LLEN for list length in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of LLEN on a non-empty list?

Given a Redis list stored with key fruits containing ["apple", "banana", "cherry"], what will the command LLEN fruits return?

Redis

LPUSH fruits apple
LPUSH fruits banana
LPUSH fruits cherry
LLEN fruits

A 3

B 0

C 1

D Error: key does not exist

💡 Hint

LLEN returns the number of items in the list stored at the given key.

The list fruits has three items after the LPUSH commands, so LLEN returns 3.

❓ query_result

intermediate

What does LLEN return for a missing key?

If you run LLEN vegetables on a Redis key vegetables that does not exist, what is the output?

Redis

LLEN vegetables

A null

B 0

C Error: key does not exist

D -1

💡 Hint

LLEN returns zero if the key does not exist.

In Redis, LLEN returns 0 for keys that do not exist, treating them as empty lists.

📝 Syntax

advanced

Which LLEN command syntax is correct?

Which of the following Redis commands correctly returns the length of the list stored at key mylist?

A LLEN(mylist)

B LLEN:mylist

C LLEN mylist

D LLEN mylist 0

💡 Hint

Redis commands do not use parentheses or colons in this context.

The correct syntax is LLEN key. Options B, C, and D are invalid syntax.

❓ optimization

advanced

Efficiently checking if a Redis list is empty

You want to check if the Redis list tasks is empty without retrieving all elements. Which command is the most efficient way to do this?

A LLEN tasks

B LRANGE tasks 0 -1

C LPOP tasks

D EXISTS tasks

💡 Hint

Think about which command returns the list length without transferring all data.

LLEN returns the length of the list efficiently. LRANGE returns all elements, which is costly. LPOP removes an element, changing the list. EXISTS only checks if the key exists, not if the list is empty.

🧠 Conceptual

expert

Understanding LLEN behavior with different data types

What happens if you run LLEN on a Redis key user:1 that stores a string value instead of a list?

Redis

SET user:1 "John"
LLEN user:1

A Returns 1

B Returns 0

C Returns null

D Returns an error about wrong data type

💡 Hint

LLEN only works on list data types.

If the key stores a string, LLEN returns a Redis error about wrong data type because it expects a list.