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ZCARD

Introduction​

In Dragonfly, as well as in Redis and Valkey, the ZCARD command is used to return the number of members in a sorted set stored at a given key. This is particularly useful when you need to monitor the size of a sorted set or implement pagination, leaderboards, or other order-based storage solutions.

Syntax​

ZCARD key
  • Time complexity: O(1)
  • ACL categories: @read, @sortedset, @fast

Parameter Explanations​

  • key: The key of the sorted set whose cardinality (number of members) is to be fetched.

Return Values​

The command returns an integer representing the number of members in the sorted set. If the key does not exist or is not a sorted set, 0 is returned.

Code Examples​

Basic Example​

To get the number of members in a sorted set:

dragonfly> ZADD myzset 1 "member1" 2 "member2" 3 "member3"
(integer) 3
dragonfly> ZCARD myzset
(integer) 3

When the Sorted Set is Empty​

If the sorted set is empty, the ZCARD command returns 0.

dragonfly> ZADD emptyset
(integer) 0
dragonfly> ZCARD emptyset
(integer) 0

Using ZCARD for Pagination​

Suppose you are building a leaderboard and need to know how many total players are ranked to help with pagination:

dragonfly> ZADD leaderboard 100 "player1" 150 "player2" 200 "player3"
(integer) 3
dragonfly> ZCARD leaderboard
(integer) 3

Best Practices​

  • Since ZCARD only fetches the number of elements, it is safe to use even with large sets without performance concerns.
  • Ideal for checking if a sorted set is non-empty before executing more complex operations on large sets.

Common Mistakes​

  • Using ZCARD on non-sorted set data types leads to incorrect results. For example, using it on a regular set or a list will not return the right cardinality since these are different data types.
  • Assuming ZCARD can return detailed information about members; it only returns the count of elements.

FAQs​

What happens if the key does not exist?​

If the key does not exist, ZCARD will return 0 since there are no members in a non-existent set.

Can I use ZCARD on a non-sorted set?​

No, ZCARD specifically works with sorted sets. If you use it on a key that holds a different data type, Dragonfly will throw an error. For counting members of a regular set, you should use the SCARD command.