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.