ZCARD for set size in Redis - Step-by-Step Execution

Concept Flow - ZCARD for set size

Start

↓

Call ZCARD with key

↓

Check if key exists

↓

Count elements

↓

Return count

The ZCARD command checks if the sorted set key exists, counts its elements if yes, or returns 0 if no.

Execution Sample

Redis

ZADD myset 1 one 2 two 3 three
ZCARD myset
ZCARD unknownset

Add three elements to a sorted set 'myset', then get its size with ZCARD, and check size of a non-existing set.

Execution Table

Step	Command	Key	Key Exists?	Action	Output
1	ZADD	myset	No	Create 'myset' and add 3 elements	3
2	ZCARD	myset	Yes	Count elements in 'myset'	3
3	ZCARD	unknownset	No	Return 0 because key does not exist	0

💡 Execution stops after returning the count or 0 for each ZCARD command.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
myset	does not exist	sorted set with 3 elements	unchanged	unchanged
unknownset	does not exist	does not exist	does not exist	does not exist

Key Moments - 2 Insights

Why does ZCARD return 0 for a key that does not exist?

Because as shown in execution_table row 3, when the key is missing, Redis returns 0 instead of an error.

Does ZCARD count the number of elements or sum their scores?

ZCARD counts the number of elements only, not their scores, as shown in row 2 where output is 3 for three elements.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of ZCARD for 'myset' at step 2?

A Error

B 0

C 3

D 1

💡 Hint

Check the 'Output' column in execution_table row 2.

At which step does Redis create the sorted set 'myset'?

A Step 3

B Step 1

C Step 2

D No creation happens

💡 Hint

Look at the 'Action' column in execution_table row 1.

If 'unknownset' existed with 5 elements, what would ZCARD return at step 3?

A 5

B 0

C Error

D 1

💡 Hint

ZCARD returns the count of elements if the key exists, see execution_table row 2 for example.

Concept Snapshot

ZCARD key
- Returns the number of elements in a sorted set.
- If key does not exist, returns 0.
- Does not sum scores, only counts elements.
- Useful to check size of sorted sets quickly.

Full Transcript

The ZCARD command in Redis returns the number of elements in a sorted set stored at the given key. If the key does not exist, it returns 0 instead of an error. For example, after adding three elements to 'myset', ZCARD 'myset' returns 3. If you call ZCARD on a non-existing key like 'unknownset', it returns 0. This command helps you quickly find out how many members are in a sorted set without retrieving the elements themselves.

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