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.

Practice

1. What does the Redis command ZCARD do when used on a sorted set?

easy

A. Deletes all elements in the sorted set

B. Returns the highest score in the sorted set

C. Returns the elements with the lowest score

D. Returns the number of elements in the sorted set

Solution

1. Step 1: Understand the purpose of ZCARD

   The ZCARD command is designed to count how many elements are in a sorted set.

2. Step 2: Compare with other options

   Other options describe different commands like fetching scores or deleting, which ZCARD does not do.

3. Final Answer:

   Returns the number of elements in the sorted set -> Option D

4. Quick Check:

   ZCARD = count of elements [OK]

Hint: ZCARD counts items in sorted set, not scores or values [OK]

Common Mistakes:

• Confusing ZCARD with ZRANGE or ZSCORE
• Thinking it deletes elements
• Assuming it returns scores instead of count

2. Which of the following is the correct syntax to get the size of a sorted set named myzset in Redis?

easy

A. ZCOUNT myzset

B. ZCARD myzset

C. ZSIZE myzset

D. ZLEN myzset

Solution

1. Step 1: Recall the correct command name

   The command to get the number of elements in a sorted set is ZCARD.

2. Step 2: Verify syntax correctness

   The syntax is ZCARD key, so ZCARD myzset is correct.

3. Final Answer:

   ZCARD myzset -> Option B

4. Quick Check:

   Correct command syntax = ZCARD myzset [OK]

Hint: Use ZCARD followed by set name to get size [OK]

Common Mistakes:

• Using ZCOUNT which counts by score range
• Using non-existent commands like ZSIZE or ZLEN
• Forgetting to include the key name

3. Given the following Redis commands executed in order:

ZADD fruits 1 apple 2 banana 3 cherry
ZADD fruits 4 date
ZREM fruits banana

What will ZCARD fruits return?

medium

A. 4

B. 1

C. 3

D. 2

Solution

1. Step 1: Count elements after additions

   After ZADD fruits 1 apple 2 banana 3 cherry, the set has 3 elements: apple, banana, cherry.

2. Step 2: Add one more element and remove one

   ZADD fruits 4 date adds date, total 4 elements. Then ZREM fruits banana removes banana, leaving 3 elements.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Elements after ops = 3 [OK]

Hint: Additions minus removals = final count [OK]

Common Mistakes:

• Counting removed elements
• Ignoring the last removal command
• Assuming ZADD replaces instead of adds

4. You run ZCARD myset but get an error: WRONGTYPE Operation against a key holding the wrong kind of value. What is the most likely cause?

medium

A. The key 'myset' is not a sorted set but another data type

B. The key 'myset' does not exist

C. The ZCARD command is misspelled

D. Redis server is down

Solution

1. Step 1: Understand the error message

   The error WRONGTYPE means the key exists but is not the expected data type.

2. Step 2: Match error to cause

   Since ZCARD works only on sorted sets, the key must be a different type like string or list.

3. Final Answer:

   The key 'myset' is not a sorted set but another data type -> Option A

4. Quick Check:

   WRONGTYPE = wrong data type key [OK]

Hint: WRONGTYPE means key is not a sorted set [OK]

Common Mistakes:

• Assuming key does not exist causes this error
• Thinking command spelling causes WRONGTYPE
• Blaming server status for this error

5. You have a sorted set scores with 1000 elements. You want to check if it is empty before processing. Which Redis command and check is the best and most efficient way?

hard

A. Use ZCARD scores and check if result is 0

B. Use ZRANGE scores 0 -1 and check if result is empty

C. Use ZCOUNT scores -inf +inf and check if result is 0

D. Use GET scores and check if result is null

Solution

1. Step 1: Understand command efficiency

   ZCARD returns the count quickly without fetching all elements, ideal for checking emptiness.

2. Step 2: Compare other options

   ZRANGE fetches all elements, costly for large sets. ZCOUNT counts by score range but is less direct. GET is for strings, not sorted sets.

3. Final Answer:

   Use ZCARD scores and check if result is 0 -> Option A

4. Quick Check:

   ZCARD = fast size check [OK]

Hint: ZCARD quickly returns size, best for emptiness check [OK]

Common Mistakes:

• Using ZRANGE wastes resources fetching all elements
• Using GET on a sorted set key causes errors
• Using ZCOUNT unnecessarily when ZCARD suffices