ZRANK and ZREVRANK for position in Redis

Introduction

ZRANK and ZREVRANK help you find the position of an item in a sorted list, so you know where it stands compared to others.

You want to see a player's rank in a game leaderboard.

You need to find the position of a product in a list sorted by price.

You want to check the order of tasks by priority in a to-do list.

You want to know the rank of a student based on their score.

You want to find the position of a word in a sorted dictionary.

Syntax

Redis

ZRANK key member
ZREVRANK key member

ZRANK returns the position of the member with scores ordered from low to high (0 is the top).

ZREVRANK returns the position with scores ordered from high to low (0 is the top).

Examples

Finds the rank of 'player1' in 'leaderboard' sorted from lowest to highest score.

Redis

ZRANK leaderboard player1

Finds the rank of 'player1' in 'leaderboard' sorted from highest to lowest score.

Redis

ZREVRANK leaderboard player1

Gets the position of 'item42' in the sorted set 'scores' from low to high.

Redis

ZRANK scores item42

Sample Program

We add three players with scores. Then we find player3's rank from low to high and from high to low.

Redis

ZADD leaderboard 100 player1 200 player2 150 player3
ZRANK leaderboard player3
ZREVRANK leaderboard player3

Important Notes

If the member does not exist, both commands return null (no rank).

Ranks start at 0, so the top item has rank 0.

ZRANK and ZREVRANK are fast even with many items.

Summary

ZRANK shows position with lowest score first (ascending).

ZREVRANK shows position with highest score first (descending).

Both help find where an item stands in a sorted set.

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