LRANGE

Introduction and Use Case(s)

The LRANGE command in Redis is used to return a specified range of elements from a list. This command is particularly useful when you need to retrieve a subset of list elements, such as the most recent items in a log or a specific page of results in a paginated dataset.

Syntax

  1. LRANGE key start stop

Parameter Explanations

  • key: The name of the list from which to retrieve elements.
  • start: The starting index of the range (0-based). Negative values can be used to specify offsets from the end of the list; for example, -1 represents the last element.
  • stop: The ending index of the range (inclusive). Negative values can also be used here.

Return Values

The LRANGE command returns a list of elements within the specified range. If the start or stop indices are out of bounds, it will simply return an empty list or the truncated part of the list that exists within the bounds.

Example Outputs

  • If the list contains the elements [“a”, “b”, “c”, “d”, “e”]:

    1. dragonfly> LRANGE mylist 0 2
    2. 1) "a"
    3. 2) "b"
    4. 3) "c"

  • If start is greater than the length of the list, an empty list is returned:

    1. dragonfly> LRANGE mylist 10 20
    2. (empty list)

Code Examples

  1. dragonfly> RPUSH mylist "a" "b" "c" "d" "e"
  2. (integer) 5
  3. dragonfly> LRANGE mylist 0 2
  4. 1) "a"
  5. 2) "b"
  6. 3) "c"
  7. dragonfly> LRANGE mylist -3 -1
  8. 1) "c"
  9. 2) "d"
  10. 3) "e"
  11. dragonfly> LRANGE mylist 1 -2
  12. 1) "b"
  13. 2) "c"
  14. 3) "d"
  15. dragonfly> LRANGE mylist 10 20
  16. (empty list)

Best Practices

  • Use negative indices to easily access elements from the end of the list without needing to know its exact length.
  • Be mindful of list size and the impact on performance when accessing large ranges. Consider breaking down large lists or using pagination techniques with appropriate start and stop indices.

Common Mistakes

  • Confusing the start and stop parameters can lead to unexpected results. Always ensure start comes before stop logically.
  • Using non-existent keys will result in an empty list rather than an error, which might hide issues in your code logic.

FAQs

What happens if the list is empty?

If the list is empty, the LRANGE command will return an empty list.

Can I use LRANGE with non-list data types?

No, LRANGE is specific to lists. Using it with other data types will result in an error.

How does LRANGE handle out-of-bound indices?

Out-of-bound indices do not cause an error; instead, they return the existing portion of the list within those bounds or an empty list if completely outside any valid range.