ZCARD for set size in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of ZCARD on a sorted set?

Given a Redis sorted set named myzset with the following members and scores:

zadd myzset 1 one 2 two 3 three

What will be the output of the command ZCARD myzset?

Redis

ZCARD myzset

A 1

B 0

C 3

D Error: wrong type

💡 Hint

ZCARD returns the number of elements in a sorted set.

The sorted set myzset has three members: 'one', 'two', and 'three'. ZCARD returns the count of members, which is 3.

📝 Syntax

intermediate

Which command syntax correctly returns the size of a sorted set?

Which of the following Redis commands correctly returns the number of elements in the sorted set myzset?

A ZCARD myzset

B ZCOUNT myzset

C ZSIZE myzset

D ZLEN myzset

💡 Hint

The command to get the count of elements in a sorted set is a five-letter command starting with Z and ending with CARD.

ZCARD is the correct Redis command to get the number of elements in a sorted set. ZCOUNT requires min and max score arguments, and ZSIZE and ZLEN are invalid commands.

❓ query_result

advanced

What happens when ZCARD is used on a non-existent key?

If you run ZCARD nosuchkey on a Redis server where nosuchkey does not exist, what will be the output?

A 0

B Error: key does not exist

C 1

D null

💡 Hint

Redis commands often return zero or empty results for non-existent keys instead of errors.

ZCARD returns 0 if the key does not exist, indicating the sorted set has zero elements.

🔧 Debug

advanced

Why does ZCARD return an error on this key?

You have a Redis key mystring holding a string value. When you run ZCARD mystring, you get an error. Why?

A Because the key is empty

B Because the key does not exist

C Because ZCARD requires a score range argument

D Because ZCARD only works on sorted sets, not strings

💡 Hint

ZCARD is specific to sorted sets and cannot be used on other data types.

ZCARD returns an error if the key exists but is not a sorted set. Here, mystring is a string, so the command fails.

❓ optimization

expert

Efficiently checking if a sorted set is empty

You want to check if a sorted set myzset is empty or not in the most efficient way. Which Redis command or approach is best?

A Use <code>ZRANK myzset anymember</code> and check if it returns null

B Use <code>ZCARD myzset</code> and check if the result is 0

C Use <code>ZRANGE myzset 0 0</code> and check if the result is empty

D Use <code>ZCOUNT myzset -inf +inf</code> and check if the result is 0

💡 Hint

Think about which command directly returns the count without extra overhead.

ZCARD directly returns the number of elements in the sorted set efficiently. ZCOUNT requires score range arguments and is less direct. ZRANGE returns elements but involves more data transfer. ZRANK requires a member name and does not check emptiness directly.