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

Concept Flow - SCARD for set size

Start

↓

Receive SCARD command with key

↓

Check if key exists

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Count members

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Return count as result

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End

The SCARD command checks if the set key exists, counts its members if yes, or returns 0 if no, then returns the count.

Execution Sample

Redis

SADD myset apple banana cherry
SCARD myset

Add three members to a set 'myset' and then get the count of members using SCARD.

Execution Table

Step	Command	Action	Set State	Output
1	SADD myset apple banana cherry	Add 'apple', 'banana', 'cherry' to 'myset'	{myset: ['apple', 'banana', 'cherry']}	3
2	SCARD myset	Check if 'myset' exists	{myset: ['apple', 'banana', 'cherry']}	3
3	SCARD unknownset	Check if 'unknownset' exists	{myset: ['apple', 'banana', 'cherry']}	0

💡 SCARD returns the number of members if set exists, otherwise 0 if set does not exist.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
myset	undefined	['apple', 'banana', 'cherry']	['apple', 'banana', 'cherry']	['apple', 'banana', 'cherry']
unknownset	undefined	undefined	undefined	undefined

Key Moments - 2 Insights

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

Because SCARD checks if the set exists first (see execution_table step 3). If the set is missing, it returns 0 instead of an error.

Does SCARD change the set contents?

No, SCARD only counts members without modifying the set, as shown in execution_table step 2 where the set remains unchanged.

Visual Quiz - 3 Questions

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

A 3

B 0

C Error

D 1

💡 Hint

Check the Output column at step 2 in the execution_table.

At which step does the set 'myset' get its members added?

A Step 3

B Step 1

C Step 2

D No step adds members

💡 Hint

Look at the Command and Action columns in execution_table step 1.

If the set 'unknownset' was created with 2 members before step 3, what would SCARD return at step 3?

A Error

B 0

C 2

D 1

💡 Hint

SCARD returns the count of members if the set exists, see execution_table step 3.

Concept Snapshot

SCARD key
- Returns the number of members in the set stored at key.
- If key does not exist, returns 0.
- Does not modify the set.
- Useful to quickly get set size.
- Example: SCARD myset -> returns count.

Full Transcript

The SCARD command in Redis returns the number of members in a set stored at a given key. When you run SCARD with a key, Redis first checks if the set exists. If it does, it counts the members and returns that number. If the set does not exist, SCARD returns 0 instead of an error. For example, after adding three members to 'myset', SCARD myset returns 3. If you ask for a set that does not exist, like 'unknownset', SCARD returns 0. SCARD does not change the set contents; it only counts members. This makes it a safe and fast way to get the size of a set.