Question

Show what is produced by the following \(\mathrm{C}++\) code. Assume the node is in the usual info-link form with the info of type int. (list and ptr are pointers of type nodeType.) 1ist \(=\) new nodeType; \(1 \mathrm{ist} \rightarrow \mathrm{info}=20\) ptr \(=\) new nodeType; \(p t r->i n f 0=28\) \(p t r->1\) ink \(=N U L L\) 1 ist \(->\operatorname{link}=\) ptr ptr \(=\) new nodeType; \(p t r->i n f 0=30\) \(p t r->1\) ink \(=1\) ist list \(=\operatorname{ptr}\) ptr \(=\) new nodeType; \(p t r->i n f 0=42\) \(p t r->1 i n k=1 i s t->1 i n k\) \(1 i s t->1 i n k=p t r\) \(p t r=1\) ist while (ptr != NULL) $$\\{$$ $$\begin{array}{l} \text { cout }<<\operatorname{ptr} \rightarrow \inf _{0}<<\text { end } 1 ; \\\ \operatorname{ptr}=\operatorname{ptr} \rightarrow \operatorname{lin} \mathrm{k} ; \end{array}$$ $$\\}$$

Short answer

The code produces the output: 30, 42, 28.

Step-by-step solution

Initializing and Linking the First Node

The first statement, `list = new nodeType; list->info = 20;`, initializes a new node and sets the data, or info, part of this node to the value 20. At this point, `list` acts as a pointer to this node. The structure of this node is effectively: info = 20, link = NULL, but link is not explicitly set yet.

Creating and Linking the Second Node

Next, `ptr = new nodeType; ptr->info = 28; ptr->link = NULL;` creates a second node, assigns the data part to 28, and sets its link part to NULL, making it the end of the list currently. Then, `list->link = ptr;` links the first node to this second node. Now, the structure is: [20] -> [28] -> NULL.

Creating a Loop with Third Node

The code `ptr = new nodeType; ptr->info = 30; ptr->link = list; list = ptr;` creates a third node with info = 30 and links it back to the first node (list). The pointer `list` is updated to point to this new node, effectively making it the head node. The structure becomes: [30] -> [20] -> [28] -> NULL, closing a loop on the first node.

Modifying List with a Fourth Node

The next segment `ptr = new nodeType; ptr->info = 42; ptr->link = list->link;` creates a fourth node with info = 42, linking it to the node that `list` was pointing to after the first node (list->link, which was previously the node `ptr = [28]`). Then, `list->link = ptr;` modifies the first node's link to point to this new fourth node, making the new structure: [30] -> [42] -> [28] -> NULL.

Printing the Linked List

The while loop iterates through the nodes from `list`, printing each node's info followed by a newline until a node with a `NULL` link is encountered. Starting from the updated head, this prints the sequence of values found by following each link: 30, 42, 28, and then ends as the pointer encounters the `NULL` of the last node.

Key concepts

Node Structure

In C++ linked lists, the node structure is a fundamental building block. A node is typically an object or a struct containing at least two components: an `info` part and a `link` part.
- The `info` contains the data (e.g., an integer, string, or object) that the node holds.
- The `link` part holds the address of the next node in the sequence, forming a chain.
This type of setup is often termed an "info-link form." The first node, often called the head, is where traversal of the entire list begins. In our example, each node is of a `nodeType` structure or class, with an integer `info` and a nodeType pointer `link`. Understanding this structure is crucial.
It ensures that lists can dynamically grow and shrink as memory is allocated for new nodes and freed upon removal.

Pointer Manipulation

Pointer manipulation is essential in handling linked lists in C++. Pointers allow us to directly access and modify the memory address of nodes.
- When we write `list = new nodeType;`, we create a new node. Here, `list` points to the memory location where this new node is stored.
- With `list->info = 20;`, we set the data for this node, and `list->link = ptr;` connects it to another node by assigning the address of `ptr` to `list->link`.
Pointer manipulation enables the creation of links between nodes and allows us to traverse, modify, or expand the list seamlessly. Mismanaging pointers can lead to errors like memory leaks or segmentation faults.

Info-Link Form

The info-link form of nodes is a standard structure used in linked lists. This form emphasizes two primary elements within a node:
- `info`: The data component that holds the valuable data the node is supposed to keep.
- `link`: The pointer or reference that indicates the next node in the list, or `NULL` if it is the last node.
In the original exercise, we see nodes chaining together using the info-link form, starting with an `info` of 30 leading to another node with `info` 42, and ending with `info` 28. This structure is simple yet effective for creating dynamic data sequences, allowing efficient insertion and deletion of elements without needing to shift any of the subsequent data entries in memory.

Loop and Iteration in C++

Looping through a linked list is crucial for accessing or printing each node's data. This is typically done using a while loop that iterates until it encounters a `NULL` link, which signals the end of the list.
Consider the while loop: `while (ptr != NULL)` where the loop continues as long as `ptr` points to a valid node:
- Each iteration outputs the `info` of the current node.
- `ptr = ptr->link;` moves the pointer to the next node by assigning it the address stored in the current node's `link`.
Looping ensures we can efficiently access each node in sequence, performing tasks like printing, searching, or modifying nodes within the list as needed.