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copied!<p>It appears to be exactly what the comment says. They are arrays. Presumably <code>rowList</code> will be an array of <code>rows</code> elements, and <code>columnList</code> will be an array of <code>cols</code> elements. The reason it's a <code>Node**</code> is that each item in the array is a <code>Node*</code>. A pointer to an array always has an extra level of indirection (an extra <code>*</code>). That means when you index a single element out of that array you get a value of type <code>Node*</code> again.</p> <p>The arrays are created like this:</p> <pre><code>rowList = new Node* [rows]; columnList = new Node* [cols]; // Don't forget to initialise the values to NULL! Here's the dull way: for( int i = 0; i < rows; i++ ) rowList[i] = NULL; for( int i = 0; i < cols; i++ ) columnList[i] = NULL; </code></pre> <p>When you need to delete them (in the destructor for <code>Matrix</code>):</p> <pre><code>delete [] rowList; delete [] colList; </code></pre> <p>As for your question on how to implement your matrix from that, that's really up to you. Presumably when you create a node at position <code>(i, j)</code>, you append that node to each of <code>rowList</code> and <code>columnList</code>. <em>ie</em>:</p> <pre><code>Node * node = new Node(i, j, 123.0); rowList[i] = node; columnList[j] = node; </code></pre> <p>But it's not that simple, because the node obviously must be linked into both a row and column list. At the very basic level, and using the structures you've provided, here's one way:</p> <pre><code>// Inserts newNode at the head of the list and modifies the head pointer. void insert_row( Node* & r, Node *newNode ) { newNode->rowPtr = r; if( r != NULL ) r->rowPtr = newNode; r = newNode; } // Similarly with insert_col()... </code></pre> <p>Now using the above with my original example:</p> <pre><code>Node * node = new Node(i, j, 123.0); insert_row( rowList[i], node ); insert_col( columnList[j], node ); </code></pre> <hr> <h2>For ordered insert</h2> <p>Since you have code already, I will offer my take on it. But you still need to do some work yourself.</p> <blockquote> <p>I just try to understand the concept but it's so confusing for me.</p> </blockquote> <p>Let's just clean things up to begin with. It's a class, and you're using C++ so please use your C++ knowledge:</p> <pre><code>class Node { public: Node( int i, int j, int val ); void InsertRowAfter( Node* node ); void InsertColAfter( Node* node ); int iValue, jValue; // Row and column index, 1-based float value; // Element value Node *rowPtr; // Next element in this row (sorted by jValue) Node *colPtr; // Next element in this column (sorted by iValue) }; Node::Node( int i, int j, int val ) : iValue(i) , jValue(j) , value(val) , rowPtr(NULL) , colPtr(NULL) {} // Inserts the given node to follow this node in the row list void Node::InsertRowAfter( Node* node ) { // [go on, you do it] } // Inserts the given node to follow this node in the column list void Node::InsertColAfter( Node* node ); { // [go on, you do it] } </code></pre> <p>So, now you need to implement the <code>Matrix::inputData</code> function... Essentially you do what your friend was trying to do, but without the errors and memory leaks. That means you start like this:</p> <pre><code>// Use 'horz' and 'vert' to search through the row and column lists. If a // node needs to be created, it will be stored in 'node'. Node *horz = rowList[iValue - 1]; Node *vert = columnList[jValue - 1]; Node *node; // If there is no row list or smallest jValue, insert at the head. // Otherwise, search for an insert point. if( !horz || horz->jValue > jValue ) { // [go on, you do it] } else { // Move 'horz' pointer to position at which we will append a node. Node *next = horz->rowPtr; while( next && next->jValue <= jValue ) { horz = next; next = next->rowPtr; } // If replacing an existing value, there's nothing else to do. if( horz->jValue == jValue ) { horz->value = value; return; } // Otherwise append a new node. // [go on, you do it] } </code></pre> <p>Now, you finish the function off, and don't forget to do the column indexing...</p>

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