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LTRIM key start stop

Time complexity: O(N) where N is the number of elements to be removed by the operation.

ACL categories: @write, @list, @slow

Trim an existing list so that it will contain only the specified range of elements specified. Both start and stop are zero-based indexes, where 0 is the first element of the list (the head), 1 the next element and so on.

For example: LTRIM foobar 0 2 will modify the list stored at foobar so that only the first three elements of the list will remain.

start and end can also be negative numbers indicating offsets from the end of the list, where -1 is the last element of the list, -2 the penultimate element and so on.

Out of range indexes will not produce an error: if start is larger than the end of the list, or start > end, the result will be an empty list (which causes key to be removed). If end is larger than the end of the list, Dragonfly will treat it like the last element of the list.

A common use of LTRIM is together with LPUSH / RPUSH. For example:

LPUSH mylist someelement
LTRIM mylist 0 99

This pair of commands will push a new element on the list, while making sure that the list will not grow larger than 100 elements. It is important to note that when used in this way LTRIM is an O(1) operation, because in the average case just one element is removed from the tail of the list.


Simple string reply


dragonfly> RPUSH mylist "one"
(integer) 1
dragonfly> RPUSH mylist "two"
(integer) 2
dragonfly> RPUSH mylist "three"
(integer) 3
dragonfly> LTRIM mylist 1 -1
dragonfly> LRANGE mylist 0 -1
1) "two"
2) "three"