LLEN for list length in Redis - Step-by-Step Execution

Concept Flow - LLEN for list length

Start

↓

Send LLEN command with key

↓

Check if key exists

↓

Get length

↓

Return length

↓

End

The LLEN command asks Redis for the length of a list stored at a given key. If the key exists, it returns the number of elements; if not, it returns 0.

Execution Sample

Redis

LLEN mylist

Returns the number of elements in the list stored at key 'mylist'.

Execution Table

Step	Action	Key Exists?	Length Retrieved	Output
1	Send LLEN command with key 'mylist'	Unknown	N/A	N/A
2	Check if 'mylist' exists in Redis	Yes	N/A	N/A
3	Retrieve length of list at 'mylist'	Yes	3	N/A
4	Return length to client	Yes	3	3
5	End	N/A	N/A	N/A

💡 Execution stops after returning the length of the list.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
key_exists	Unknown	Yes	Yes	Yes
list_length	N/A	N/A	3	3
output	N/A	N/A	N/A	3

Key Moments - 2 Insights

What happens if the key does not exist in Redis?

If the key does not exist, Redis returns 0 as the length, as shown in the flow where the 'No' branch returns 0 immediately.

Does LLEN count elements or bytes?

LLEN counts the number of elements in the list, not the bytes or size in memory. This is clear in step 3 where the length retrieved is the count of elements.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 4?

A N/A

B 0

C 3

D Error

💡 Hint

Check the 'Output' column in row for step 4 in the execution_table.

At which step does Redis confirm the key exists?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Key Exists?' column in execution_table to find when the key is checked.

If the key 'mylist' did not exist, what would the output be?

A 3

B 0

C Error

D Null

💡 Hint

Refer to the concept_flow where the 'No' branch returns 0 if key does not exist.

Concept Snapshot

LLEN key
Returns the number of elements in the list stored at 'key'.
If the key does not exist, returns 0.
Used to quickly find list length in Redis.
Example: LLEN mylist → 3

Full Transcript

The LLEN command in Redis returns the length of a list stored at a given key. When you send LLEN with a key, Redis first checks if the key exists. If it does, Redis counts the number of elements in the list and returns that number. If the key does not exist, Redis returns 0. This command helps you quickly find out how many items are in a list without retrieving the entire list.