LLEN for list length in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the length of the list stored at key "mylist".

Redis

LLEN [1]

Drag options to blanks, or click blank then click option'

Aitems

Blist

Clength

Dmylist

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong key name like 'list' or 'length' instead of the actual list key.

Forgetting to provide the key name after LLEN.

The LLEN command requires the key name of the list to return its length. Here, the list key is "mylist".

2fill in blank

medium

Complete the code to get the length of the list stored at key "tasks".

Redis

LLEN [1]

Drag options to blanks, or click blank then click option'

Atasks

Bitems

Clist

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic word like 'list' or 'items' instead of the actual key.

Leaving the key name blank.

To get the length of the list, use LLEN followed by the list's key name, which is "tasks" here.

3fill in blank

hard

Fix the error in the command to get the length of the list "queue".

Redis

LLEN [1]

Drag options to blanks, or click blank then click option'

A"queue"

Bqueue

Clist

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Adding quotes around the key name causing syntax errors.

Using a wrong key name.

In Redis commands, keys are given without quotes. So use LLEN queue, not LLEN "queue".

4fill in blank

hard

Fill both blanks to get the length of the list "shopping" and store it in variable length.

Redis

length = [1] [2]

Drag options to blanks, or click blank then click option'

ALLEN

Bshopping

Cmylist

DGET

Attempts:

3 left

💡 Hint

Common Mistakes

Using GET instead of LLEN.

Using wrong key names.

Use LLEN shopping to get the length of the list named "shopping". Assign it to the variable length.

5fill in blank

hard

Fill all three blanks to get the length of the list "tasks" and print it.

Redis

length = [1] [2]
PRINT [3]

Drag options to blanks, or click blank then click option'

ALLEN

Btasks

Clength

DGET

Attempts:

3 left

💡 Hint

Common Mistakes

Using GET instead of LLEN.

Printing the key name instead of the variable.

Using wrong key names.

Use LLEN tasks to get the list length, store it in variable length, then print length.

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