SCARD for set size in Redis

Introduction

SCARD helps you find out how many unique items are in a set. It tells you the size of the set quickly.

You want to count how many friends are in your friend list stored as a set.

You need to know how many unique visitors came to your website today.

You want to check how many different tags are assigned to a blog post.

You want to find out how many unique products a customer has bought.

Syntax

Redis

SCARD key

key is the name of the set you want to check.

The command returns the number of elements in the set.

Examples

Returns the number of elements in the set named 'myset'.

Redis

SCARD myset

Returns how many users are currently online if stored in the 'users_online' set.

Redis

SCARD users_online

Sample Program

First, we add some fruits to the set named 'fruits'. Notice 'banana' is added twice but counted once because sets store unique items. Then we use SCARD to get the total number of unique fruits.

Redis

SADD fruits apple banana orange
SADD fruits banana grape
SCARD fruits

Important Notes

If the set does not exist, SCARD returns 0.

Sets only store unique values, so duplicates do not increase the count.

Summary

SCARD returns the number of unique elements in a set.

It is useful to quickly check the size of a set without retrieving all elements.

Practice

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

easy

A. Returns all elements in the set

B. Deletes all elements from the set

C. Adds a new element to the set

D. Returns the number of unique elements in the set

Solution

1. Step 1: Understand the purpose of SCARD

   The SCARD command is designed to count elements in a set, not modify or list them.

2. Step 2: Identify the correct behavior

   SCARD returns the count of unique elements stored in the set.

3. Final Answer:

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

4. Quick Check:

   SCARD = set size count [OK]

Hint: SCARD counts set items without listing them [OK]

Common Mistakes:

• Confusing SCARD with commands that list or delete elements
• Thinking SCARD adds elements
• Assuming SCARD returns elements instead of count

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

easy

A. SCARD myset

B. SCARD(myset)

C. GETSIZE myset

D. SIZE myset

Solution

1. Step 1: Recall Redis command syntax

   Redis commands use the format COMMAND key without parentheses.

2. Step 2: Match correct command for set size

   SCARD is the command to get set size, so SCARD myset is correct.

3. Final Answer:

   SCARD myset -> Option A

4. Quick Check:

   Correct Redis syntax = SCARD myset [OK]

Hint: Redis commands have no parentheses, just command and key [OK]

Common Mistakes:

• Using parentheses like a function call
• Using non-existent commands like GETSIZE or SIZE
• Confusing SCARD with GET

3. Given the following Redis commands executed in order:

SADD fruits apple banana orange
SADD fruits banana grape
SCARD fruits

What will be the output of SCARD fruits?

medium

A. 3

B. 5

C. 4

D. 2

Solution

1. Step 1: Identify unique elements added to the set

   First SADD adds apple, banana, orange (3 unique). Second SADD adds banana (already exists), grape (new).

2. Step 2: Count unique elements after both commands

   Unique elements are apple, banana, orange, grape = 4 total.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Unique fruits count = 4 [OK]

Hint: SCARD counts unique items, duplicates ignored [OK]

Common Mistakes:

• Counting duplicates multiple times
• Assuming SADD overwrites set
• Confusing SCARD output with list of elements

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

medium

A. myset is not a set but another data type like string or list

B. The key myset does not exist

C. SCARD command is misspelled

D. Redis server is down

Solution

1. Step 1: Understand the error message

   The error indicates the key exists but is not a set type.

2. Step 2: Identify cause of WRONGTYPE error

   SCARD only works on sets; if myset is string or list, this error occurs.

3. Final Answer:

   myset is not a set but another data type like string or list -> Option A

4. Quick Check:

   WRONGTYPE = wrong data type for SCARD [OK]

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

Common Mistakes:

• Assuming key does not exist causes this error
• Thinking SCARD command spelling causes this error
• Ignoring data type mismatch

5. You have two sets in Redis: setA with 3 elements and setB with 4 elements, where 2 elements overlap. You want to find the total number of unique elements across both sets without retrieving all elements. Which Redis command or combination should you use?

hard

A. SCARD setA + SCARD setB

B. SUNIONSTORE tempSet setA setB followed by SCARD tempSet

C. SINTER setA setB

D. SDIFF setA setB

Solution

1. Step 1: Understand the problem of counting unique elements across two sets

   Simply adding SCARD results counts duplicates twice.

2. Step 2: Use SUNIONSTORE to combine sets uniquely

   SUNIONSTORE creates a new set with all unique elements from both sets.

3. Step 3: Use SCARD on the combined set

   SCARD on the new set gives total unique elements count.

4. Final Answer:

   SUNIONSTORE tempSet setA setB followed by SCARD tempSet -> Option B

5. Quick Check:

   Use SUNIONSTORE + SCARD for unique combined count [OK]

Hint: Combine sets with SUNIONSTORE, then SCARD the result [OK]

Common Mistakes:

• Adding SCARD counts directly causing duplicates
• Using SINTER or SDIFF which don't give total unique count
• Trying to count without combining sets