ZINCRBY for score updates in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the new score of 'player1' after this command?

Given a sorted set 'game_scores' with 'player1' having a score of 50, what will be the score of 'player1' after running:

ZINCRBY game_scores 20 player1

Redis

ZINCRBY game_scores 20 player1

A 70

B 20

C 50

D Error: member not found

💡 Hint

ZINCRBY adds the increment to the current score of the member.

ZINCRBY increases the score of 'player1' by 20. Since the original score was 50, the new score is 70.

❓ query_result

intermediate

What happens if the member does not exist?

What will be the result of running:

ZINCRBY game_scores 15 playerX

when 'playerX' is not in the sorted set 'game_scores'?

Redis

ZINCRBY game_scores 15 playerX

A 0

B Error: member not found

C No change, returns nil

D 15

💡 Hint

ZINCRBY adds the member if it does not exist, starting with the increment as score.

If the member does not exist, ZINCRBY adds it with the increment as its score, so the score becomes 15.

📝 Syntax

advanced

Which command syntax is correct to increase 'player2' score by 10?

Choose the correct Redis command to increase the score of 'player2' by 10 in the sorted set 'game_scores'.

A ZINCRBY game_scores player2 10

B ZINCRBY 10 game_scores player2

C ZINCRBY game_scores 10 player2

D ZINCRBY player2 game_scores 10

💡 Hint

The syntax is ZINCRBY key increment member.

The correct syntax is ZINCRBY followed by the key, then the increment, then the member name.

❓ optimization

advanced

How to efficiently update multiple player scores?

You want to increase scores of multiple players in 'game_scores' by different amounts. Which approach is best for performance?

A Use a Redis pipeline to send multiple ZINCRBY commands at once.

B Run multiple ZINCRBY commands one by one.

C Use ZINCRBY with multiple members in one command.

D Delete and recreate the sorted set with updated scores.

💡 Hint

Pipelining reduces network round trips.

Redis pipelines allow sending multiple commands in one go, reducing latency and improving performance.

🧠 Conceptual

expert

What is the effect of using a negative increment with ZINCRBY?

If you run:

ZINCRBY game_scores -30 player3

and 'player3' has a current score of 25, what will be the new score and what happens if the score becomes negative?

A Member is removed if score goes below zero.

B New score is -5; Redis allows negative scores in sorted sets.

C Error; Redis does not allow negative increments.

D New score is 0; Redis sets negative scores to zero automatically.

💡 Hint

Redis sorted sets can have negative scores.

ZINCRBY allows negative increments and scores can be negative; Redis does not remove members or clamp scores automatically.

Practice

1.

What does the Redis command ZINCRBY do?

easy

A. It creates a new sorted set without any members.

B. It increases the score of a member in a sorted set by a given amount.

C. It returns the rank of a member in a sorted set.

D. It deletes a member from a sorted set.

Solution

1. Step 1: Understand the purpose of ZINCRBY

   ZINCRBY is used to increase the score of a member in a sorted set by a specified increment.

2. Step 2: Compare with other options

   Other options describe different commands: deletion, rank retrieval, or creation, which are not what ZINCRBY does.

3. Final Answer:

   It increases the score of a member in a sorted set by a given amount. -> Option B

4. Quick Check:

   ZINCRBY increments score of member [OK]

Hint: ZINCRBY always adds to a member's score in sorted sets [OK]

Common Mistakes:

• Confusing ZINCRBY with ZREM (removal)
• Thinking it returns rank instead of updating score
• Assuming it creates a new sorted set without members

2.

Which of the following is the correct syntax to increase the score of member user1 by 5 in sorted set leaderboard?

?

easy

A. ZINCRBY leaderboard 5 user1

B. ZINCRBY leaderboard user1 5

C. ZINCRBY 5 leaderboard user1

D. ZINCRBY user1 leaderboard 5

Solution

1. Step 1: Recall ZINCRBY syntax

   The correct syntax is: ZINCRBY key increment member.

2. Step 2: Match syntax with options

   ZINCRBY leaderboard 5 user1 matches the correct order: ZINCRBY leaderboard 5 user1.

3. Final Answer:

   ZINCRBY leaderboard 5 user1 -> Option A

4. Quick Check:

   Command order = ZINCRBY key increment member [OK]

Hint: Remember: ZINCRBY key increment member [OK]

Common Mistakes:

• Swapping member and increment positions
• Putting member before key
• Using increment before key

3.

Given the following Redis commands executed in order:

ZINCRBY scores 10 alice
ZINCRBY scores 5 bob
ZINCRBY scores -3 alice
ZINCRBY scores 2 charlie

What is the score of alice after these commands?

medium

A. 7

B. 10

C. 12

D. -3

Solution

1. Step 1: Calculate alice's score changes

   First increment: +10, then decrement: -3, total = 10 - 3 = 7.

2. Step 2: Confirm final score

   Other members' scores do not affect alice's score.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   10 - 3 = 7 [OK]

Hint: Add increments for the same member to get final score [OK]

Common Mistakes:

• Ignoring negative increments
• Adding scores of other members
• Using only the first increment

4.

What is wrong with this Redis command to increase user2's score by 4 in sorted set game_scores?

ZINCRBY game_scores user2 4

medium

A. The increment value must be negative.

B. The key name is missing.

C. The command should be ZADD instead of ZINCRBY.

D. The increment and member arguments are swapped.

Solution

1. Step 1: Check argument order for ZINCRBY

   ZINCRBY syntax is: ZINCRBY key increment member. Here, 'user2' and '4' are swapped.

2. Step 2: Identify the error

   The command should be: ZINCRBY game_scores 4 user2.

3. Final Answer:

   The increment and member arguments are swapped. -> Option D

4. Quick Check:

   Order must be key, increment, member [OK]

Hint: Increment always comes before member in ZINCRBY [OK]

Common Mistakes:

• Swapping increment and member
• Using ZADD instead of ZINCRBY for increment
• Thinking increment must be negative

5.

You have a sorted set ranking with members and scores:

"alice" 15
"bob" 20
"carol" 10

You want to increase carol's score by 12 and decrease bob's score by 5 using Redis commands. Which sequence of commands correctly updates the scores?

hard

A. ZADD ranking 22 carol
ZADD ranking 15 bob

B. ZINCRBY ranking carol 12
ZINCRBY ranking bob -5

C. ZINCRBY ranking 12 carol
ZINCRBY ranking -5 bob

D. ZINCRBY ranking 5 carol
ZINCRBY ranking -12 bob

Solution

1. Step 1: Apply correct ZINCRBY syntax for increments

   To increase carol's score by 12: ZINCRBY ranking 12 carol.

2. Step 2: Apply correct ZINCRBY syntax for decrement

   To decrease bob's score by 5: ZINCRBY ranking -5 bob.

3. Step 3: Verify other options

   ZINCRBY ranking carol 12 ZINCRBY ranking bob -5 swaps increment and member, ZADD ranking 22 carol ZADD ranking 15 bob uses ZADD which replaces scores, ZINCRBY ranking 5 carol ZINCRBY ranking -12 bob uses wrong increments.

4. Final Answer:

   ZINCRBY ranking 12 carol ZINCRBY ranking -5 bob -> Option C

5. Quick Check:

   Use ZINCRBY with positive and negative increments [OK]

Hint: Use positive or negative increments with ZINCRBY to adjust scores [OK]

Common Mistakes:

• Swapping member and increment arguments
• Using ZADD which overwrites scores instead of incrementing
• Using wrong increment values