ZRANGE and ZREVRANGE for reading in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command ZRANGE do?

It returns a range of members in a sorted set, ordered from the lowest to the highest score.

beginner

How does ZREVRANGE differ from ZRANGE?

ZREVRANGE returns members in a sorted set ordered from the highest to the lowest score, reversing the order of ZRANGE.

beginner

What do the start and stop parameters in ZRANGE key start stop represent?

They specify the index range of elements to return, where 0 is the first element, 1 the second, and so on. Negative numbers count from the end (-1 is last).

intermediate

How can you include the scores of members when using ZRANGE or ZREVRANGE?

Add the WITHSCORES option to the command to get each member's score along with the member.

beginner

If you want to get the top 3 highest scored members from a sorted set, which command would you use?

Use ZREVRANGE key 0 2 to get the first 3 members with the highest scores.

What order does ZRANGE return members from a sorted set?

A Alphabetical order

B From highest to lowest score

C Random order

D From lowest to highest score

ZRANGE returns members ordered by increasing score, from lowest to highest.

Which command returns members from highest to lowest score?

A ZRANGE

B ZREVRANGE

C ZADD

D ZREM

ZREVRANGE returns members ordered from highest to lowest score.

What does the WITHSCORES option do when used with ZRANGE or ZREVRANGE?

A Returns members and their scores

B Sorts members alphabetically

C Returns only scores

D Deletes members with scores

WITHSCORES returns each member along with its score.

If you want the last 2 members in a sorted set using ZRANGE, what start and stop values do you use?

A 1 2

B 0 1

C -2 -1

D -1 0

Negative indexes count from the end, so -2 -1 gets the last two members.

Which command would you use to get the top 5 lowest scored members?

A ZRANGE key 0 4

B ZREVRANGE key 0 4

C ZRANGE key -5 -1

D ZREVRANGE key -5 -1

ZRANGE key 0 4 returns the first 5 members with the lowest scores.

Explain how ZRANGE and ZREVRANGE commands work in Redis sorted sets.

Think about the order and how you specify which members to get.

Describe how to retrieve the top 3 highest scored members from a Redis sorted set including their scores.

Remember ZREVRANGE reverses the order to highest first.

Practice

1. What does the Redis command ZRANGE myset 0 2 do?

easy

A. Deletes the first three elements from the sorted set 'myset'.

B. Returns the first three elements with the lowest scores from the sorted set 'myset'.

C. Returns the last three elements with the highest scores from the sorted set 'myset'.

D. Returns all elements from the sorted set 'myset' without any order.

Solution

1. Step 1: Understand ZRANGE command behavior

   ZRANGE returns elements in ascending order by score, from the start index to the end index.

2. Step 2: Apply the index range 0 to 2

   Indexes 0 to 2 mean the first three elements with the lowest scores are returned.

3. Final Answer:

   Returns the first three elements with the lowest scores from the sorted set 'myset'. -> Option B

4. Quick Check:

   ZRANGE with 0 2 = first 3 lowest scored items [OK]

Hint: ZRANGE gets items low to high by index range [OK]

Common Mistakes:

• Confusing ZRANGE with ZREVRANGE order
• Thinking it deletes elements
• Assuming it returns all elements unordered

2. Which of the following is the correct syntax to get the top 5 highest scored elements from a sorted set named 'scores'?

easy

A. ZREVRANGE scores 0 4

B. ZRANGE scores -5 -1

C. ZRANGE scores 0 4 WITHSCORES

D. ZREVRANGE scores 5 0

Solution

1. Step 1: Identify command for highest scores first

   ZREVRANGE returns elements from highest to lowest score.

2. Step 2: Use index range 0 to 4 for top 5 elements

   Indexes 0 to 4 select the first 5 elements in descending order.

3. Final Answer:

   ZREVRANGE scores 0 4 -> Option A

4. Quick Check:

   Top 5 highest = ZREVRANGE 0 4 [OK]

Hint: Use ZREVRANGE 0 N-1 for top N highest scores [OK]

Common Mistakes:

• Using negative indexes incorrectly with ZRANGE
• Confusing ZRANGE and ZREVRANGE
• Wrong index order like 5 0

3. Given a sorted set 'players' with elements and scores: {"Alice":10, "Bob":20, "Carol":15}, what is the output of ZRANGE players 0 1 WITHSCORES?

medium

A. ["Bob", "20", "Carol", "15"]

B. ["Alice", "10", "Bob", "20"]

C. ["Carol", "15", "Bob", "20"]

D. ["Alice", "10", "Carol", "15"]

Solution

1. Step 1: Sort elements by score ascending

   Sorted order by score: Alice(10), Carol(15), Bob(20).

2. Step 2: Select indexes 0 to 1 with scores

   Indexes 0 and 1 correspond to Alice and Carol with their scores.

3. Final Answer:

   ["Alice", "10", "Carol", "15"] -> Option D

4. Quick Check:

   ZRANGE 0 1 WITHSCORES = Alice and Carol with scores [OK]

Hint: ZRANGE returns low to high; WITHSCORES adds scores after each element [OK]

Common Mistakes:

• Mixing order of elements
• Forgetting WITHSCORES doubles output
• Assuming ZREVRANGE order

4. You run ZREVRANGE leaderboard 0 2 but get unexpected results. Which of these is a likely cause?

medium

A. ZREVRANGE does not support index ranges.

B. You used ZRANGE instead of ZREVRANGE.

C. The sorted set 'leaderboard' is empty or does not exist.

D. The index range 0 2 is invalid for ZREVRANGE.

Solution

1. Step 1: Check if sorted set exists and has elements

   If 'leaderboard' is empty or missing, ZREVRANGE returns empty result.

2. Step 2: Validate command and index usage

   ZREVRANGE supports index ranges; 0 2 is valid; using ZRANGE instead would change order but not cause empty result.

3. Final Answer:

   The sorted set 'leaderboard' is empty or does not exist. -> Option C

4. Quick Check:

   Empty set causes empty ZREVRANGE output [OK]

Hint: Empty or missing set returns empty list [OK]

Common Mistakes:

• Thinking index range is invalid
• Confusing ZRANGE and ZREVRANGE effects
• Believing ZREVRANGE lacks index support

5. You want to get the middle 3 elements by score from a sorted set 'scores' with 10 elements, ordered from lowest to highest. Which command correctly retrieves elements ranked 4th to 6th (0-based index) with their scores?

hard

A. ZRANGE scores 3 5 WITHSCORES

B. ZREVRANGE scores 3 5 WITHSCORES

C. ZRANGE scores 4 6 WITHSCORES

D. ZREVRANGE scores 4 6 WITHSCORES

Solution

1. Step 1: Identify index range for middle 3 elements

   Elements ranked 4th to 6th correspond to indexes 3, 4, 5 (0-based).

2. Step 2: Use ZRANGE for ascending order and WITHSCORES to get scores

   ZRANGE scores 3 5 WITHSCORES returns elements at indexes 3 to 5 with their scores.

3. Final Answer:

   ZRANGE scores 3 5 WITHSCORES -> Option A

4. Quick Check:

   Middle 3 elements = ZRANGE 3 5 WITHSCORES [OK]

Hint: ZRANGE with correct indexes gets middle elements low to high [OK]

Common Mistakes:

• Using ZREVRANGE which reverses order
• Off-by-one errors in index range
• Confusing rank with index