ZRANK and ZREVRANK for position in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find the rank of a member using ZRANK

Given a sorted set with members and scores, what is the rank of member 'apple' using ZRANK?

Redis

ZADD fruits 10 apple 20 banana 15 cherry
ZRANK fruits apple

A 1

B 2

C 0

D nil

💡 Hint

ZRANK returns the zero-based rank of the member ordered from lowest to highest score.

The sorted set 'fruits' has 'apple' with score 10, which is the lowest score, so its rank is 0.

❓ query_result

intermediate

Find the reverse rank of a member using ZREVRANK

Given a sorted set with members and scores, what is the reverse rank of member 'banana' using ZREVRANK?

Redis

ZADD fruits 10 apple 20 banana 15 cherry
ZREVRANK fruits banana

A 0

B 1

C 2

D nil

💡 Hint

ZREVRANK returns the zero-based rank of the member ordered from highest to lowest score.

The sorted set 'fruits' has 'banana' with the highest score 20, so its reverse rank is 0.

📝 Syntax

advanced

Identify the error in using ZRANK with a non-existing member

What is the output of the command ZRANK fruits orange if 'orange' is not a member of the sorted set?

Redis

ZADD fruits 10 apple 20 banana 15 cherry
ZRANK fruits orange

A -1

B 0

C Error: member not found

D nil

💡 Hint

ZRANK returns nil if the member does not exist in the sorted set.

Since 'orange' is not in the sorted set, ZRANK returns nil indicating no rank.

❓ query_result

advanced

Compare ZRANK and ZREVRANK outputs

Given the sorted set fruits with scores, what are the outputs of ZRANK fruits cherry and ZREVRANK fruits cherry?

Redis

ZADD fruits 10 apple 20 banana 15 cherry
ZRANK fruits cherry
ZREVRANK fruits cherry

A ZRANK: 1, ZREVRANK: 1

B ZRANK: 2, ZREVRANK: 0

C ZRANK: 1, ZREVRANK: 0

D ZRANK: 0, ZREVRANK: 2

💡 Hint

ZRANK orders from lowest to highest score, ZREVRANK from highest to lowest.

'cherry' has score 15, which is the middle score. So rank is 1 and reverse rank is also 1.

🧠 Conceptual

expert

Understanding rank behavior with duplicate scores

If a sorted set has members with duplicate scores, how does ZRANK determine the rank of a member?

A ZRANK returns the average rank of all members with the same score.

B ZRANK orders members with the same score lexicographically and returns the zero-based rank accordingly.

C ZRANK returns the highest rank among members with the same score.

D ZRANK returns a random rank among members with the same score.

💡 Hint

Redis sorts members with the same score by their member name in alphabetical order.

When scores tie, Redis orders members lexicographically by their member name to assign ranks.

Practice

1. What does the Redis command ZRANK return when used on a sorted set?

easy

A. The total number of members in the sorted set

B. The zero-based rank of a member with scores in ascending order

C. The score of the member in the sorted set

D. The member with the highest score

Solution

1. Step 1: Understand what ZRANK does

   ZRANK returns the position of a member in a sorted set ordered by score from lowest to highest.

2. Step 2: Identify the rank type

   The rank is zero-based, meaning the lowest score member has rank 0.

3. Final Answer:

   The zero-based rank of a member with scores in ascending order -> Option B

4. Quick Check:

   ZRANK = ascending zero-based rank [OK]

Hint: ZRANK ranks ascending, starting at zero [OK]

Common Mistakes:

• Confusing rank with score
• Thinking rank starts at 1
• Mixing ZRANK with ZREVRANK

2. Which of the following is the correct syntax to get the rank of member 'user1' in sorted set 'scores' using ZREVRANK?

easy

A. ZRANK user1 scores

B. ZREVRANK user1 scores

C. ZRANK scores user1

D. ZREVRANK scores user1

Solution

1. Step 1: Recall ZREVRANK command syntax

   The syntax is: ZREVRANK key member

2. Step 2: Apply to given example

   Here, key is 'scores' and member is 'user1', so command is ZREVRANK scores user1

3. Final Answer:

   ZREVRANK scores user1 -> Option D

4. Quick Check:

   ZREVRANK key member syntax [OK]

Hint: Command syntax: ZREVRANK key member [OK]

Common Mistakes:

• Swapping key and member order
• Using ZRANK instead of ZREVRANK
• Adding extra parameters

3. Given a sorted set 'players' with members and scores: {"alice": 50, "bob": 70, "carol": 60}, what is the result of ZRANK players bob?

medium

A. 2

B. 1

C. 0

D. None (member not found)

Solution

1. Step 1: Sort members by ascending score

   Sorted by score ascending: alice(50), carol(60), bob(70)

2. Step 2: Find zero-based rank of 'bob'

   bob is third in order, so rank is 2 (0-based)

3. Final Answer:

   2 -> Option A

4. Quick Check:

   ZRANK ascending order rank = 2 [OK]

Hint: ZRANK counts from lowest score, zero-based [OK]

Common Mistakes:

• Assuming rank is score
• Confusing ascending with descending order
• Using ZREVRANK result instead

4. You run ZREVRANK leaderboard "playerX" but get a null reply. What is the most likely reason?

medium

A. "playerX" is not a member of the sorted set "leaderboard"

B. The command syntax is incorrect

C. The sorted set "leaderboard" is empty

D. ZREVRANK only works on lists, not sorted sets

Solution

1. Step 1: Understand ZREVRANK null reply meaning

   Null reply means the member does not exist in the sorted set.

2. Step 2: Check other options

   Syntax is correct, empty set would also return null but question implies member missing, and ZREVRANK works only on sorted sets.

3. Final Answer:

   "playerX" is not a member of the sorted set "leaderboard" -> Option A

4. Quick Check:

   Null reply means member missing [OK]

Hint: Null reply means member not in set [OK]

Common Mistakes:

• Thinking syntax error causes null
• Assuming empty set returns error
• Confusing data types for ZREVRANK

5. You have a sorted set 'scores' with members and scores: {"x": 10, "y": 20, "z": 30}. You want to find the rank of member 'y' counting from highest score first. Which command and expected result is correct?

hard

A. ZRANK scores y returns 1

B. ZREVRANK scores y returns 2

C. ZREVRANK scores y returns 1

D. ZRANK scores y returns 2

Solution

1. Step 1: Understand ZREVRANK ranks descending

   ZREVRANK ranks members from highest to lowest score, zero-based.

2. Step 2: Sort members descending and find 'y' rank

   Descending order: z(30), y(20), x(10). 'y' is second, so rank is 1.

3. Final Answer:

   ZREVRANK scores y returns 1 -> Option C

4. Quick Check:

   ZREVRANK descending zero-based rank = 1 [OK]

Hint: ZREVRANK ranks from highest score, zero-based [OK]

Common Mistakes:

• Mixing ZRANK and ZREVRANK results
• Counting rank starting at 1 instead of 0
• Confusing ascending and descending order