LPOS key element [RANK rank] [COUNT num-matches] [MAXLEN len]

Available since: 6.0.6

Time complexity: O(N) where N is the number of elements in the list, for the average case. When searching for elements near the head or the tail of the list, or when the MAXLEN option is provided, the command may run in constant time.

ACL categories: @read @list @slow

The command returns the index of matching elements inside a Redis list. By default, when no options are given, it will scan the list from head to tail, looking for the first match of “element”. If the element is found, its index (the zero-based position in the list) is returned. Otherwise, if no match is found, nil is returned.

> RPUSH mylist a b c 1 2 3 c c
> LPOS mylist c
2

The optional arguments and options can modify the command’s behavior. The RANK option specifies the “rank” of the first element to return, in case there are multiple matches. A rank of 1 means to return the first match, 2 to return the second match, and so forth.

For instance, in the above example the element “c” is present multiple times, if I want the index of the second match, I’ll write:

> LPOS mylist c RANK 2
6

That is, the second occurrence of “c” is at position 6. A negative “rank” as the RANK argument tells LPOS to invert the search direction, starting from the tail to the head.

So, we want to say, give me the first element starting from the tail of the list:

> LPOS mylist c RANK -1
7

Note that the indexes are still reported in the “natural” way, that is, considering the first element starting from the head of the list at index 0, the next element at index 1, and so forth. This basically means that the returned indexes are stable whatever the rank is positive or negative.

Sometimes we want to return not just the Nth matching element, but the position of all the first N matching elements. This can be achieved using the COUNT option.

> LPOS mylist c COUNT 2
[2,6]

We can combine COUNT and RANK, so that COUNT will try to return up to the specified number of matches, but starting from the Nth match, as specified by the RANK option.

> LPOS mylist c RANK -1 COUNT 2
[7,6]

When COUNT is used, it is possible to specify 0 as the number of matches, as a way to tell the command we want all the matches found returned as an array of indexes. This is better than giving a very large COUNT option because it is more general.

> LPOS mylist c COUNT 0
[2,6,7]

When COUNT is used and no match is found, an empty array is returned. However when COUNT is not used and there are no matches, the command returns nil.

Finally, the MAXLEN option tells the command to compare the provided element only with a given maximum number of list items. So for instance specifying MAXLEN 1000 will make sure that the command performs only 1000 comparisons, effectively running the algorithm on a subset of the list (the first part or the last part depending on the fact we use a positive or negative rank). This is useful to limit the maximum complexity of the command. It is also useful when we expect the match to be found very early, but want to be sure that in case this is not true, the command does not take too much time to run.

When MAXLEN is used, it is possible to specify 0 as the maximum number of comparisons, as a way to tell the command we want unlimited comparisons. This is better than giving a very large MAXLEN option because it is more general.

@return

The command returns the integer representing the matching element, or nil if there is no match. However, if the COUNT option is given the command returns an array (empty if there are no matches).

@examples

redis> RPUSH mylist a b c d 1 2 3 4 3 3 3
TBD
redis> LPOS mylist 3
TBD
redis> LPOS mylist 3 COUNT 0 RANK 2