Errors in Decode() method for huffman_test --> ascii_text

Question

I'm a begginer Java programmer and am trying to write a Decode() method that is supposed to take an encoded (Encode() method included below) huffman_text within a Huffman tree and decode it back into ascii_text. I think I'm almost there but I am having some trouble with compilation errors, mostly regarding charAt usage, but don't know what to use instead of charAt to get my code to work.

I'd appreciate some help if anyone knows how to proceed.

ERRORS

Tree.java:96: error: cannot find symbol if (huffman_text.chartAt(i) == "1") ^ symbol: method chartAt(int) location: variable huffman_text of type String

Tree.java:103: error: cannot find symbol if (root.charAt(i).length() == 1) ^ symbol: variable i location: class Tree

Tree.java:104: error: cannot find symbol ascii_text += root.charAt(i); ^ symbol: variable i location: class Tree

Tree.java:105: error: cannot find symbol else if (leaf_nodes[huffman_code] == null) ^ symbol: variable huffman_code location: class Tree

Tree.java:107: error: cannot find symbol huffman_text.charAt(i)); ^ symbol: variable i location: class Tree

class Tree
{
    Node root;
    Node leaf_nodes[];

    public void CreateLeafNodes(String ascii_text)
    {
        // Array of leaf nodes indexed by ASCII code
        leaf_nodes = new Node[256];

        // Parse text
        for (int i = 0; i< ascii_text.length();i++)
        {
            // Get ASCII code for current character
            int ascii_code = ascii_text.charAt(i);

            // Create node if it does not exist
            if (leaf_nodes[ascii_code] == null)
                leaf_nodes[ascii_code] = new Node(ascii_code);

            // Increment frequncy
            leaf_nodes[ascii_code].frequency++;
        }
    }

    public void BuildTree()
    {
        // Create heap
        Heap heap = new Heap();

        // Insert all leaf nodes
        for (int i = 0; i < 256; i++)
            if (leaf_nodes[i] != null)
                heap.Insert(leaf_nodes[i]);

        // Build tree
        while(heap.GetLength() > 1)
        {
            // Extract 2 nodes with minimum frequency
            Node left = (Node) heap.ExtractMax();
            Node right = (Node) heap.ExtractMax();

            // Create new node and make it the root for now
            root = new Node(0);
            root.left = left;
            root.right = right;
            root.frequency = left.frequency + right.frequency;

            // Insert new node in heap
            heap.Insert(root);
        }

        // Set Huffman codes
        root.SetHuffmanCode("");
    }

    public String Encode(String ascii_text)
    {
        // Initialize result
        String huffman_text = "";

        // Traverse ASCII text
        for (int i = 0; i < ascii_text.length(); i++)
        {
            // Get ASCII code
            int ascii_code = ascii_text.charAt(i);

            // Check if character is supported
            if (leaf_nodes[ascii_code] == null)
                throw new RuntimeException("Character not supported: " + 
                        ascii_text.charAt(i));

            // Get Huffman code
            String huffman_code = leaf_nodes[ascii_code].huffman_code;

            // Add it
            huffman_text += huffman_code;

            // Message
            System.out.println(ascii_text.charAt(i) + " -> " + huffman_code);
        }

        // Result
        return huffman_text;
    }

    public String Decode(String huffman_text)
    {
        // Initialize result
        String ascii_text = "";

        // Traverse huffman text
        for (int i = 0; i < huffman_text.length(); i++)
        {
            while(root.left != null && root.right != null && i < huffman_text.length())
                if (huffman_text.chartAt(i) == "1")
                    root = root.right;
                else
                    root = root.left;
                i++;
        }
        if (root != null)
            if (root.charAt(i).length() == 1)
                ascii_text += root.charAt(i);
            else if (leaf_nodes[huffman_code] == null)
                throw new RuntimeException("Character not supported: " + 
                        huffman_text.charAt(i));

        // Result
        return ascii_text;
    }
}

class Test
{
    public static void main(String args[])
    {
        float compression;

        Tree tree = new Tree();

        if (args.length == 0)
            throw new RuntimeException("Please enter an argument. ");

        String ascii_text = args[0];
        tree.CreateLeafNodes(ascii_text);
        tree.BuildTree();

        System.out.println(ascii_text);

        String huffman_text = tree.Encode(ascii_text);
        System.out.println(huffman_text);

        compression = huffman_text.length() * 100/ascii_text.length();
        System.out.println(compression);    

        String ascii_text_2 = tree.Decode(huffman_text);
    }
}

Please provide the compilation errors you encounter! – itzFlubby Apr 17 '20 at 17:52 — itzFlubby, Apr 17 '20 at 17:52

score 1 · Accepted Answer · edited Apr 19 '20 at 10:59

I'll just go through the errors one by one.

Error line 96
cannot find symbol if (huffman_text.chartAt(i) == "1") ^ symbol: method chartAt(int)

Typo: method is called charAt() not chartAt()

Error lines 103, 104 and 107
cannot find symbol { ... } ^ symbol: variable i

You used i outside the for-loop: it's not declared in this scope

Error line 105
cannot find symbol else if (leaf_nodes[huffman_code] == null) ^ symbol: variable huffman_code

huffman_code is not declared anywhere.

These are errors, you should be able to solve. Or at least know what to do, just by looking at them. Understanding errors-messages is a needed skill in programming!

Regardless, there are some more problems with your code which actually should also throw error-messages:

if (huffman_text.chartAt(i) == "1") in this statement, you're comparing huffman_text.chartAt(i) with "1". This should not be possible, as the method charAt() returns a char, but "1" is of type String.
The correct way to compare this, is by using only single quotes, which represent char:
if (huffman_text.chartAt(i) == '1')

Errors in Decode() method for huffman_test --> ascii_text

1 Answers1