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DiceDB

HLEN

The HLEN command in DiceDB is used to obtain the number of fields contained within a hash stored at a specified key. This command is particularly useful for understanding the size of a hash and for performing operations that depend on the number of fields in a hash.

Syntax

Terminal window
HLEN key

Parameters

ParameterDescriptionTypeRequired
keyThe key associated with the hash for which the number of fields is to be retrievedStringYes

Return Value

ConditionReturn Value
If specified key existsnumber of fields in the hash at key
If key doesn’t exist0

Behaviour

  • DiceDB checks if the specified key exists.
  • If the key exists and is associated with a hash, DiceDB counts the number of fields in the hash and returns this count.
  • If the key does not exist, DiceDB returns 0.
  • If the key exists but is not associated with a hash, an error is returned.

Errors

  1. Wrong type of key:

    • Error Message: (error) WRONGTYPE Operation against a key holding the wrong kind of value
    • Occurs when attempting to use the command on a key that contains a non-hash value.

  2. Wrong number of arguments:

    • Error Message: (error) -ERR wrong number of arguments for 'HLEN' command
    • Occurs if key isn’t specified in the command.

Example Usage

Basic Usage

Creating hash myhash with two fields field1 and field2. Getting hash length of myhash.

Terminal window
127.0.0.1:7379> HSET myhash field1 "value1" field2 "value2"
(integer) 2


127.0.0.1:7379> HLEN myhash
(integer) 2

Invalid Usage on non-existent key

Getting hash length from a non-existent hash key nonExistentHash.

Terminal window
127.0.0.1:7379> HLEN nonExistentHash
(integer) 0

Invalid Usage on non-hash key

Getting hash length from a key mystring associated with a non-hash type.

Terminal window
127.0.0.1:7379> SET mystring "This is a string"
OK


127.0.0.1:7379> HLEN mystring
(error) WRONGTYPE Operation against a key holding the wrong kind of value

Notes

  • The HLEN command is a constant-time operation, meaning its execution time is O(1) regardless of the number of fields in the hash.
  • This command is useful for quickly determining the size of a hash without needing to retrieve all the fields and values.

By understanding the HLEN command, you can efficiently manage and interact with hash data structures in DiceDB, ensuring that your applications can handle hash-based data effectively.