List vs sorted set for sequences in Redis - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the end of a Redis list.

Redis

redis-cli RPUSH mylist [1]

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

Aelement

Bmylist

CZADD

DSADD

Attempts:

3 left

💡 Hint

Common Mistakes

Using the list name instead of the element as the second argument.

Confusing RPUSH with ZADD or SADD commands.

RPUSH adds the specified element to the end of the list named 'mylist'.

2fill in blank

medium

Complete the code to add an element with a score to a Redis sorted set.

Redis

redis-cli ZADD myzset [1] element

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

Ascore

BZREM

Cmyzset

Delement

Attempts:

3 left

💡 Hint

Common Mistakes

Using the element name instead of the score.

Confusing ZADD with ZREM or other commands.

ZADD requires the sorted set name, a score, and the element to add. The score determines the order.

3fill in blank

hard

Fix the error in the command to get the first 3 elements from a Redis list.

Redis

redis-cli LRANGE mylist [1] 2

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

A-1

B0

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 1 instead of 0, which skips the first element.

Using 3 as start index which is out of range.

LRANGE uses zero-based indexes. To get the first 3 elements, start at 0 and end at 2.

4fill in blank

hard

Fill both blanks to retrieve elements with scores between 10 and 20 from a sorted set.

Redis

redis-cli ZRANGEBYSCORE myzset [1] [2]

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

A10

B20

C-inf

D+inf

Attempts:

3 left

💡 Hint

Common Mistakes

Using -inf or +inf instead of specific score values.

Swapping min and max values.

ZRANGEBYSCORE returns elements with scores between the given min and max values, inclusive.

5fill in blank

hard

Fill all three blanks to add an element with score 15 and then retrieve elements with scores between 10 and 20.

Redis

redis-cli [1] myzset [2] element && redis-cli ZRANGEBYSCORE myzset 10 20

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

AZADD

B15

Cmyzset

DSADD

Attempts:

3 left

💡 Hint

Common Mistakes

Using SADD instead of ZADD for sorted sets.

Mixing up the order of arguments.

First, use ZADD to add 'element' with score 15 to 'myzset'. Then use ZRANGEBYSCORE to get elements with scores between 10 and 20.

Practice

1. Which Redis data structure allows storing duplicate elements while preserving their order?

easy

A. Hash

B. List

C. Sorted Set

D. Set

Solution

1. Step 1: Understand List behavior

   Lists in Redis keep elements in the order they were added and allow duplicates.

2. Step 2: Compare with Sorted Set

   Sorted sets store unique elements sorted by score, so duplicates are not allowed.

3. Final Answer:

   List -> Option B

4. Quick Check:

   Duplicates allowed and order preserved = List [OK]

Hint: Duplicates allowed? Use List, not Sorted Set. [OK]

Common Mistakes:

• Thinking Sorted Set allows duplicates
• Confusing Sets with Lists
• Assuming Hashes store sequences

2. Which of the following Redis commands correctly adds an element with a score to a sorted set?

easy

A. HSET myhash field element

B. LPUSH mylist element

C. SADD myset element

D. ZADD myzset 10 element

Solution

1. Step 1: Identify command for sorted set insertion

   ZADD adds elements with a score to a sorted set.

2. Step 2: Check other commands

   LPUSH adds to list, SADD adds to set, HSET adds to hash field.

3. Final Answer:

   ZADD myzset 10 element -> Option D

4. Quick Check:

   ZADD = add with score [OK]

Hint: ZADD is for sorted sets with scores. [OK]

Common Mistakes:

• Using LPUSH for sorted sets
• Confusing SADD with ZADD
• Using HSET for sequences

3. Given the following Redis commands executed in order:
LPUSH mylist a
LPUSH mylist b
LPUSH mylist c
What will LRANGE mylist 0 -1 return?

medium

A. ["a", "b", "c"]

B. ["b", "c", "a"]

C. ["c", "b", "a"]

D. Error

Solution

1. Step 1: Understand LPUSH order

   LPUSH adds elements to the head (start) of the list, so last pushed is first.

2. Step 2: Trace the list after pushes

   LPUSH mylist a: ["a"], then LPUSH mylist b: ["b", "a"], then LPUSH mylist c: ["c", "b", "a"]. LRANGE mylist 0 -1 returns ["c", "b", "a"].

3. Final Answer:

   ["c", "b", "a"] -> Option C

4. Quick Check:

   LPUSH adds to head, so newest first [OK]

Hint: LPUSH adds to front; last pushed appears first. [OK]

Common Mistakes:

• Assuming LPUSH adds to tail
• Mixing order of elements
• Expecting error from LRANGE

4. You tried to add duplicate elements with different scores to a sorted set using:
ZADD myzset 1 element
ZADD myzset NX 2 element
But the score did not update as expected. What is the likely issue?

medium

A. Sorted sets do not allow duplicate elements; score updates require special flag

B. ZADD command syntax is incorrect

C. You must remove the element before adding it again

D. Redis lists should be used instead

Solution

1. Step 1: Understand sorted set uniqueness

   Sorted sets store unique elements; adding the same element updates its score by default.

2. Step 2: Check if score updates require flags

   By default, ZADD updates score if element exists; if it did not update, likely a misunderstanding or missing flag like NX or XX.

3. Final Answer:

   Sorted sets do not allow duplicate elements; score updates require special flag -> Option A

4. Quick Check:

   Sorted sets unique, score updates need care [OK]

Hint: Sorted sets unique; score updates may need flags. [OK]

Common Mistakes:

• Thinking duplicates allowed in sorted sets
• Assuming ZADD syntax is wrong
• Trying to remove before adding

5. You want to store a sequence of user actions with timestamps and retrieve them sorted by time, allowing multiple actions at the same timestamp. Which Redis data structure and approach is best?

hard

A. Use a Sorted Set with timestamps as scores; append a unique suffix to actions to allow duplicates

B. Use a List and store actions with timestamps as strings; order is insertion order

C. Use a Hash with timestamps as fields and actions as values

D. Use a Set to store actions; order does not matter

Solution

1. Step 1: Analyze requirement for sorting by timestamp

   Sorted sets sort elements by numeric score, perfect for timestamps.

2. Step 2: Handle duplicates with same timestamp

   Sorted sets require unique elements, so append unique suffix to actions to allow duplicates with same score.

3. Step 3: Compare with other options

   Lists preserve order but cannot sort by timestamp easily; hashes and sets do not maintain order or allow sorting by score.

4. Final Answer:

   Use a Sorted Set with timestamps as scores; append a unique suffix to actions to allow duplicates -> Option A

5. Quick Check:

   Sorted set + unique elements for timestamp sorting [OK]

Hint: Sorted set + unique suffix for duplicates with same score. [OK]

Common Mistakes:

• Using List but losing timestamp sorting
• Ignoring uniqueness in sorted sets
• Using Hash or Set which don't sort