Command: LIST.LEN

Overview

The LIST.LEN command is used to retrieve the number of elements in a list stored at a given key. It validates the key, checks for the appropriate data type, and returns the size of the list. If the key is not associated with a list, or if the key doesn’t exist, a relevant error or response is returned.

Command Name

LIST.LEN

Description

Returns the length of the list stored at the specified key.

  • If the key does not exist, the command returns 0 .

  • If the key exists but is not a list, an error is returned.

  • Supports validation for proper key format.

Use Cases

  1. Monitoring list sizes: Determine how many items are currently queued or stored.

  2. Pre-validation before iteration: Check if a list contains any elements before processing.

  3. Debugging data structure issues: Confirm the data structure associated with a key is a list and inspect its length.

  4. Efficient length retrieval: Avoid fetching the entire list for size checks.

Syntax 

  LIST.LEN <key>
  • <key> : The name of the list key to query.

Permissions

  • Read access is sufficient.

  • No special permissions required beyond access to the key.

Examples

Example 1: Successful Retrieval

  localhost:9219> LIST.ISET myKey 0 "This is my first element"
Ok

localhost:9219> LIST.ISET myKey 1 "{"name":"John", "address": "kolkata"}"
Ok

localhost:9219> LIST.LEN myKey
Ok 2

Example 2: Key Does Not Exist

  localhost:9219> LIST.LEN unknownKey
Ok 0

Example 3: Key Is Not a List

  localhost:9219> SET myKey "not a list"
Ok

localhost:9219> LIST.LEN myKey
InvalidValueError: The current value associated with the provided key must be a list

Example 4: Invalid Arguments

  localhost:9219> LIST.LEN
InvalidKeyError: Key must be provided

Behavior on Error

Scenario Error Message
No key provided InvalidKeyError: Key must be provided
Incorrect number of arguments InvalidArgsError: invalid number of arguments, Syntax: LIST.LEN <key>
Invalid key format InvalidKeyError: <reason>
Key not associated with a list InvalidValueError: The current value associated with the provided key must be a list