Question

What does the following function do? void mystery(queueType& q) { stackType s; while (!q.isEmptyQueue()) { s.push(q.front()); q.deleteQueue(); } while (!s.isEmptyStack()) { q.addQueue(2 * s.top()); s.pop(); } }

Short answer

The function reverses the queue's elements, doubles them, and stores them back.

Step-by-step solution

Understand Function Parameters

The function `mystery` takes a reference to a queue of integers, `q`, as its parameter. This means any changes made to the queue inside the function will affect the original queue.

Introduce Stack

Inside the function, a stack `s` of integers is initialized. This stack will be used to temporarily hold data from the queue.

Transfer Elements from Queue to Stack

Using a while loop, the function continuously checks if the queue `q` is not empty with `!q.isEmptyQueue()`. The front element of the queue is accessed, pushed onto the stack `s`, and then removed from the queue until the queue is empty.

Process Stack Elements

Once the queue is empty, another while loop is used to check if the stack `s` is not empty with `!s.isEmptyStack()`. Inside the loop, the top element of the stack is accessed, doubled, and added back to the queue `q` using `q.addQueue(2 * s.top())`. The top element is then removed from the stack using `s.pop()`.

Describe Result of Function

The function essentially reverses the order of elements from the queue into the stack, processes them by doubling each element, and then places them back into the queue. This means the elements in the queue are now twice their original values and are in the reverse order compared to the starting configuration.

Key concepts

Queue

In the realm of data structures, a queue is a linear collection of elements following a specific order in which operations are performed. The primary guiding principle of a queue is the First-In-First-Out (FIFO) methodology. It means that the first element added to the queue will be the first one to be removed.
For instance, it resembles a real-life queue, like a line of people waiting for a bus, where the first person in line is the first to board the bus.
Queues are commonly used in situations where maintaining the order of elements is crucial. Typical operations performed on a queue are:

• Enqueue: Adding an element to the end of the queue.
• Dequeue: Removing the element from the front of the queue.
• Front: Accessing the first element, without removing it.
• isEmptyQueue: Checking if the queue has no elements.

In programming, queues are instrumental in scheduling and managing tasks efficiently.

Stack

A stack is another fundamental data structure commonly used to manage and organize data. Its primary operation principle is known as Last-In-First-Out (LIFO).
This means the last element added to the stack is the first one to be removed. Imagine a stack of plates in a cafeteria, where you can only take the top plate and not the one beneath it.
In a stack, you can perform the following operations:

• Push: Adding an element onto the top of the stack.
• Pop: Removing the top element from the stack.
• Top: Accessing the top element without removing it.
• isEmptyStack: Checking if the stack is empty.

Stacks are widely used in software development for tasks such as parsing expressions, backtracking algorithms, and managing function calls within programming languages.

Reversal Algorithm

The reversal algorithm employed in the `mystery` function primarily involves transferring elements between two data structures: a queue and a stack. This algorithm ensures that elements in the queue are reversed in their order. It makes clever use of the LIFO property of stacks.
Here’s a simplied breakdown of how the reversal happens:

• Elements are dequeued from the queue and pushed onto the stack. This act reverses their order because the first element dequeued goes to the bottom of the stack, and so on.


• Once all elements are in the stack, they're popped from the stack and enqueued back into the queue. Now, the first element that was dequeued is the last to be enqueued, effectively reversing the order.

This technique is useful when you need to reorder elements but need to maintain linear operations and minimize additional space usage beyond the stack.

Function Analysis

Analyzing a function involves breaking down its components and operation sequences to understand its behavior and effect on the input data. For the `mystery` function, our analysis focuses on understanding how it manipulates the queue.

• The function begins by taking a reference to a queue and initializes an empty stack. By taking the queue as a reference, any changes directly affect the original data.


• Through the first while loop, elements are removed from the queue and pushed onto the stack until the queue is empty. This step implies the elements' order is getting reversed.


• In the second loop, each element in the stack is accessed, doubled, and then added back to the queue. The elements are added back in reverse order with doubled values.

Overall, the function modifies the queue by reversing the order of elements and doubling each value, showcasing how basic data structure operations can bring substantial transformation in a structured manner.