ZRANK and ZREVRANK for position in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command ZRANK do?

It returns the rank (position) of a member in a sorted set, ordered from the lowest to highest score, starting at 0.

beginner

How does ZREVRANK differ from ZRANK?

ZREVRANK returns the rank of a member in a sorted set ordered from highest to lowest score, starting at 0.

beginner

If a member is not found in the sorted set, what do ZRANK and ZREVRANK return?

Both commands return null (or nil) if the member does not exist in the sorted set.

intermediate

Example: Given a sorted set with members and scores: {"apple": 10, "banana": 20, "cherry": 15}, what is the result of ZRANK myset "cherry"?

The rank of "cherry" is 1 because the sorted order by score ascending is: apple(10), cherry(15), banana(20).

intermediate

Example: Using the same sorted set, what does ZREVRANK myset "cherry" return?

The rank of "cherry" is 1 because the descending order by score is: banana(20), cherry(15), apple(10).

What does ZRANK return if the member is the lowest scored in the sorted set?

A The highest rank number

B 1

C null

D 0

ZRANK ranks from lowest to highest score starting at 0, so the lowest scored member has rank 0.

Which command gives the rank starting from the highest score?

A ZRANGE

B ZRANK

C ZREVRANK

D ZADD

ZREVRANK ranks members from highest to lowest score.

If a member is not in the sorted set, what will ZREVRANK return?

A null

B 0

C The last rank

D An error

Both ZRANK and ZREVRANK return null if the member is missing.

Given sorted set scores: {a:5, b:10, c:15}, what is ZRANK myset "b"?

A 1

B null

C 2

D 0

Sorted ascending: a(5), b(10), c(15). "b" is at position 1.

What is the rank of the highest scored member using ZREVRANK?

A 1

B 0

C The last rank

D null

ZREVRANK starts ranking from highest score at 0.

Explain how ZRANK and ZREVRANK determine the position of a member in a Redis sorted set.

Think about the order of scores and what 0 means.

Describe a real-life example where you might use ZRANK and ZREVRANK in an application.

Consider games or scoring systems.

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