SCARD for set size in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the number of elements in the set named 'myset'.

Redis

SCARD [1]

Drag options to blanks, or click blank then click option'

Acountset

Bsetcount

Celements

Dmyset

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong set name that does not exist.

Forgetting to provide the set name after SCARD.

The SCARD command returns the number of elements in the specified set. Here, the set is named myset.

2fill in blank

medium

Complete the code to find the size of the set stored in the variable 'setKey'.

Redis

SCARD [1]

Drag options to blanks, or click blank then click option'

AsetKey

Bsetkey

CSetKey

Dset_key

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the case of the key name.

Using underscores or other characters not in the key.

Redis commands are case-sensitive for keys. Use the exact variable name setKey as the set name.

3fill in blank

hard

Fix the error in the command to get the set size of 'users'.

Redis

SCARD [1]

Drag options to blanks, or click blank then click option'

Ausers

Buser

CuserSet

Duser_set

Attempts:

3 left

💡 Hint

Common Mistakes

Using a singular form instead of plural.

Adding extra characters or underscores.

The set name is users. Using user or other variants will cause Redis to look for a different key.

4fill in blank

hard

Fill both blanks to get the size of the set named 'activeUsers' and store it in variable 'count'.

Redis

count = [1]([2]) activeUsers

Drag options to blanks, or click blank then click option'

Aredis.call

BSCARD

Cget

DSET

Attempts:

3 left

💡 Hint

Common Mistakes

Using get which is for strings, not sets.

Using SET which is for setting values, not getting size.

In Redis scripting, redis.call is used to run commands like SCARD to get the size of a set.

5fill in blank

hard

Fill all three blanks to write a Lua script that returns the size of the set given by the first key argument.

Redis

return [1]([2], [3][1])

Drag options to blanks, or click blank then click option'

Aredis.call

BKEYS

CSCARD

DARGV

Attempts:

3 left

💡 Hint

Common Mistakes

Using ARGV instead of KEYS for keys.

Using get instead of SCARD.

This Lua script calls redis.call with the command SCARD on the first key in KEYS to get the set size.

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