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Queue operations: Class 12 practice

A queue is first-in, first-out (FIFO): enqueue adds at the rear and dequeue removes from the front.. Read the method, follow the three worked examples, then try the six questions.

What you should know first

  • Lists

The idea behind queue operations

A queue is first-in, first-out (FIFO): enqueue adds at the rear and dequeue removes from the front.

Step-by-step method

  1. Start with the given queue (front at the left).
  2. Apply operations in order.
  3. Record dequeued values and the final queue.

Worked example 1 (Basic)

Starting with an empty queue, perform: enqueue(77), enqueue(37), enqueue(5), enqueue(30), dequeue(), enqueue(94). What is removed and what is left (front → rear)?

  1. Removed: 77
  2. Left: [37, 5, 30, 94]
  3. Answer: Removed 77; left [37, 5, 30, 94]

Worked example 2 (Standard)

Starting with an empty queue, perform: enqueue(86), dequeue(), enqueue(70), dequeue(), enqueue(1), dequeue(), enqueue(17), dequeue(). What is removed and what is left (front → rear)?

  1. Removed: 86, 70, 1, 17
  2. Left: []
  3. Answer: Removed 86, 70, 1, 17; left []

Worked example 3 (Challenge)

Starting with an empty queue, perform: enqueue(32), enqueue(8), enqueue(41), dequeue(), enqueue(17), enqueue(1), enqueue(96), enqueue(57), dequeue(), enqueue(92). What is removed and what is left (front → rear)?

  1. Removed: 32, 8
  2. Left: [41, 17, 1, 96, 57, 92]
  3. Answer: Removed 32, 8; left [41, 17, 1, 96, 57, 92]

Common mistakes and misconceptions

  • Removing from the rear
  • Mixing up front and rear

Practice questions

  1. (Basic) Starting with an empty queue, perform: enqueue(69), enqueue(35), enqueue(89), enqueue(60), enqueue(74), enqueue(99). What is removed and what is left (front → rear)?
  2. (Basic) Starting with an empty queue, perform: enqueue(3), enqueue(67), dequeue(), enqueue(9), enqueue(60), enqueue(31). What is removed and what is left (front → rear)?
  3. (Standard) Starting with an empty queue, perform: enqueue(65), enqueue(85), enqueue(11), enqueue(3), enqueue(14), dequeue(), enqueue(95), dequeue(). What is removed and what is left (front → rear)?
  4. (Standard) Starting with an empty queue, perform: enqueue(3), dequeue(), enqueue(90), dequeue(), enqueue(59), dequeue(), enqueue(42), enqueue(99). What is removed and what is left (front → rear)?
  5. (Challenge) Starting with an empty queue, perform: enqueue(25), dequeue(), enqueue(41), enqueue(49), dequeue(), enqueue(24), enqueue(83), dequeue(), dequeue(), enqueue(2). What is removed and what is left (front → rear)?
  6. (Challenge) Starting with an empty queue, perform: enqueue(41), enqueue(34), dequeue(), enqueue(21), enqueue(3), enqueue(92), dequeue(), enqueue(75), dequeue(), enqueue(43). What is removed and what is left (front → rear)?

Answer key with full working

  • Q1: Removed: none → Left: [69, 35, 89, 60, 74, 99] → Answer: Removed none; left [69, 35, 89, 60, 74, 99]
  • Q2: Removed: 3 → Left: [67, 9, 60, 31] → Answer: Removed 3; left [67, 9, 60, 31]
  • Q3: Removed: 65, 85 → Left: [11, 3, 14, 95] → Answer: Removed 65, 85; left [11, 3, 14, 95]
  • Q4: Removed: 3, 90, 59 → Left: [42, 99] → Answer: Removed 3, 90, 59; left [42, 99]
  • Q5: Removed: 25, 41, 49, 24 → Left: [83, 2] → Answer: Removed 25, 41, 49, 24; left [83, 2]
  • Q6: Removed: 41, 34, 21 → Left: [3, 92, 75, 43] → Answer: Removed 41, 34, 21; left [3, 92, 75, 43]

Syllabus note

This skill is listed under "Data Structures" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Related practice and notes

Frequently asked questions

Which chapter is queue operations in?

"Data Structures", Class 12 Computer Science (NCERT/CBSE). Other boards may place it differently.

How were the answers checked?

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

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