LPOP and RPOP for removal in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of LPOP on a list?

Given a Redis list mylist with elements ["a", "b", "c", "d"], what does the command LPOP mylist return?

Redis

LPUSH mylist a b c d
LPOP mylist

A "a"

B "d"

C "b"

D nil

💡 Hint

LPOP removes and returns the first element from the left side of the list.

LPOP removes the leftmost element of the list. Since the list is ["a", "b", "c", "d"], the first element is "a".

❓ query_result

intermediate

What does RPOP return from a list?

If a Redis list colors contains ["red", "green", "blue"], what is the result of RPOP colors?

Redis

RPUSH colors red green blue
RPOP colors

A nil

B "red"

C "green"

D "blue"

💡 Hint

RPOP removes and returns the last element from the right side of the list.

RPOP removes the rightmost element. The list is ["red", "green", "blue"], so the last element is "blue".

📝 Syntax

advanced

Which command syntax correctly removes the leftmost element?

Which of the following Redis commands correctly removes and returns the leftmost element of the list tasks?

A REMOVE LEFT tasks

B LPOP tasks

C RPOP tasks

D POP tasks LEFT

💡 Hint

The command to remove from the left is LPOP.

LPOP tasks is the correct syntax. RPOP removes from the right. The other commands are invalid in Redis.

❓ query_result

advanced

What happens when LPOP is called on an empty list?

If the Redis list queue is empty, what does LPOP queue return?

Redis

DEL queue
LPOP queue

A Error

B "" (empty string)

C nil

D 0

💡 Hint

LPOP returns nil if the list does not exist or is empty.

When the list is empty or missing, LPOP returns nil to indicate no element was removed.

🧠 Conceptual

expert

How to atomically remove and get multiple elements from the left?

Which Redis command or combination can atomically remove and return the first 3 elements from the left of a list mylist?

A Use LPOP mylist 3

B Use RPOP mylist 3

C Use LRANGE mylist 0 2 followed by LTRIM mylist 3 -1

D Use LPOP mylist three times in a transaction

💡 Hint

Recent Redis versions support a count argument for LPOP.

Starting Redis 6.2, LPOP mylist 3 atomically removes and returns up to 3 elements from the left.

Option A is not atomic and may cause race conditions. Option A is atomic but less efficient. Option A removes from the right.

Practice

1. What does the Redis command LPOP do to a list?

easy

A. Removes and returns the first item from the list

B. Removes and returns the last item from the list

C. Adds an item to the start of the list

D. Adds an item to the end of the list

Solution

1. Step 1: Understand LPOP command

   LPOP removes the first (leftmost) item from a Redis list.

2. Step 2: Compare with other options

   Only Removes and returns the first item from the list correctly describes removal of the first item; others describe adding or removing last item.

3. Final Answer:

   Removes and returns the first item from the list -> Option A

4. Quick Check:

   LPOP = Remove first item [OK]

Hint: LPOP = Left POP (first item) [OK]

Common Mistakes:

• Confusing LPOP with RPOP
• Thinking LPOP adds items
• Assuming LPOP removes last item

2. Which of the following is the correct syntax to remove the last item from a Redis list named mylist?

easy

A. RPOP mylist

B. LPOP mylist

C. POP mylist

D. REMOVE mylist

Solution

1. Step 1: Identify command for last item removal

   RPOP removes the last (rightmost) item from a Redis list.

2. Step 2: Check syntax correctness

   RPOP mylist is the correct syntax; other options are invalid or remove first item.

3. Final Answer:

   RPOP mylist -> Option A

4. Quick Check:

   RPOP = Remove last item [OK]

Hint: RPOP = Right POP (last item) [OK]

Common Mistakes:

• Using LPOP instead of RPOP
• Using non-existent POP command
• Using REMOVE which is invalid

3. Given a Redis list colors with items ["red", "green", "blue"], what will be the result of LPOP colors followed by RPOP colors?

medium

A. "green" then "blue"

B. ["red", "blue"]

C. "red" then "blue"

D. ["green"]

Solution

1. Step 1: Apply LPOP on the list

   LPOP colors removes and returns the first item: "red". Remaining list: ["green", "blue"]

2. Step 2: Apply RPOP on the updated list

   RPOP colors removes and returns the last item: "blue". Remaining list: ["green"]

3. Final Answer:

   "red" then "blue" -> Option C

4. Quick Check:

   LPOP = "red", RPOP = "blue" [OK]

Hint: LPOP removes first, RPOP removes last [OK]

Common Mistakes:

• Thinking LPOP returns the whole list
• Confusing order of removal
• Expecting list output instead of removed items

4. You run the command LPOP mylist but get null as a result. What is the most likely reason?

medium

A. LPOP command syntax is incorrect

B. The list mylist does not exist or is empty

C. LPOP removes the last item, not the first

D. Redis server is down

Solution

1. Step 1: Understand LPOP return behavior

   If the list is empty or does not exist, LPOP returns null.

2. Step 2: Check other options

   Syntax is correct, LPOP removes first item, and server down would cause connection errors, not null.

3. Final Answer:

   The list mylist does not exist or is empty -> Option B

4. Quick Check:

   LPOP returns null if list empty [OK]

Hint: Null means empty or missing list [OK]

Common Mistakes:

• Assuming syntax error causes null
• Confusing LPOP with RPOP behavior
• Thinking server down returns null

5. You have a Redis list tasks with items ["task1", "task2", "task3", "task4"]. You want to remove the first and last tasks and get their names. Which sequence of commands will do this correctly?

hard

A. RPOP tasks; RPOP tasks

B. RPOP tasks; LPOP tasks

C. LPOP tasks; LPOP tasks

D. LPOP tasks; RPOP tasks

Solution

1. Step 1: Remove first task with LPOP

   LPOP tasks removes and returns "task1" (first item).

2. Step 2: Remove last task with RPOP

   RPOP tasks removes and returns "task4" (last item).

3. Final Answer:

   LPOP tasks; RPOP tasks -> Option D

4. Quick Check:

   First and last removed by LPOP then RPOP [OK]

Hint: LPOP first, RPOP last to remove both ends [OK]

Common Mistakes:

• Reversing command order
• Using same command twice
• Expecting one command to remove both ends