Checking if a value exists in a list already Redis

I am surprised that no one mentioned the set, which perfectly solved the question.
Using the sismember key value in set, it checks if the value is a member of the key.
Here is the example:

redis 127.0.0.1:6379> SADD myset1 "hello"
(integer) 1
redis 127.0.0.1:6379> SISMEMBER myset1 "hello"
(integer) 1
redis 127.0.0.1:6379> SISMEMBER myset1 "world"
(integer) 0

Your options are as follows:

  1. Using LREM and replacing it if it was found.
  2. Maintaining a separate SET in conjunction with your LIST
  3. Looping through the LIST until you find the item or reach the end.

Redis lists are implemented as a http://en.wikipedia.org/wiki/Linked_list, hence the limitations.

I think your best option is maintaining a duplicate SET. This is what I tend to do. Just think of it as an extra index. Regardless, make sure your actions are atomic with MULTI-EXEC or Lua scripts.


You can also use LPOS.

When the item exists it returns a number indicating the position:

LPOS mylist myitem
3

When not it returns nill:

LPOS mylist myitem_which_do_not_exits
(nil)

  • Notes:

    • Available since 6.0.6.
    • Time complexity: O(N)
    • https://redis.io/commands/lpos

Lists allow duplicates but do not provide a simple way to check for existence and as @Fritzy advised, you either need to:

  • Make multiple operations (remove then add again if found during removal) for simple checks = Cost in time
  • Maintain a separate set = Cost in memory

I am surprised no one advised you to use either a Hash Table or a Sorted Set which combine advantages of allowing duplicity (by storing the number of elements as value - Hash Table, or score - Sorted Set) and indexing members by nature of a hash table/set.

Hash Table

To check for a key existence, use HEXISTS for a specific field which returns 0 if the specified member does not exist. You could also use the HGETcommand. It returns a nil answer if the specified member does not exist.

To add a new member, simply use HINCRBY which will either update the value (ie the number of elements with the member name) or create a new member if it does not exist.

Sorted Set

To check for a key existence, use either one of the three following commands:

  • ZSCORE
  • ZRANK
  • ZREVRANK

They return a nil answer if the specified member does not exist.

To add a new member, simply use ZINCRBY which will either update the score (ie the number of elements with the member name) or create a new member if it does not exist.

To sum up: Sorted Sets or Hash Tables allow you to make all the operations with your requirements with a single command.

Tags:

Redis

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