ZCARD for set size in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command ZCARD do?

The ZCARD command returns the number of elements in a sorted set stored at a given key.

beginner

How do you use ZCARD to find the size of a sorted set named myset?

You run the command: ZCARD myset. It returns the count of elements in myset.

beginner

What type of Redis data structure does ZCARD work with?

ZCARD works only with sorted sets, which are collections of unique elements ordered by score.

intermediate

What will ZCARD return if the key does not exist?

If the key does not exist, ZCARD returns 0, meaning the sorted set is empty or missing.

intermediate

Can ZCARD be used on Redis sets or lists?

No, ZCARD only works on sorted sets. For regular sets, use SCARD, and for lists, use LLEN.

What does the Redis command ZCARD myset return?

A The number of elements in the sorted set <code>myset</code>

B The sum of scores in <code>myset</code>

C The first element in <code>myset</code>

D The last element in <code>myset</code>

ZCARD returns the count of elements in the sorted set.

If the key myset does not exist, what does ZCARD myset return?

A null

B An error

C 0

D 1

ZCARD returns 0 when the key does not exist.

Which Redis data type is ZCARD designed for?

A Sorted Set

B Set

C List

D Hash

ZCARD works only with sorted sets.

Which command should you use to get the size of a regular Redis set?

A ZCARD

B SCARD

C LLEN

D HGETALL

SCARD returns the number of elements in a regular set.

What will ZCARD return if the sorted set has 5 elements?

A 0

B An error

C The highest score

D 5

ZCARD returns the count of elements, which is 5 in this case.

Explain what the Redis command ZCARD does and when you would use it.

Think about counting items in a special ordered collection.

Describe the difference between ZCARD and SCARD in Redis.

Focus on the data structure each command works with.

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