Question

Write a function double lastValue(ListNode *ptr) that returns the value stored in the last node of a nonempty list passed to it as parameter. The function should print an error message and terminate the program if the list passed to it is empty.

Short answer

Question: Write a function that takes a non-empty linked list as input and returns the value stored in the last node of the list. Answer: To write a function that returns the value stored in the last node of a non-empty linked list, follow these steps: 1. Declare the function with the name 'double lastValue(ListNode *ptr)', where 'ptr' is a pointer to the head of the linked list. 2. Check if the list is empty. If it is, print an error message and terminate the program. 3. Create a loop that iterates through the linked list until the last node is reached. 4. Return the value of the last node, which should be stored in the 'current' pointer after iteration.

Step-by-step solution

1. Function Declaration

Declare the function with the name 'double lastValue(ListNode *ptr)', where 'ptr' is a pointer to the head of the linked list.

2. Check if the list is empty

Inside the function, first check if the list passed to the function is empty. If it is, print an error message and terminate the program. To do this, you can use the 'if' statement: ```cpp if(ptr == nullptr){ cout << "Error: The list is empty." << endl; exit(EXIT_FAILURE); } ```

3. Iterate through the list

Now, create a loop that iterates through the linked list until the last node is reached. You can use a 'while' loop for this purpose: ```cpp ListNode *current = ptr; while(current->next != nullptr){ current = current->next; } ```

4. Return the value of the last node

At this point, the 'current' pointer should be pointing to the last node in the linked list. Simply return the value stored in this node: ```cpp return current->value; ``` Putting everything together, the final function will look like this: ```cpp double lastValue(ListNode *ptr){ if(ptr == nullptr){ cout << "Error: The list is empty." << endl; exit(EXIT_FAILURE); } ListNode *current = ptr; while(current->next != nullptr){ current = current->next; } return current->value; } ```

Key concepts

Function Implementation in C++

In C++, functions are reusable blocks of code that perform a specific task. When implementing a function, it is crucial to be clear about the function's purpose and its parameters. For the task at hand, we're implementing a function named `lastValue()` that accepts a pointer to the first node of a linked list and returns the value stored in the last node.

Here are the main steps involved in function implementation:

• Function Declaration: This is the first line of any function and includes the return type, name, and parameters. In our case, the function is declared with `double lastValue(ListNode *ptr)`, indicating it returns a `double` and takes a `ListNode` pointer as its argument.
• Function Definition: The body of the function follows the declaration and contains the logic to achieve the function's purpose. The key activities within `lastValue()` include checking for an empty list, traversing the list, and returning the last node's value.

Function implementation in C++ relies on understanding exactly what the function needs to do, and writing clear and concise code to achieve that goal.

Linked List Traversal

Traversing a linked list involves moving through the list from the first node to the last. This is a fundamental operation in many algorithms that work with linked lists.

To implement linked list traversal in the `lastValue()` function:

• Initialize a Pointer: We start by setting a pointer (such as `current`) to point at the head of the list, which is given by `ptr`.
• Iterate with a Loop: We use a `while` loop to move the pointer from one node to the next until the end of the list is reached. The condition for the loop `while(current->next != nullptr)` ensures that the loop continues until the `current` pointer reaches the last node (which does not have a subsequent node).
• Read the Value: At the conclusion of the loop, `current` points to the last node, allowing us to read or return its value.

Each of these steps ensures that the linked list is traversed efficiently from the beginning to the end without omitting any nodes.

Error Handling in C++

Error handling is a critical aspect of robustness in C++ programming. It involves identifying potential error conditions and managing them gracefully rather than allowing them to cause unexpected behavior or crashes.

In the `lastValue()` function, a significant error-handling step involves checking if the linked list is empty.

• Check for Null Pointer: The first thing the function does is check if the pointer to the head of the list (`ptr`) is `nullptr`. This is accomplished using an `if` statement.
• Output an Error Message: If `ptr` is `nullptr`, the function outputs an error message indicating the list is empty and cannot be processed.
• Terminate the Program: Following the error message, the function calls `exit(EXIT_FAILURE);` to terminate the program. This stops execution and indicates an error occurred, which can be crucial for debugging and user awareness.

By incorporating error handling, you ensure the linked list operations are safe and that users are informed if something goes wrong, allowing for more manageable and maintainable code.