Question

Recall that within the ArrayBoundedQueue the front variable and the rear variable hold the indices of the elements array where the current front and rear elements, respectively, of the queue are stored. Which of the following code sequences could be used to correctly enqueue element into the queue, assuming that enqueue is called on a non-full queue and that the code also correctly increments numElements?

a. numElements++; elements[rear) - element:
b. front++; elements(front) - element:
c. rear = (rear + 1) % elements.length; elements[rear) - element;
d. front = (front + 1) % elements.length; elements[front) - element;

Answer 1

c. rear = (rear + 1) % elements.length; elements[rear] = element;

Step-by-step explanation:

In the above statement:

Name of the array is elements.

rear holds current index of elements array where current rear element of queue is stored. Front are rear are two open ends of the queue and the end from which the element is inserted into the queue is called rear.

element is the element that is required to enqueue into the queue

Enqueue basically mean to add an element to a queue.

Here it is assumed that the queue is not full. This means an element can be added to the queue.

It is also assumed that code also correctly increments numElements.

rear = (rear + 1) % elements.length; This statement adds 1 to the rear and takes the modulus of rear+1 to the length of the array elements[]. This statement specifies the new position of the rear.

Now that the new position of rear is found using the above statement. Next the element can be enqueued to that new rear position using the following statement:

elements[rear] = element; this statement sets the element at the rear-th (new position) index of elements[] array.

For example we have a queue of length 5 and there are already 4 elements inserted into this queue. We have to add a new element i.e. 6 to the queue. There are four elements in elements[] array and length of the array is 5 so this means the queue is not full. Lets say that rear = 3

elements.length = 5

rear = 3

Using above two statements we get.

rear = (rear + 1) % elements.length;

= 3 + 1 % 5

= 4%5

= 4

This computes the new position of rear. So the new position of rear is the 4-th index of elements[]. Now next statement:

elements[rear] = element;

elements[4] = 6

This statement adds element 6 to the 4-th index of elements[] array.

Thus the above statement enqueues element (element 6 in above example) into the queue.