtensionMethods)
 
-#pragma mark Children
+#pragma mark Working With the Expression Tree
 
 - (NSArray *)children
 {
@@ -102,6 +104,26 @@
     return [children copy];
 }
 
+- (NSUInteger)indexOfChild:(MPExpression *)child
+{
+    NSUInteger index = 0;
+    for (; index < [self numberOfChildren]; index++) {
+        if ([[self childAtIndex:index] isEqualToExpression:child]) {
+            return index;
+        }
+    }
+    return NSNotFound;
+}
+
+- (id)symbolAtIndexPath:(NSIndexPath *)indexPath
+{
+    if (indexPath.length == 0) {
+        return self;
+    }
+    MPExpression *child = [self childAtIndex:[indexPath indexAtPosition:0]];
+    return [child symbolAtIndexPath:[indexPath indexPathByRemovingFirstIndex]];
+}
+
 #pragma mark Evaluating Functions
 
 - (float)floatValue