Checking if a value exists in a list already Redis

Question

I'm wondering if there's a way to check if a key already exists in a redis list?

I can't use a set because I don't want to enforce uniqueness, but I do want to be able to check if the string is actually there.

Answer 1

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.

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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.

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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.

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

Answer 2

No, there is no way to check if a redis list contains a given value. See Redis list commands for reference.

I guess you could use LREM to (try to) remove the value, and check the return value to see if it was removed. But then you would have to put it back in, and this seems iffy. There is probably some better solution to your problem - what are you trying to accomplish?

Answer 3

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.

Answer 4

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