Peek Front Element of Queue in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'peek' operation do in a queue?

It returns the front element of the queue without removing it, allowing you to see which element will be dequeued next.

beginner

In a queue implemented with an array, which index usually holds the front element?

The front element is usually at the index pointed to by the 'front' variable, which tracks the start of the queue.

beginner

What should the peek function return if the queue is empty?

It should indicate that the queue is empty, often by returning a special value like -1 or NULL, or by printing an error message.

intermediate

Why is peek operation in a queue considered O(1) time complexity?

Because it only accesses the front element directly without traversing or modifying the queue, so it takes constant time.

intermediate

Show the basic steps to peek the front element in a queue implemented with a linked list.

1. Check if the queue is empty (front pointer is NULL).
2. If not empty, return the data at the front node.
3. Do not remove the node.

What does the peek operation in a queue do?

A Returns the front element without removing it

B Removes the front element

C Adds an element to the rear

D Checks if the queue is full

Peek returns the front element without removing it, so you can see the next element to be dequeued.

If a queue is empty, what should peek return?

A An error or special value indicating empty

B The rear element

C The last element

D The size of the queue

Peek should indicate the queue is empty, often by returning -1 or NULL.

In an array-based queue, which variable usually points to the front element?

A rear

B capacity

C size

D front

The 'front' variable points to the index of the front element in the queue.

What is the time complexity of the peek operation in a queue?

A O(n)

B O(log n)

C O(1)

D O(n^2)

Peek accesses the front element directly, so it runs in constant time O(1).

Which of these is NOT a correct step in peeking the front element of a linked list queue?

A Check if front pointer is NULL

B Remove the front node

C Return data at front node

D Do not modify the queue

Peek should not remove the front node; it only returns its data.

Explain how to implement the peek operation in a queue using an array.

Think about the front index and empty condition.

Describe why peek operation is useful in queue data structures.

Consider when you want to see the next item without changing the queue.

Practice

1. What does the peek operation do in a queue?

easy

A. Returns the front element without removing it

B. Removes the front element

C. Adds an element to the back

D. Checks if the queue is empty

Solution

1. Step 1: Understand the peek operation

   The peek operation looks at the front element of the queue without changing the queue.

2. Step 2: Compare with other operations

   Removing is done by dequeue, adding by enqueue, and checking empty is a separate check.

3. Final Answer:

   Returns the front element without removing it -> Option A

4. Quick Check:

   Peek = front element without removal [OK]

Hint: Peek shows front without removing [OK]

Common Mistakes:

• Confusing peek with dequeue
• Thinking peek adds elements
• Assuming peek checks emptiness

2. Which of the following is the correct C syntax to peek the front element of a queue named q implemented with an array and front index front?

easy

A. int x = q[front];

B. int x = q[rear];

C. int x = q[front + 1];

D. int x = q[0];

Solution

1. Step 1: Identify front element in array queue

   The front element is at index front in the array q.

2. Step 2: Check other options

   rear points to the back, front + 1 is next element, q[0] may not be front if front moved.

3. Final Answer:

   int x = q[front]; -> Option A

4. Quick Check:

   Peek front = q[front] [OK]

Hint: Peek uses front index, not rear or zero [OK]

Common Mistakes:

• Using rear index instead of front
• Adding 1 to front index incorrectly
• Assuming front is always zero

3. What will be the output of the following C code snippet?

int q[5] = {10, 20, 30, 40, 50};
int front = 2;
printf("%d", q[front]);

medium

A. 10

B. 20

C. 30

D. 40

Solution

1. Step 1: Identify the element at index front

   Index 2 in array q is the third element, which is 30.

2. Step 2: Confirm output of printf

   Printing q[front] prints 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   q[2] = 30 [OK]

Hint: Array index starts at 0, so q[2] is third element [OK]

Common Mistakes:

• Confusing index 2 with value 20
• Printing q[front + 1] by mistake
• Assuming front is zero

4. Identify the error in this C code snippet that tries to peek the front element of a queue:

int q[3] = {5, 10, 15};
int front = 3;
printf("%d", q[front]);

medium

A. Queue array is empty

B. front index is out of bounds

C. printf syntax error

D. No error, code is correct

Solution

1. Step 1: Check front index validity

   Array q has indices 0 to 2. front = 3 is outside bounds.

2. Step 2: Understand consequences

   Accessing q[3] causes undefined behavior or runtime error.

3. Final Answer:

   front index is out of bounds -> Option B

4. Quick Check:

   front must be within 0 to 2 [OK]

Hint: Front index must be inside array bounds [OK]

Common Mistakes:

• Ignoring array bounds
• Assuming front can be equal to array size
• Thinking printf syntax is wrong

5. Given a circular queue implemented with array q[5], front index front = 4, and rear index rear = 2, which expression correctly peeks the front element?

hard

A. q[front]

B. q[rear]

C. q[(rear + 1) % 5]

D. q[(front + 1) % 5]

Solution

1. Step 1: Understand circular queue front

   In a circular queue, the front element is at index (front + 1) % size, because front points to the last dequeued position.

2. Step 2: Check modulo usage

   Modulo is used to wrap around the array indices correctly.

3. Final Answer:

   q[(front + 1) % 5] -> Option D

4. Quick Check:

   Peek front = q[(front + 1) % size] in circular queue [OK]

Hint: In circular queue, front points behind the front element [OK]

Common Mistakes:

• Using q[front] directly in circular queue
• Confusing rear with front
• Assuming front is always zero