queues

Implementing a queue

How do you program a queue?

• A queue is a linear ADT that follows the First In, First Out (FIFO) principle:

  • Enqueue: Add item to the back

  • Dequeue: Remove item from the front

  • Peek/front: Look at the front item without removing

  • isEmpty / isFull: Check queue status

• Assuming a record-based circular queue implementation:

Enqueue

PROCEDURE enqueue(q : REF Queue, item : STRING) IF NOT isFull(q) THEN q.rear ← (q.rear + 1) MOD q.capacity q.items[q.rear] ← item q.size ← q.size + 1 ELSE OUTPUT "Queue is full" ENDIF
ENDPROCEDURE

Dequeue

FUNCTION dequeue(q : REF Queue) RETURNS STRING IF NOT isEmpty(q) THEN DECLARE value : STRING value ← q.items[q.front] q.front ← (q.front + 1) MOD q.capacity q.size ← q.size - 1 RETURN value ELSE RETURN "Queue is empty" ENDIF
ENDFUNCTION

isEmpty / isFull

FUNCTION isEmpty(q : Queue) RETURNS BOOLEAN RETURN q.size = 0
ENDFUNCTION FUNCTION isFull(q : Queue) RETURNS BOOLEAN RETURN q.size = q.capacity
ENDFUNCTION

Find (linear search)

FUNCTION find(q : Queue, target : STRING) RETURNS BOOLEAN DECLARE i : INTEGER DECLARE pos : INTEGER pos ← q.front FOR i ← 1 TO q.size IF q.items[pos] = target THEN RETURN TRUE ENDIF pos ← (pos + 1) MOD q.capacity NEXT i RETURN FALSE
ENDFUNCTION

Python

from collections import deque class Queue: def __init__(self, capacity): self.items = deque() self.capacity = capacity def isEmpty(self): return len(self.items) == 0 def isFull(self): return len(self.items) == self.capacity def enqueue(self, item): if not self.isFull(): self.items.append(item) else: print("Queue is full") def dequeue(self): if not self.isEmpty(): return self.items.popleft() else: return "Queue is empty" def peek(self): if not self.isEmpty(): return self.items[0] else: return None def find(self, target): return target in self.items # Example usage
q = Queue(5)
q.enqueue("Alice")
q.enqueue("Bob")
q.enqueue("Charlie")
print("Front item:", q.peek())
print("Popped:", q.dequeue())
print("Is 'Bob' in queue?", q.find("Bob"))

Java

import java.util.LinkedList;
import java.util.Queue; public class QueueExample { public static void main(String[] args) { Queue<String> q = new LinkedList<>(); // Enqueue q.offer("Alice"); q.offer("Bob"); q.offer("Charlie"); // Peek System.out.println("Front item: " + q.peek()); // Dequeue System.out.println("Popped: " + q.poll()); // Find System.out.println("Is 'Bob' in queue? " + q.contains("Bob")); }
}