SCARD for set size in Redis - Mini Project: Build & Apply

Counting Unique Items with Redis SCARD

📖 Scenario: You are managing a small online store. You want to keep track of unique customers who have visited your website each day. You will use Redis sets to store customer IDs and then count how many unique customers visited on a particular day.

🎯 Goal: Build a Redis data structure to store unique customer IDs for a day and use the SCARD command to find out how many unique customers visited that day.

📋 What You'll Learn

Create a Redis set with a specific key for storing customer IDs

Add multiple customer IDs to the set

Use a variable to hold the key name for the set

Use the SCARD command to get the count of unique customers

Complete the Redis commands in the correct order

💡 Why This Matters

🌍 Real World

Tracking unique visitors or users is common in websites and apps to understand user engagement.

💼 Career

Knowing how to use Redis sets and SCARD helps in building efficient real-time analytics and user tracking systems.

1

Create a Redis set with customer IDs

Create a Redis set with the key "customers:2024-06-01" and add these customer IDs: "cust1", "cust2", "cust3" using the SADD command.

Redis

# Use SADD to add 'cust1', 'cust2', and 'cust3' to the set 'customers:2024-06-01'
# Your code here

Hint

Use the SADD command followed by the set key and the customer IDs separated by spaces.

2

Set a variable for the Redis set key

Create a variable called key and set it to the string "customers:2024-06-01" to hold the Redis set key.

Redis

SADD customers:2024-06-01 cust1 cust2 cust3
# Set the variable 'key' to 'customers:2024-06-01'
# Your code here

Hint

Use the SET command to create a variable named key with the value of the set key.

3

Add more customer IDs to the set

Use the SADD command to add two more customer IDs: "cust4" and "cust5" to the existing set.

Redis

SADD customers:2024-06-01 cust1 cust2 cust3
SET key customers:2024-06-01
# Add 'cust4' and 'cust5' to the set
# Your code here

Hint

Use SADD with the same set key to add new customer IDs.

4

Get the count of unique customers with SCARD

Use the SCARD command to get the number of unique customers in the set.

Redis

SADD customers:2024-06-01 cust1 cust2 cust3
SET key customers:2024-06-01
SADD customers:2024-06-01 cust4 cust5
# Use SCARD to get the count of unique customers
# Your code here

Hint

Use SCARD followed by the set key to get the count of unique members.

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