XML Parser with Binary Trees in C++: CSCI 340 Assignment Guide

Introduction: Why Binary Trees for XML?

XML is everywhere: from configuration files to web APIs. In this C++ assignment (CSCI 340 at NIU), you'll build a simplified XML parser using binary trees. But wait—XML elements can have many children, not just two. That's where tilted trees come in: a clever transformation that represents any multi-child tree as a binary tree. Think of it like a tournament bracket where each node's left child is its first opponent (child) and right child is the next match (sibling).

By the end of this guide, you'll understand how to implement binary tree traversals, handle XML tags and attributes, and navigate tilted trees. Let's break it down step by step, using timely examples from gaming leaderboards and AI apps to keep things relevant.

Binary Tree Basics: Traversals

Your bintree.h must implement four traversals: inorder, preorder, postorder, and levelorder. Each takes a root pointer and a function fn to apply to each node's data.

Inorder Traversal

Inorder visits left subtree, then node, then right subtree. For a tilted tree (children left, siblings right), inorder gives you: first child, then parent, then next sibling. This is handy for printing XML in document order.

template <typename T>
void inorder(Node<T>* root, std::function<void(T&)> fn) {
    if (!root) return;
    inorder(root->left, fn);
    fn(root->data);
    inorder(root->right, fn);
}

Preorder and Postorder

Preorder visits node before children: useful for printing opening tags. Postorder visits children before node: perfect for closing tags. Implement them similarly.

Levelorder (BFS)

Levelorder visits nodes level by level. Use a queue. For tilted trees, you must treat left as child and right as sibling, so BFS will process all siblings at the same depth before moving deeper.

template <typename T>
void levelorder(Node<T>* root, std::function<void(T&)> fn) {
    if (!root) return;
    std::queue<Node<T>*> q;
    q.push(root);
    while (!q.empty()) {
        Node<T>* cur = q.front(); q.pop();
        fn(cur->data);
        // Enqueue children (left) then siblings (right) 
        for (Node<T>* child = cur->left; child; child = child->right)
            q.push(child);
    }
}

Don't forget delete_tree to free memory recursively.

Tilted Trees: Left = Child, Right = Sibling

In a tilted tree, the left pointer points to the first child of the original multi-way tree, and the right pointer points to the next sibling. This is like a file system: a folder's left child is its first subfolder, and that subfolder's right child is the next subfolder in the same parent.

You need three functions:

tilted_find_parent: Given a node, find its parent in the tilted tree. Since right links are siblings, traverse up via parent pointers (if you store them) or by searching from root.
tilted_get_children: Return a list of all children of a given node. Start from left child and follow right pointers until null.
tilted_levelorder: Same as levelorder but respects tilted structure (left=child, right=sibling).

Example: For an XML element <book> with children <title>, <author>, the tilted tree has book.left = title, title.right = author.

XML Parser Implementation

Your xml.cc will implement functions called by the provided parse_xml. The parser reads tags and plaintext, building a tilted binary tree of xml_element nodes.

Key Functions

to_string: Convert an xml_element to string. For opening tags, include attributes; for closing, just the name.
xml_handle_tag: Called when an opening tag is encountered. Create a new xml_element with type tag, parse attributes, and add to tree via xml_add_node.
xml_handle_plaintext: Create a plain text element and add as child of current node.
xml_handle_attributes: Parse key="value" pairs from the tag string into a map.
xml_add_node: Insert a new node as a child of the current node (state.cur). If current has no children, set left child; else find the last sibling (follow right pointers) and attach there.
xml_close_tag: When a closing tag is found, verify it matches the current node's tag name, then set closed=true and move state.cur up to parent (you may need a stack or parent pointers).
xml_print_subtree: Recursively print the tree with indentation (preorder).
xml_find_by_name: Search the tree (any traversal) for a tag with given name.
xml_find_with_attr: Find a tag that has a specific attribute key-value pair.

Parsing Algorithm Flow

Start with state.root = nullptr, state.cur = nullptr.
When an opening tag is encountered, create a new node, add it as child of state.cur (if any), then set state.cur to that new node.
When plaintext is encountered, add as child of state.cur.
When closing tag is encountered, set closed=true on the current node, then move state.cur back to its parent. If no parent, root is closed.

Error Handling for Well-Formed XML

Your parser must handle errors: mismatched closing tags, unclosed tags, malformed attributes. Use exceptions or error messages. For example, if a closing tag doesn't match the current node's name, print an error and exit.

Putting It All Together: A Gaming Leaderboard Example

Imagine an XML file storing a gaming leaderboard:

<leaderboard game="Valorant">
  <player rank="1">
    <name>Phoenix</name>
    <score>2500</score>
  </player>
  <player rank="2">
    <name>Jett</name>
    <score>2400</score>
  </player>
</leaderboard>

Your parser will build a tilted tree: leaderboard node with left child player[1], which has right sibling player[2]. Each player has left child name, and name's right sibling score. Using xml_find_by_name, you could locate the score element for a player. This is exactly how modern AI apps parse structured data from APIs.

Conclusion

You've now seen how to implement a simplified XML parser using binary trees and the tilted tree transformation. The key is to remember that left means child, right means sibling. With these building blocks, you can handle any hierarchical data format. Good luck with your NIU CSCI 340 assignment!