Dequeue Operation in DSA C - Build from Scratch

Dequeue Operation

📖 Scenario: Imagine you are managing a line of customers waiting for service. Sometimes customers leave from the front or the back of the line. You want to keep track of this line using a data structure called a dequeue (double-ended queue).

🎯 Goal: You will create a dequeue using an array, set its size, perform removal operations from both ends, and then print the current state of the dequeue.

📋 What You'll Learn

Create an array called dequeue with exactly these 5 integers: 10, 20, 30, 40, 50

Create an integer variable called size and set it to 5

Remove one element from the front and one element from the back of the dequeue

Print the remaining elements of the dequeue separated by spaces

💡 Why This Matters

🌍 Real World

Dequeue operations are useful in real-world scenarios like managing customer lines, undo-redo features in software, and task scheduling where elements can be added or removed from both ends.

💼 Career

Understanding dequeue operations is important for software developers working with data structures, especially in system design, performance optimization, and real-time processing.

1

Create the initial dequeue array

Create an integer array called dequeue with these exact values: 10, 20, 30, 40, 50

DSA C

// Your code here

Hint

Use array initialization syntax with the exact values inside curly braces.

2

Set the size of the dequeue

Create an integer variable called size and set it to 5

DSA C

int dequeue[5] = {10, 20, 30, 40, 50};
// Your code here

Hint

Declare an integer variable and assign it the value 5.

3

Remove elements from front and back

Remove one element from the front by increasing the start index and one element from the back by decreasing size. Use an integer variable start initialized to 0 to track the front index. Increase start by 1 and decrease size by 1 to remove elements from both ends.

DSA C

int dequeue[5] = {10, 20, 30, 40, 50};
int size = 5;
// Your code here

Hint

Use a variable start to track the front index and update it to remove the front element. Decrease size to remove the last element.

4

Print the current dequeue elements

Use a for loop with an integer variable i from start to size - 1 to print each element of dequeue separated by spaces.

DSA C

int dequeue[5] = {10, 20, 30, 40, 50};
int size = 5;
int start = 0;
start = start + 1;
size = size - 1;
// Your code here

Hint

Use a for loop to print elements from start to size - 1 with a space after each number.

Practice

1. What is a dequeue in data structures?

easy

A. A double-ended queue allowing insertion and deletion at both ends

B. A queue where elements are inserted only at the front

C. A stack that allows only last-in-first-out operations

D. A linked list with only one pointer per node

Solution

1. Step 1: Understand the definition of dequeue

   A dequeue is a data structure where elements can be added or removed from both front and rear ends.

2. Step 2: Compare options with definition

   A double-ended queue allowing insertion and deletion at both ends correctly describes a dequeue. Options B, C, and D describe other data structures.

3. Final Answer:

   A double-ended queue allowing insertion and deletion at both ends -> Option A

4. Quick Check:

   Dequeue = double-ended queue [OK]

Hint: Dequeue means double-ended queue, both ends usable [OK]

Common Mistakes:

• Confusing dequeue with stack or simple queue
• Thinking insertion is only at one end
• Mixing dequeue with linked list structure

2. Which of the following is the correct syntax to insert an element 10 at the front of a dequeue named dq in C?

easy

A. dq.insertFront(10);

B. insertFront(dq, 10);

C. dq->insertFront(10);

D. insertFront(10, dq);

Solution

1. Step 1: Identify typical C function call syntax

   In C, functions are called with function_name(arguments). Here, insertFront is a function taking dequeue and value.

2. Step 2: Match options with C syntax

   insertFront(dq, 10); matches C style: insertFront(dq, 10);. Options B and C use object syntax (C++ style). insertFront(10, dq); reverses argument order.

3. Final Answer:

   insertFront(dq, 10); -> Option B

4. Quick Check:

   C function call = insertFront(dq, 10) [OK]

Hint: C uses function_name(dequeue, value) syntax [OK]

Common Mistakes:

• Using object method syntax in C
• Swapping argument order
• Using arrow operator without pointer

3. Consider the following C code for dequeue operations:

initialize dequeue dq
insertRear(dq, 5);
insertFront(dq, 3);
insertRear(dq, 7);
deleteFront(dq);
insertFront(dq, 2);
deleteRear(dq);

What is the final state of the dequeue (front to rear)?

medium

A. 2 -> 3 -> null

B. 3 -> 5 -> 7 -> null

C. 2 -> 5 -> null

D. 5 -> 7 -> null

Solution

1. Step 1: Trace each operation step-by-step

   Start empty: [] insertRear 5: [5] insertFront 3: [3,5] insertRear 7: [3,5,7] deleteFront: remove 3 -> [5,7] insertFront 2: [2,5,7] deleteRear: remove 7 -> [2,5]

2. Step 2: Write final dequeue state front to rear

   Final state is 2 at front, then 5, then null.

3. Final Answer:

   2 -> 5 -> null -> Option C

4. Quick Check:

   Trace ops carefully = 2 -> 5 [OK]

Hint: Track each insert/delete step in order [OK]

Common Mistakes:

• Forgetting which end deleteFront/deleteRear affects
• Mixing order of insertFront and insertRear
• Skipping an operation

4. The following code snippet for dequeue operations has a bug. What is the error?

initialize dequeue dq
insertFront(dq, 10);
deleteRear(dq);
deleteRear(dq);

medium

A. Deleting from rear when dequeue is empty causes underflow

B. insertFront syntax is incorrect

C. Cannot deleteRear twice in a row

D. Dequeue must be initialized with size

Solution

1. Step 1: Analyze operations and dequeue state

   Start empty: [] insertFront 10: [10] deleteRear: removes 10 -> [] deleteRear: tries to remove from empty dequeue

2. Step 2: Identify error from empty dequeue deletion

   Deleting from empty dequeue causes underflow error. This is the bug.

3. Final Answer:

   Deleting from rear when dequeue is empty causes underflow -> Option A

4. Quick Check:

   Deleting empty dequeue = underflow error [OK]

Hint: Check if dequeue is empty before delete [OK]

Common Mistakes:

• Ignoring empty dequeue before delete
• Assuming multiple deletes always safe
• Confusing insert syntax with error

5. You have a dequeue initially empty. You perform these operations:

insertRear(dq, 1);
insertRear(dq, 2);
insertFront(dq, 3);
deleteRear(dq);
insertFront(dq, 4);
deleteFront(dq);
insertRear(dq, 5);

What is the final state of the dequeue from front to rear?

hard

A. 4 -> 3 -> 1 -> null

B. 1 -> 5 -> null

C. 4 -> 1 -> 5 -> null

D. 3 -> 1 -> 5 -> null

Solution

1. Step 1: Execute each operation stepwise

   Start: [] insertRear 1: [1] insertRear 2: [1,2] insertFront 3: [3,1,2] deleteRear: remove 2 -> [3,1] insertFront 4: [4,3,1] deleteFront: remove 4 -> [3,1] insertRear 5: [3,1,5]

2. Step 2: Write final dequeue state front to rear

   Final state is 3 at front, then 1, then 5, then null.

3. Final Answer:

   3 -> 1 -> 5 -> null -> Option D

4. Quick Check:

   Trace carefully = 3 -> 1 -> 5 [OK]

Hint: Write state after each operation [OK]

Common Mistakes:

• Deleting wrong end element
• Mixing insertFront and insertRear order
• Skipping delete operations