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.

Practice

1. What does the Redis command LLEN mylist do?

easy

A. Deletes the list named 'mylist'.

B. Returns the number of elements in the list named 'mylist'.

C. Adds an element to the list named 'mylist'.

D. Returns the first element of the list named 'mylist'.

Solution

1. Step 1: Understand the LLEN command

   LLEN returns the length (number of elements) of a Redis list.

2. Step 2: Apply LLEN to 'mylist'

   Using LLEN on 'mylist' returns how many items it contains.

3. Final Answer:

   Returns the number of elements in the list named 'mylist'. -> Option B

4. Quick Check:

   LLEN = list length [OK]

Hint: LLEN always gives list size count quickly [OK]

Common Mistakes:

• Confusing LLEN with commands that modify the list
• Thinking LLEN returns list elements instead of count
• Assuming LLEN deletes or adds items

2. Which of the following is the correct syntax to get the length of a Redis list named 'tasks'?

easy

A. LISTLEN tasks

B. LEN tasks

C. LENGTH tasks

D. LLEN tasks

Solution

1. Step 1: Recall Redis list length command syntax

   The correct command to get list length is LLEN key.

2. Step 2: Match syntax with 'tasks'

   Using LLEN tasks correctly queries the length of the 'tasks' list.

3. Final Answer:

   LLEN tasks -> Option D

4. Quick Check:

   LLEN syntax = LLEN key [OK]

Hint: LLEN is the only correct Redis list length command [OK]

Common Mistakes:

• Using LEN or LENGTH which are invalid Redis commands
• Adding extra words like LISTLEN
• Confusing with other data structure commands

3. Given the following Redis commands executed in order:

LPUSH fruits apple
LPUSH fruits banana
LPUSH fruits cherry
LLEN fruits

What will be the output of LLEN fruits?

medium

A. 3

B. 0

C. 1

D. Error

Solution

1. Step 1: Count elements added to 'fruits'

   Three LPUSH commands add 'apple', 'banana', and 'cherry' to the list.

2. Step 2: Use LLEN to get list length

   LLEN returns the total number of elements, which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 items pushed, LLEN = 3 [OK]

Hint: Count LPUSH commands to find LLEN result [OK]

Common Mistakes:

• Assuming LLEN returns 0 if list exists
• Confusing LPUSH order with count
• Expecting an error instead of a number

4. You run LLEN mylist but get an error: "WRONGTYPE Operation against a key holding the wrong kind of value". What is the most likely cause?

medium

A. 'mylist' is not a list but another data type like string or set.

B. Redis server is down.

C. The LLEN command is misspelled.

D. The key 'mylist' does not exist.

Solution

1. Step 1: Understand the error message

   "WRONGTYPE" means the key exists but is not a list type.

2. Step 2: Identify cause for LLEN error

   LLEN only works on lists; if 'mylist' is a string or set, error occurs.

3. Final Answer:

   'mylist' is not a list but another data type like string or set. -> Option A

4. Quick Check:

   Wrong type error = key not a list [OK]

Hint: Check key type before LLEN to avoid WRONGTYPE error [OK]

Common Mistakes:

• Assuming key absence causes WRONGTYPE error
• Thinking LLEN syntax error causes this
• Ignoring Redis server status

5. You have a Redis list 'orders' that may or may not exist. You want to check if it has at least 5 items before processing. Which Redis command sequence is best to do this efficiently?

hard

A. Use EXISTS orders to check existence, then LLEN orders.

B. Use LRANGE orders 0 4 and count returned items.

C. Use LLEN orders and check if result >= 5 in your application.

D. Use GET orders and parse the result.

Solution

1. Step 1: Understand LLEN behavior on missing keys

   LLEN returns 0 if the list does not exist, so no error occurs.

2. Step 2: Choose efficient way to check length

   LLEN directly returns list length quickly; checking if >= 5 is simple.

3. Final Answer:

   Use LLEN orders and check if result >= 5 in your application. -> Option C

4. Quick Check:

   LLEN returns length or 0 if missing [OK]

Hint: LLEN returns 0 if list missing, so just check length [OK]

Common Mistakes:

• Using LRANGE which is slower and returns full items
• Checking EXISTS first wastes a command
• Using GET on a list key causes errors