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

Recall & Review

beginner

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

It shows the front element of the queue without removing it, so you can see who is next without changing the queue.

beginner

In a queue, which end is the 'front'?

The front is the end where elements are removed (dequeued). It's like the front of a line where the first person is served.

intermediate

Why is peeking useful in a queue?

Peeking lets you check the next item to be processed without changing the queue, so you can decide what to do next without losing data.

intermediate

What happens if you peek an empty queue?

Usually, you get a special value like nil or an error because there is no front element to show.

beginner

Show a simple Go code snippet to peek the front element of a queue implemented as a slice.

if len(queue) > 0 { front := queue[0] fmt.Println("Front element:", front) } else { fmt.Println("Queue is empty") }

What does the peek operation in a queue do?

A Checks if the queue is empty

B Removes the front element

C Adds an element to the back

D Returns the front element without removing it

Peek shows the front element without removing it, unlike dequeue which removes it.

If a queue is empty, what should peek return?

A The last element

B An error or nil value

C The first element added

D The size of the queue

Peeking an empty queue usually returns an error or nil because no front element exists.

In Go, how do you access the front element of a queue stored as a slice named 'queue'?

A queue[0]

B queue[len(queue)-1]

C queue.front()

D queue.peek()

The front element is at index 0 in a slice representing a queue.

Which operation changes the queue by removing the front element?

A Peek

B Enqueue

C Dequeue

D IsEmpty

Dequeue removes the front element, changing the queue.

Why might you want to peek instead of dequeue?

A To check the next element without removing it

B To add an element

C To empty the queue

D To reverse the queue

Peeking lets you see the next element without removing it, useful for decisions.

Explain how the peek operation works in a queue and why it is useful.

Think about checking the next person in line without letting them go.

Write a simple Go code snippet to peek the front element of a queue implemented as a slice, including handling the empty queue case.

Use if len(queue) > 0 before accessing queue[0].

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 from the queue

C. Adds an element to the back of the queue

D. Checks if the queue is empty

Solution

1. Step 1: Understand the queue operations

   A queue supports adding elements at the back and removing from the front.

2. Step 2: Define Peek operation

   Peek returns the front element without removing it, unlike dequeue which removes it.

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 removal, unlike dequeue [OK]

Common Mistakes:

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

2. Which of the following is the correct Go syntax to peek the front element of a queue implemented as a slice named queue?

easy

A. front := queue[len(queue)-1]

B. front := queue[len(queue)]

C. front := queue[0]

D. front := queue.pop()

Solution

1. Step 1: Understand Go slice indexing

   Go slices are zero-indexed, so the first element is at index 0.

2. Step 2: Identify front element access

   The front element of a queue is the first element, so queue[0] gives it.

3. Final Answer:

   front := queue[0] -> Option C

4. Quick Check:

   First element index = 0 [OK]

Hint: Front element is always at index 0 in slice queue [OK]

Common Mistakes:

• Using len(queue) which is out of range
• Using last element instead of first
• Using pop() which is not a Go slice method

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

queue := []int{10, 20, 30}
front := queue[0]
fmt.Println(front)

medium

A. runtime error

B. 10

C. 30

D. 20

Solution

1. Step 1: Initialize queue slice

   The queue slice contains elements [10, 20, 30].

2. Step 2: Access front element

   front := queue[0] assigns 10 to front.

3. Step 3: Print front

   fmt.Println(front) prints 10.

4. Final Answer:

   10 -> Option B

5. Quick Check:

   queue[0] = 10 [OK]

Hint: queue[0] gives first element value [OK]

Common Mistakes:

• Printing queue[1] instead of queue[0]
• Expecting last element output
• Thinking it causes runtime error

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

var queue []int
front := queue[0]
fmt.Println(front)

medium

A. No error, prints 0

B. Prints nil

C. Compile-time syntax error

D. Runtime panic: index out of range

Solution

1. Step 1: Check queue initialization

   queue is declared but not initialized, so length is 0.

2. Step 2: Accessing queue[0]

   Accessing queue[0] causes runtime panic because slice is empty.

3. Final Answer:

   Runtime panic: index out of range -> Option D

4. Quick Check:

   Empty slice access causes panic [OK]

Hint: Empty slice access causes runtime panic [OK]

Common Mistakes:

• Assuming default zero value is returned
• Expecting compile-time error
• Thinking it prints nil

5. Given a queue implemented as a slice queue := []int{5, 10, 15}, write the correct Go code snippet to safely peek the front element and handle the empty queue case by printing "Empty" if no elements exist.

hard

A. if len(queue) > 0 {
fmt.Println(queue[0])
} else {
fmt.Println("Empty")
}

B. fmt.Println(queue[0])

C. if queue != nil {
fmt.Println(queue[0])
} else {
fmt.Println("Empty")
}

D. if len(queue) == 0 {
fmt.Println(queue[0])
} else {
fmt.Println("Empty")
}

Solution

1. Step 1: Check queue length before access

   Use len(queue) > 0 to ensure queue has elements.

2. Step 2: Print front or "Empty"

   If queue has elements, print queue[0]; else print "Empty".

3. Final Answer:

   if len(queue) > 0 { fmt.Println(queue[0]) } else { fmt.Println("Empty") } -> Option A

4. Quick Check:

   Check length before access to avoid panic [OK]

Hint: Always check length before peeking front [OK]

Common Mistakes:

• Not checking length before access
• Checking nil instead of length
• Reversing if-else logic