Peek Front Element of Queue in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to peek the front element of the queue.

DSA C++

int frontElement = myQueue.[1]();

Drag options to blanks, or click blank then click option'

Apop

Bpush

Cback

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of front() causes the element to be removed.

Using back() returns the last element, not the front.

The front() function returns the element at the front of the queue without removing it.

2fill in blank

medium

Complete the code to check if the queue is empty before peeking the front element.

DSA C++

if (!myQueue.[1]()) {
    int frontElement = myQueue.front();
}

Drag options to blanks, or click blank then click option'

Asize

Bempty

Cfront

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using front() as a condition causes errors because it returns an element, not a boolean.

Using pop() removes elements and is not for checking emptiness.

The empty() function checks if the queue has no elements.

3fill in blank

hard

Fix the error in the code to correctly peek the front element of the queue.

DSA C++

std::cout << myQueue.[1]() << std::endl;

Drag options to blanks, or click blank then click option'

Afront

Bpop

Cpush

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of front() causes the element to be removed.

Using push() or empty() here causes compilation errors.

Use front() to access the front element without removing it. pop() removes the element, so it cannot be used here.

4fill in blank

hard

Fill both blanks to safely peek the front element only if the queue is not empty.

DSA C++

if (!myQueue.[1]()) {
    int frontElement = myQueue.[2]();
}

Drag options to blanks, or click blank then click option'

Aempty

Bpop

Cfront

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of front() removes the element.

Not checking if the queue is empty can cause runtime errors.

First check if the queue is empty using empty(). Then use front() to get the front element safely.

5fill in blank

hard

Fill all three blanks to print the front element only if the queue is not empty, else print "Empty".

DSA C++

if (!myQueue.[1]()) {
    std::cout << myQueue.[2]() << std::endl;
} else {
    std::cout << [3] << std::endl;
}

Drag options to blanks, or click blank then click option'

Aempty

Bfront

C"Empty"

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of front() removes the element.

Printing Empty without quotes causes compilation errors.

Check if the queue is empty with empty(). If not empty, print the front element using front(). Otherwise, print the string "Empty".

Practice

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

easy

A. Removes the front element from the queue

B. Returns the front element without removing it

C. Adds an element to the back of the queue

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

   Unlike dequeue, peek does not remove the element; it only returns it.

3. Final Answer:

   Returns the front element without removing it -> Option B

4. Quick Check:

   peek() = front element [OK]

Hint: Peek shows front without removal [OK]

Common Mistakes:

• Confusing peek with dequeue
• Thinking peek adds elements
• Assuming peek empties queue

2. Which of the following is the correct syntax to peek the front element of a std::queue<int> named q in C++?

easy

A. q.front();

B. q.peek();

C. q.top();

D. q.getFront();

Solution

1. Step 1: Recall C++ queue methods

   In C++, std::queue uses front() to access the front element.

2. Step 2: Check other options

   peek() and getFront() are not valid methods; top() is for stacks, not queues.

3. Final Answer:

   q.front(); -> Option A

4. Quick Check:

   std::queue front() method = q.front() [OK]

Hint: Use front() to peek in std::queue [OK]

Common Mistakes:

• Using peek() which doesn't exist
• Using top() which is for stacks
• Trying getFront() which is invalid

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

#include <iostream>
#include <queue>

int main() {
    std::queue<int> q;
    q.push(10);
    q.push(20);
    q.push(30);
    std::cout << q.front() << std::endl;
    q.pop();
    std::cout << q.front() << std::endl;
    return 0;
}

medium

A. 10\n20

B. 10\n10

C. 20\n30

D. 30\n20

Solution

1. Step 1: Trace initial queue state

   After pushes, queue is [10 (front), 20, 30].

2. Step 2: Output front and pop

   First output is 10 (front). Then pop removes 10, queue becomes [20 (front), 30]. Next output is 20.

3. Final Answer:

   10\n20 -> Option A

4. Quick Check:

   First front=10, after pop front=20 [OK]

Hint: Pop removes front; peek shows new front [OK]

Common Mistakes:

• Not updating front after pop
• Printing popped element
• Confusing push order

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

#include <queue>
#include <iostream>

int main() {
    std::queue<int> q;
    q.push(5);
    std::cout << q.peek() << std::endl;
    return 0;
}

medium

A. q.push() requires two arguments

B. Queue is empty before peek

C. Using q.peek() which is not a valid method

D. Missing #include <vector>

Solution

1. Step 1: Check method used to peek

   The code uses q.peek(), but std::queue does not have a peek() method.

2. Step 2: Identify correct method

   The correct method to access the front element is q.front().

3. Final Answer:

   Using q.peek() which is not a valid method -> Option C

4. Quick Check:

   std::queue peek method = front() [OK]

Hint: Use front(), not peek(), to access front element [OK]

Common Mistakes:

• Using peek() method
• Assuming peek() exists
• Ignoring compiler errors

5. Given a queue q containing integers, write a C++ function int peekFrontOrDefault(std::queue<int>& q, int defaultVal) that returns the front element if the queue is not empty, or defaultVal if it is empty. What is the correct implementation?

hard

A. return q.front();

B. if (q.empty()) return q.front(); else return defaultVal;

C. if (q.size() > 1) return q.front(); else return defaultVal;

D. if (!q.empty()) return q.front(); else return defaultVal;

Solution

1. Step 1: Check if queue is empty

   Use q.empty() to verify if queue has elements.

2. Step 2: Return front or default value

   If not empty, return q.front(); otherwise, return defaultVal.

3. Final Answer:

   if (!q.empty()) return q.front(); else return defaultVal; -> Option D

4. Quick Check:

   Check empty before front() to avoid errors [OK]

Hint: Always check empty before front() to avoid errors [OK]

Common Mistakes:

• Not checking empty before front()
• Wrong condition on size
• Returning front when empty