Leaderboard implementation in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve top 3 players from leaderboard

Given a Redis sorted set game_leaderboard where player scores are stored, which command returns the top 3 players with the highest scores?

Redis

ZADD game_leaderboard 1500 player1 1800 player2 1200 player3 2000 player4 1700 player5

A ZREVRANGE game_leaderboard 0 2 WITHSCORES

B ZRANGEBYSCORE game_leaderboard 0 3 WITHSCORES

C ZRANGE game_leaderboard 0 2 WITHSCORES

D ZREVRANGEBYSCORE game_leaderboard 0 3 WITHSCORES

💡 Hint

Remember that higher scores mean higher rank, and ZREVRANGE returns elements from highest to lowest score.

ZREVRANGE returns elements in descending order of score. Using 0 2 gets the top 3 players. WITHSCORES includes their scores in the output.

🧠 Conceptual

intermediate

Understanding score updates in Redis leaderboard

Which Redis command correctly increases the score of player3 by 300 points in the game_leaderboard sorted set?

A ZADD game_leaderboard player3 300

B ZADD game_leaderboard 300 player3

C ZINCRBY player3 300 game_leaderboard

D ZINCRBY game_leaderboard 300 player3

💡 Hint

Look for the command that increments a member's score by a given amount.

ZINCRBY increments the score of a member in a sorted set by the specified amount. The syntax is ZINCRBY key increment member.

📝 Syntax

advanced

Identify the syntax error in leaderboard score retrieval

Which option contains a syntax error when trying to get the rank of player2 in game_leaderboard?

A ZRANK game_leaderboard player2

B ZREVRANK game_leaderboard player2

C ZRANK player2 game_leaderboard

D 2reyalp draobredael_emag KNARVERZ

💡 Hint

Check the order of arguments for the rank commands.

ZRANK and ZREVRANK require the sorted set key first, then the member. Option C reverses this order, causing a syntax error.

❓ optimization

advanced

Efficiently retrieve players ranked 5 to 10 with scores

Which Redis command efficiently retrieves players ranked from 5 to 10 (inclusive) with their scores from game_leaderboard, sorted from highest to lowest score?

A ZREVRANGE game_leaderboard 4 9 WITHSCORES

B ZREVRANGEBYSCORE game_leaderboard 10 5 WITHSCORES

C ZRANGEBYSCORE game_leaderboard 5 10 WITHSCORES

D ZRANGE game_leaderboard 4 9 WITHSCORES

💡 Hint

Remember that ranks start at 0 and ZREVRANGE returns highest scores first.

Ranks start at 0, so rank 5 is index 4. ZREVRANGE returns members from highest to lowest score. Using 4 9 gets ranks 5 to 10.

🔧 Debug

expert

Why does this leaderboard update fail?

Consider this Redis command intended to update player6's score to 2500 in game_leaderboard:

ZINCRBY game_leaderboard 2500 player6

Why might this command not set player6's score to exactly 2500?

A Because <code>ZINCRBY</code> requires the member name before the increment value

B Because <code>ZINCRBY</code> adds 2500 to the existing score instead of setting it directly

C Because <code>ZINCRBY</code> only works if the member already exists

D Because <code>ZINCRBY</code> cannot be used on sorted sets

💡 Hint

Think about what ZINCRBY does to the score value.

ZINCRBY increases the current score by the given increment. It does not set the score to that value. To set a score directly, use ZADD with the new score.

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