ZRANGE and ZREVRANGE for reading in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the first 3 elements from a sorted set named 'myzset'.

Redis

ZRANGE myzset 0 [1]

Drag options to blanks, or click blank then click option'

A2

B3

C-1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as the end index returns 4 elements instead of 3.

Using negative indices without understanding their meaning.

The command ZRANGE myzset 0 2 returns the first 3 elements because the range is inclusive and zero-based.

2fill in blank

medium

Complete the code to get the last 2 elements from a sorted set named 'myzset' using ZREVRANGE.

Redis

ZREVRANGE myzset [1] -1

Drag options to blanks, or click blank then click option'

A0

B-2

C-1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as the start index returns elements from the beginning, not the end.

Using -1 as the start index returns only the last element.

The command ZREVRANGE myzset -2 -1 returns the last 2 elements in reverse order.

3fill in blank

hard

Fix the error in the command to get elements from index 1 to 3 in 'myzset'.

Redis

ZRANGE myzset [1] 3

Drag options to blanks, or click blank then click option'

A1

B0

C2

D-3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as the start index returns elements from index 0, not 1.

Using negative indices incorrectly.

The correct start index is 1 to get elements from index 1 to 3 inclusive.

4fill in blank

hard

Fill both blanks to get elements from index 2 to 4 in reverse order from 'myzset'.

Redis

ZREVRANGE myzset [1] [2]

Drag options to blanks, or click blank then click option'

A4

B2

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping start and end indices incorrectly.

Using negative indices without understanding their effect.

Using ZREVRANGE myzset 2 4 returns elements from index 2 to 4 in reverse order.

5fill in blank

hard

Fill all three blanks to get elements from index 1 to 3 with scores included from 'myzset'.

Redis

ZRANGE myzset [1] [2] [3]

Drag options to blanks, or click blank then click option'

A0

B1

CWITHSCORES

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Placing WITHSCORES before indices.

Using incorrect indices that do not match the desired range.

The command ZRANGE myzset 1 3 WITHSCORES returns elements from index 1 to 3 along with their scores.

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