3.py

class Empty(Exception):
    pass

class ArrayQueue:
    DEFAULT_CAPACITY = 10

    def __init__(self):
        self._data = [None] * ArrayQueue.DEFAULT_CAPACITY
        self._size = 0
        self._front = 0

    def __len__(self):
        return self._size

    def is_empty(self):
        return self._size == 0

    def first(self):
        if self.is_empty():
            raise Empty("Queue is Empty!")
        return self._data[self._front]

    def dequeue(self):
        if self.is_empty():
            raise Empty("Queue is Empty!")
        answer = self._data[self._front]
        self._data[self._front] = None
        self._front = (self._front + 1) % len(self._data)
        self._size -= 1
        return answer

    def enqueue(self, e):
        if self._size == len(self._data):
            self._resize(2 * len(self._data))
        avail = (self._front + self._size) % len(self._data)
        self._data[avail] = e
        self._size += 1
        print("Element", e, "is enqueued successfully")

    def _resize(self, cap):
        old = self._data
        self._data = [None] * cap
        walk = self._front
        for k in range(self._size):
            self._data[k] = old[walk]
            walk = (walk + 1) % len(old)
        self._front = 0


def main():
    q = ArrayQueue()
    while True:
        print("\n\tQUEUE OPERATIONS")
        print("1-Enqueue")
        print("2-Dequeue")
        print("3-First Element")
        print("4-Length of Queue")
        print("5-Is Empty")
        print("6-Display Queue")
        print("7-Exit")

        try:
            ch = int(input("Enter your choice: "))

            if ch == 1:
                e = int(input("Enter an element: "))
                q.enqueue(e)

            elif ch == 2:
                print("The front element:", q.dequeue(), "is dequeued successfully")

            elif ch == 3:
                print("First element in the queue is", q.first())

            elif ch == 4:
                print("Queue length =", len(q))

            elif ch == 5:
                if q.is_empty():
                    print("Queue is empty!")
                else:
                    print("Queue is not empty!")

            elif ch == 6:
                if q.is_empty():
                    print("Queue is empty!")
                else:
                    print("Queue elements (front to rear):")
                    index = q._front
                    print("|", end=" ")
                    for _ in range(len(q)):
                        print(q._data[index], "|", end=" ")
                        index = (index + 1) % len(q._data)
                    print()

            elif ch == 7:
                print("Program exited")
                break

            else:
                print("Invalid choice")

        except Empty as e:
            print(e)
        except ValueError:
            print("Please enter a valid number")


if __name__ == "__main__":
    main()