LINSERT

The LINSERT command in DiceDB inserts an element in the list stored at key either before or after the reference value pivot. When key does not exist, it is considered an empty list and no operation is performed. An error is returned when key exists but does not hold a list value.

Syntax

LINSERT key <BEFORE | AFTER> pivot element

Parameters

Parameter	Description	Type	Required
key	The key associated with the list whose length you want to retrieve.	String	Yes
<before / after>	Tells whether to insert the element before or after the pivot.	String	Yes
pivot	The pivot element.	String	Yes
element	The element to be inserted.	String	Yes

Return values

Condition	Return Value
Command is successful	Integer reply: Number of elements in the list after insertion.
If the key does not exist	0
Syntax or specified constraints are invalid	error

Behaviour

• If the key exists and is associated with a list, the LINSERT command returns the list length after insertion.
• If the key does not exist, the LINSERT command returns 0.
• If the key exists but is not associated with a list, an error is returned.

Errors

1. Wrong type of value or key:

   • Error Message: (error) WRONGTYPE Operation against a key holding the wrong kind of value
   • Occurs if the key exists but is not associated with a list.

2. Wrong number of arguments:

   • Error Message: (error) wrong number of arguments for LINSERT
   • Occurs when invalid arguments are passed to the command.

Example Usage

Basic Usage

Performing the LINSERT on a list mylist with values ["one", "two"].

127.0.0.1:7379> RPUSH mylist "one"
(integer) 1
127.0.0.1:7379> RPUSH mylist "two"
(integer) 2
127.0.0.1:7379> LINSERT mylist after "two" "four"
(integer) 3
127.0.0.1:7379> LRANGE mylist 0 100
1) "one"
2) "two"
3) "four"
127.0.0.1:7379> LINSERT mylist before "four" "three"
(integer) 4
127.0.0.1:7379> LRANGE mylist 0 100
1) "one"
2) "two"
3) "three"
4) "four"

Non-Existent Key

Getting the LINSERT on a list nonExistentList which does not exist.

127.0.0.1:7379> LINSERT nonExistentList before "two" "one"
(integer) 0

Invalid usage

Trying to perform LINSERT on a key myhash which doens’t hold list data type.

127.0.0.1:7379> HMSET myhash field1 "value1"
OK
127.0.0.1:7379> HGET myhash field1
"value1"
127.0.0.1:7379> LINSERT myhash before pivot element
(error) WRONGTYPE Operation against a key holding the wrong kind of value

Best Practices

• Check Key Type: Before using LINSERT, ensure that the key is associated with a list to avoid errors.
• Handle Non-Existent Keys: Be prepared to handle the case where the key does not exist, as LINSERT will return 0 in such scenarios.
• Use in Conjunction with Other List Commands: The LINSERT command is often used alongside other list commands like RPUSH, LPUSH, LPOP, and RPOP to manage and process lists effectively.