Leaderboard implementation in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a user with a score to the leaderboard.

Redis

ZADD leaderboard [1] user123

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

A1500

Bleaderboard

CZADD

DSCORE

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the member name before the score

Using a string instead of a number for the score

The ZADD command requires the score before the member name. Here, 1500 is the score for user123.

2fill in blank

medium

Complete the code to get the top 3 users from the leaderboard.

Redis

ZREVRANGE leaderboard 0 [1] WITHSCORES

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

A2

B3

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as the end index which returns 4 users

Using 1 as the end index which returns only 2 users

The ZREVRANGE command uses zero-based indexes. To get the top 3 users, use 0 to 2.

3fill in blank

hard

Fix the error in the code to increment a user's score by 50.

Redis

ZINCRBY leaderboard [1] user123

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

AINCR

Bleaderboard

CZINCRBY

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Using the command name as an argument

Putting the member name before the increment

The ZINCRBY command syntax is ZINCRBY key increment member. The increment value 50 must be the second argument.

4fill in blank

hard

Fill both blanks to remove a user from the leaderboard and then check if the user still exists.

Redis

ZREM leaderboard [1]
ZSCORE leaderboard [2]

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

Auser123

Buser456

Duser789

Attempts:

3 left

💡 Hint

Common Mistakes

Using different user names in the two commands

Using a user name that was never added

ZREM removes the member user123 from the leaderboard. Then ZSCORE checks the score of user123 to confirm removal.

5fill in blank

hard

Fill all three blanks to create a leaderboard, add two users, and get the rank of one user.

Redis

ZADD [1] 1000 [2] 1500 [3]
ZRANK [1] [2]

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

Aleaderboard

BuserA

CuserB

DuserC

Attempts:

3 left

💡 Hint

Common Mistakes

Using different leaderboard names in commands

Mixing up user names in add and rank commands

The leaderboard is named leaderboard. We add userA with score 1000 and userB with 1500. Then we get the rank of userA in the leaderboard.

Practice

1. What Redis data structure is best suited for implementing a leaderboard where members have scores?

easy

A. Hash

B. Sorted Set

C. List

D. String

Solution

1. Step 1: Understand leaderboard requirements

   A leaderboard needs to store members with scores and retrieve them sorted by score.

2. Step 2: Identify Redis data structure

   Redis Sorted Sets store members with scores and keep them sorted automatically.

3. Final Answer:

   Sorted Set -> Option B

4. Quick Check:

   Leaderboard = Sorted Set [OK]

Hint: Leaderboards need sorted data, use Sorted Sets [OK]

Common Mistakes:

• Choosing List which is ordered by insertion, not score
• Using Hash which stores key-value but not sorted by score
• Using String which holds plain text, not multiple members

2. Which Redis command correctly adds or updates a member's score in a leaderboard stored as a sorted set?

easy

A. ZADD leaderboard 100 user1

B. ZADD leaderboard user1 100

C. ZINCRBY leaderboard 100 user1

D. ZREM leaderboard user1

Solution

1. Step 1: Recall ZADD syntax

   ZADD command syntax is: ZADD key score member.

2. Step 2: Match syntax to options

   ZADD leaderboard 100 user1 uses ZADD leaderboard 100 user1 which matches the correct syntax.

3. Final Answer:

   ZADD leaderboard 100 user1 -> Option A

4. Quick Check:

   ZADD syntax = ZADD key score member [OK]

Hint: ZADD syntax: key, score, then member [OK]

Common Mistakes:

• Swapping score and member order
• Using ZINCRBY which increments score, not sets it
• Using ZREM which removes member instead of adding

3. Given the following Redis commands executed in order:

ZADD leaderboard 50 user1
ZADD leaderboard 100 user2
ZADD leaderboard 75 user3
ZREVRANGE leaderboard 0 1 WITHSCORES

What is the output of the last command?

medium

A. ["user2", "100", "user3", "75"]

B. ["user1", "50", "user3", "75"]

C. ["user3", "75", "user2", "100"]

D. ["user2", "100", "user1", "50"]

Solution

1. Step 1: Add members with scores

   user1=50, user2=100, user3=75 added to sorted set.

2. Step 2: Use ZREVRANGE 0 1 WITHSCORES

   This fetches top 2 members by highest score descending: user2(100), user3(75).

3. Final Answer:

   ["user2", "100", "user3", "75"] -> Option A

4. Quick Check:

   Top 2 by score desc = user2, user3 [OK]

Hint: ZREVRANGE 0 1 gets top 2 highest scores [OK]

Common Mistakes:

• Confusing ZREVRANGE with ZRANGE (ascending order)
• Ignoring WITHSCORES option output format
• Mixing member and score order in output

4. You run the following Redis commands:

ZADD leaderboard 30 user1
ZADD leaderboard 60 user2
ZADD leaderboard 45 user3
ZREVRANGE leaderboard 0 2

But the output is empty. What is the most likely cause?

medium

A. ZREVRANGE returns ascending order by default

B. ZREVRANGE syntax is incorrect, it needs WITHSCORES

C. ZADD commands used wrong order of arguments

D. The leaderboard key was deleted before ZREVRANGE

Solution

1. Step 1: Check command correctness

   ZADD commands have correct syntax and add members with scores.

2. Step 2: Analyze empty output from ZREVRANGE

   Empty output means the key 'leaderboard' likely does not exist or was deleted before ZREVRANGE.

3. Final Answer:

   The leaderboard key was deleted before ZREVRANGE -> Option D

4. Quick Check:

   Empty output = missing key [OK]

Hint: Empty ZREVRANGE means key missing or deleted [OK]

Common Mistakes:

• Thinking WITHSCORES is required for output (it's optional)
• Assuming ZREVRANGE returns ascending order
• Believing ZADD argument order was wrong (it was correct)

5. You want to implement a leaderboard that shows the top 3 users but excludes any user with a score below 50. Which Redis commands or approach should you use?

hard

A. Use ZRANGEBYSCORE leaderboard 50 +inf LIMIT 0 3

B. Use ZREVRANGE leaderboard 0 2 and filter scores < 50 in application

C. Use ZREVRANGEBYSCORE leaderboard +inf 50 LIMIT 0 3

D. Use ZRANGE leaderboard 0 2 WITHSCORES and exclude scores < 50

Solution

1. Step 1: Understand filtering by score

   We want top 3 users with scores >= 50, sorted descending.

2. Step 2: Choose correct Redis command

   ZREVRANGEBYSCORE leaderboard +inf 50 LIMIT 0 3 returns members with scores between +inf and 50 descending, limited to 3.

3. Final Answer:

   Use ZREVRANGEBYSCORE leaderboard +inf 50 LIMIT 0 3 -> Option C

4. Quick Check:

   Filter by score with ZREVRANGEBYSCORE [OK]

Hint: Use ZREVRANGEBYSCORE with score limits and LIMIT [OK]

Common Mistakes:

• Using ZREVRANGE without score filtering
• Using ZRANGEBYSCORE which sorts ascending, not descending
• Filtering scores in app instead of Redis command