ZRANGE and ZREVRANGE for reading in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this ZRANGE command?

Given a sorted set myzset with elements {"one":1, "two":2, "three":3, "four":4}, what does the command ZRANGE myzset 1 2 return?

Redis

ZRANGE myzset 1 2

A ["four", "three"]

B ["three", "four"]

C ["two", "three"]

D ["one", "two"]

💡 Hint

ZRANGE returns elements by rank starting at 0, inclusive.

ZRANGE returns elements by their rank in ascending order. Index 1 is "two", index 2 is "three".

❓ query_result

intermediate

What is the output of this ZREVRANGE command?

Given a sorted set myzset with elements {"one":1, "two":2, "three":3, "four":4}, what does the command ZREVRANGE myzset 0 1 return?

Redis

ZREVRANGE myzset 0 1

A ["four", "three"]

B ["one", "two"]

C ["three", "two"]

D ["two", "one"]

💡 Hint

ZREVRANGE returns elements by rank in descending order starting at 0.

ZREVRANGE returns elements starting from the highest score. Index 0 is "four", index 1 is "three".

📝 Syntax

advanced

Which command correctly returns elements with scores included?

You want to get elements and their scores from myzset using ZRANGE. Which command is correct?

A ZRANGE myzset 0 -1 WITHSCORE

B ZRANGE myzset 0 -1 WITHSCORES

C ZRANGE myzset 0 -1 WITH SCORES

D ZRANGE myzset 0 -1 SCORES

💡 Hint

The option to include scores is a single word.

The correct syntax is WITHSCORES as one word without spaces.

❓ optimization

advanced

How to efficiently get the top 3 highest scored elements with scores?

Which command is the most efficient to get the top 3 elements with highest scores and their scores from myzset?

A ZREVRANGE myzset 0 2 WITHSCORES

B ZRANGE myzset -3 -1 WITHSCORES

C ZRANGE myzset 0 2 WITHSCORES

D ZREVRANGE myzset -3 -1 WITHSCORES

💡 Hint

Top highest means descending order starting at 0.

ZREVRANGE with 0 2 returns top 3 elements in descending order efficiently.

🧠 Conceptual

expert

What happens if you use negative indexes in ZRANGE?

Given myzset with 5 elements, what does ZRANGE myzset -3 -1 return?

Redis

ZRANGE myzset -3 -1

A Returns an empty list

B Returns the first 3 elements in ascending order

C Raises a syntax error

D Returns the last 3 elements in ascending order

💡 Hint

Negative indexes count from the end backwards.

Negative indexes in ZRANGE count from the end, so -3 to -1 returns last 3 elements in ascending order.

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