ZSCORE for member score in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of ZSCORE for an existing member?

Given a sorted set named players with members and their scores, what will be the output of ZSCORE players "Alice" if Alice's score is 1500?

Redis

ZADD players 1500 Alice
ZSCORE players "Alice"

A "1500"

B Error: member not found

C nil

D 1500

💡 Hint

Remember that ZSCORE returns the score as a string if the member exists.

The ZSCORE command returns the score of the specified member as a string if the member exists in the sorted set. If the member does not exist, it returns nil.

❓ query_result

intermediate

What does ZSCORE return for a non-existing member?

If you run ZSCORE players "Bob" but Bob is not in the sorted set, what will Redis return?

Redis

ZADD players 1500 Alice
ZSCORE players "Bob"

A nil

B ""

C Error: member not found

D "0"

💡 Hint

Think about what Redis returns when a member is missing in a sorted set.

When the member does not exist in the sorted set, ZSCORE returns nil to indicate absence.

🧠 Conceptual

advanced

Why does ZSCORE return scores as strings?

Which reason best explains why Redis returns the score as a string when using ZSCORE?

A Because Redis converts scores to integers before returning them.

B Because Redis stores all data internally as strings for consistency and network transmission.

C Because Redis uses JSON format for all responses.

D Because Redis only supports string members, not numeric scores.

💡 Hint

Think about how Redis handles data types and communication.

Redis stores and transmits data as strings to keep things simple and fast. Even numeric scores are sent as strings.

📝 Syntax

advanced

Which ZSCORE command syntax is correct?

Choose the correct syntax to get the score of member "Charlie" from sorted set "game_scores".

A ZSCORE game_scores Charlie

B ZSCORE "game_scores" "Charlie"

C ZSCORE "game_scores" Charlie

D ZSCORE game_scores "Charlie"

💡 Hint

Member names with special characters or spaces need quotes.

The correct syntax requires the sorted set name without quotes and the member name in quotes if it contains special characters or to be safe.

❓ optimization

expert

How to efficiently check scores of multiple members?

You want to get scores for multiple members in a sorted set efficiently. Which approach is best?

A Run ZSCORE command separately for each member one by one.

B Use ZRANGE with WITHSCORES and filter results client-side.

C Use a Lua script to batch ZSCORE calls in Redis server.

D Use ZSCORE with multiple members in one command.

💡 Hint

Think about reducing network round-trips and server load.

Redis does not support multiple members in one ZSCORE command. Running many commands separately causes many network calls. Using a Lua script batches calls inside Redis, reducing network overhead and improving performance.

Practice

1. What does the Redis command ZSCORE do?

easy

A. Returns all members of a sorted set

B. Deletes a member from a sorted set

C. Returns the score of a member in a sorted set

D. Adds a member with a score to a sorted set

Solution

1. Step 1: Understand the purpose of ZSCORE

   The ZSCORE command is used to get the score associated with a specific member in a sorted set.

2. Step 2: Compare with other commands

   Other commands like ZADD add members, ZREM deletes members, and ZRANGE lists members, so they do not return scores directly.

3. Final Answer:

   Returns the score of a member in a sorted set -> Option C

4. Quick Check:

   ZSCORE = get member score [OK]

Hint: ZSCORE always fetches a member's score in sorted sets [OK]

Common Mistakes:

• Confusing ZSCORE with ZADD or ZREM
• Thinking ZSCORE returns members instead of scores
• Assuming ZSCORE modifies the sorted set

2. Which of the following is the correct syntax to get the score of member user42 from sorted set leaderboard in Redis?

easy

A. ZSCORE leaderboard user42

B. ZSCORE user42 leaderboard

C. ZGETSCORE leaderboard user42

D. ZGET leaderboard user42

Solution

1. Step 1: Recall the correct command format

   The correct syntax for ZSCORE is: ZSCORE key member. Here, leaderboard is the key and user42 is the member.

2. Step 2: Eliminate incorrect options

   Options B, C, and D either reverse the order or use invalid commands like ZGETSCORE or ZGET, which do not exist.

3. Final Answer:

   ZSCORE leaderboard user42 -> Option A

4. Quick Check:

   ZSCORE key member = ZSCORE leaderboard user42 [OK]

Hint: Remember: ZSCORE key member order is fixed [OK]

Common Mistakes:

• Swapping key and member positions
• Using non-existent commands like ZGET or ZGETSCORE
• Missing the key or member argument

3. Given the sorted set players with members and scores: {"alice": 50, "bob": 75, "carol": 60}, what will be the result of ZSCORE players bob?

medium

A. 75

B. 50

C. 60

D. nil

Solution

1. Step 1: Identify the member's score in the sorted set

   Member bob has a score of 75 in the players sorted set.

2. Step 2: Understand ZSCORE output

   The command ZSCORE players bob returns the score of bob, which is 75.

3. Final Answer:

   75 -> Option A

4. Quick Check:

   bob's score = 75 [OK]

Hint: ZSCORE returns the exact score of the member [OK]

Common Mistakes:

• Confusing scores of different members
• Expecting a list instead of a single score
• Assuming nil if member exists

4. You run the command ZSCORE leaderboard player1 but get (nil) as a result. What could be the reason?

medium

A. The member player1 has a score of 0

B. The member player1 is not in the sorted set

C. The command syntax is incorrect

D. Redis server is down

Solution

1. Step 1: Understand what (nil) means in ZSCORE

   When ZSCORE returns (nil), it means the member does not exist in the sorted set.

2. Step 2: Check other possibilities

   A score of 0 for player1 would return "0", not (nil). Syntax errors cause error messages, not (nil). Server down would cause connection errors.

3. Final Answer:

   The member player1 is not in the sorted set -> Option B

4. Quick Check:

   Member missing = (nil) result [OK]

Hint: (nil) means member not found in sorted set [OK]

Common Mistakes:

• Thinking (nil) means the score is 0
• Assuming (nil) means syntax error
• Thinking (nil) means server issue

5. You have a sorted set tasks with members and scores: {"task1": 10, "task2": 20, "task3": 30}. You want to check if task4 exists and get its score or return 0 if it doesn't. Which Redis command sequence achieves this?

hard

A. Use ZRANK tasks task4 to get score or 0

B. Use ZGET tasks task4 to get score or 0

C. Use ZCOUNT tasks task4 task4 to get score or 0

D. Use ZSCORE tasks task4 and if result is nil, treat score as 0

Solution

1. Step 1: Identify how to get a member's score safely

   ZSCORE returns the score or (nil) if the member does not exist.

2. Step 2: Handle missing member case

   Since ZSCORE returns (nil) for missing members, you can check this result and treat it as 0 in your application logic.

3. Step 3: Evaluate other options

   ZGET does not exist, ZRANK returns rank, not score, and ZCOUNT counts members in score range, not score value.

4. Final Answer:

   Use ZSCORE tasks task4 and if result is nil, treat score as 0 -> Option D

5. Quick Check:

   ZSCORE + nil check = safe score retrieval [OK]

Hint: Use ZSCORE and handle nil as zero in your code [OK]

Common Mistakes:

• Using non-existent commands like ZGET
• Confusing rank with score using ZRANK
• Using ZCOUNT which counts members, not scores