Create priority_queue_using_list.py (#2435)

* Create priority_queue_using_list.py

* Update priority_queue_using_list.py

* Update priority_queue_using_list.py

* Update priority_queue_using_list.py

* Maximum queue size is 100

Co-authored-by: Christian Clauss <cclauss@me.com>

data_structures/queue/priority_queue_using_list.py

@@ -0,0 +1,232 @@
+"""
+Pure Python implementations of a Fixed Priority Queue and an Element Priority Queue
+using Python lists.
+"""
+
+
+class OverFlowError(Exception):
+    pass
+
+
+class UnderFlowError(Exception):
+    pass
+
+
+class FixedPriorityQueue:
+    """
+    Tasks can be added to a Priority Queue at any time and in any order but when Tasks
+    are removed then the Task with the highest priority is removed in FIFO order.  In
+    code we will use three levels of priority with priority zero Tasks being the most
+    urgent (high priority) and priority 2 tasks being the least urgent.
+
+    Examples
+    >>> fpq = FixedPriorityQueue()
+    >>> fpq.enqueue(0, 10)
+    >>> fpq.enqueue(1, 70)
+    >>> fpq.enqueue(0, 100)
+    >>> fpq.enqueue(2, 1)
+    >>> fpq.enqueue(2, 5)
+    >>> fpq.enqueue(1, 7)
+    >>> fpq.enqueue(2, 4)
+    >>> fpq.enqueue(1, 64)
+    >>> fpq.enqueue(0, 128)
+    >>> print(fpq)
+    Priority 0: [10, 100, 128]
+    Priority 1: [70, 7, 64]
+    Priority 2: [1, 5, 4]
+    >>> fpq.dequeue()
+    10
+    >>> fpq.dequeue()
+    100
+    >>> fpq.dequeue()
+    128
+    >>> fpq.dequeue()
+    70
+    >>> fpq.dequeue()
+    7
+    >>> print(fpq)
+    Priority 0: []
+    Priority 1: [64]
+    Priority 2: [1, 5, 4]
+    >>> fpq.dequeue()
+    64
+    >>> fpq.dequeue()
+    1
+    >>> fpq.dequeue()
+    5
+    >>> fpq.dequeue()
+    4
+    >>> fpq.dequeue()
+    Traceback (most recent call last):
+    ...
+    priority_queue_using_list.UnderFlowError: All queues are empty
+    >>> print(fpq)
+    Priority 0: []
+    Priority 1: []
+    Priority 2: []
+    """
+
+    def __init__(self):
+        self.queues = [
+            [],
+            [],
+            [],
+        ]
+
+    def enqueue(self, priority: int, data: int) -> None:
+        """
+        Add an element to a queue based on its priority.
+        If the priority is invalid ValueError is raised.
+        If the queue is full an OverFlowError is raised.
+        """
+        try:
+            if len(self.queues[priority]) >= 100:
+                raise OverflowError("Maximum queue size is 100")
+            self.queues[priority].append(data)
+        except IndexError:
+            raise ValueError("Valid priorities are 0, 1, and 2")
+
+    def dequeue(self) -> int:
+        """
+        Return the highest priority element in FIFO order.
+        If the queue is empty then an under flow exception is raised.
+        """
+        for queue in self.queues:
+            if queue:
+                return queue.pop(0)
+        raise UnderFlowError("All queues are empty")
+
+    def __str__(self) -> str:
+        return "\n".join(f"Priority {i}: {q}" for i, q in enumerate(self.queues))
+
+
+class ElementPriorityQueue:
+    """
+    Element Priority Queue is the same as Fixed Priority Queue except that the value of
+    the element itself is the priority. The rules for priorities are the same the as
+    Fixed Priority Queue.
+
+    >>> epq = ElementPriorityQueue()
+    >>> epq.enqueue(10)
+    >>> epq.enqueue(70)
+    >>> epq.enqueue(4)
+    >>> epq.enqueue(1)
+    >>> epq.enqueue(5)
+    >>> epq.enqueue(7)
+    >>> epq.enqueue(4)
+    >>> epq.enqueue(64)
+    >>> epq.enqueue(128)
+    >>> print(epq)
+    [10, 70, 4, 1, 5, 7, 4, 64, 128]
+    >>> epq.dequeue()
+    1
+    >>> epq.dequeue()
+    4
+    >>> epq.dequeue()
+    4
+    >>> epq.dequeue()
+    5
+    >>> epq.dequeue()
+    7
+    >>> epq.dequeue()
+    10
+    >>> print(epq)
+    [70, 64, 128]
+    >>> epq.dequeue()
+    64
+    >>> epq.dequeue()
+    70
+    >>> epq.dequeue()
+    128
+    >>> epq.dequeue()
+    Traceback (most recent call last):
+        ...
+    priority_queue_using_list.UnderFlowError: The queue is empty
+    >>> print(epq)
+    []
+    """
+
+    def __init__(self):
+        self.queue = []
+
+    def enqueue(self, data: int) -> None:
+        """
+        This function enters the element into the queue
+        If the queue is full an Exception is raised saying Over Flow!
+        """
+        if len(self.queue) == 100:
+            raise OverFlowError("Maximum queue size is 100")
+        self.queue.append(data)
+
+    def dequeue(self) -> int:
+        """
+        Return the highest priority element in FIFO order.
+        If the queue is empty then an under flow exception is raised.
+        """
+        if not self.queue:
+            raise UnderFlowError("The queue is empty")
+        else:
+            data = min(self.queue)
+            self.queue.remove(data)
+            return data
+
+    def __str__(self) -> str:
+        """
+        Prints all the elements within the Element Priority Queue
+        """
+        return str(self.queue)
+
+
+def fixed_priority_queue():
+    fpq = FixedPriorityQueue()
+    fpq.enqueue(0, 10)
+    fpq.enqueue(1, 70)
+    fpq.enqueue(0, 100)
+    fpq.enqueue(2, 1)
+    fpq.enqueue(2, 5)
+    fpq.enqueue(1, 7)
+    fpq.enqueue(2, 4)
+    fpq.enqueue(1, 64)
+    fpq.enqueue(0, 128)
+    print(fpq)
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+    print(fpq)
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+    print(fpq.dequeue())
+
+
+def element_priority_queue():
+    epq = ElementPriorityQueue()
+    epq.enqueue(10)
+    epq.enqueue(70)
+    epq.enqueue(100)
+    epq.enqueue(1)
+    epq.enqueue(5)
+    epq.enqueue(7)
+    epq.enqueue(4)
+    epq.enqueue(64)
+    epq.enqueue(128)
+    print(epq)
+    print(epq.dequeue())
+    print(epq.dequeue())
+    print(epq.dequeue())
+    print(epq.dequeue())
+    print(epq.dequeue())
+    print(epq)
+    print(epq.dequeue())
+    print(epq.dequeue())
+    print(epq.dequeue())
+    print(epq.dequeue())
+    print(epq.dequeue())
+
+
+if __name__ == "__main__":
+    fixed_priority_queue()
+    element_priority_queue()