Why lists handle ordered sequences in Redis - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

Redis

LPUSH mylist [1]

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

Aelement

Bmylist

CLPOP

DRPOP

Attempts:

3 left

💡 Hint

Common Mistakes

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

Confusing LPUSH with LPOP or RPOP commands.

The LPUSH command adds an element to the start of the list. Here, you need to specify the element to add.

2fill in blank

medium

Complete the code to retrieve all elements from a Redis list.

Redis

LRANGE mylist [1] -1

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

A10

B-1

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using -1 as the start index, which returns an empty list.

Using 1 as the start index, which skips the first element.

The LRANGE command gets elements from a list between two indexes. Starting at 0 gets all elements from the start.

3fill in blank

hard

Fix the error in the code to remove the last element from a Redis list.

Redis

[1] mylist

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

ALPOP

BRPOP

CLPUSH

DRPUSH

Attempts:

3 left

💡 Hint

Common Mistakes

Using LPUSH or RPUSH which add elements instead of removing.

Using LPOP which removes the first element, not the last.

The RPOP command removes and returns the last element of the list.

4fill in blank

hard

Fill both blanks to add an element to the end and then get the last element from a Redis list.

Redis

RPUSH mylist [1]
[2] mylist

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

Aelement

BRPOP

CLPOP

Dmylist

Attempts:

3 left

💡 Hint

Common Mistakes

Using LPOP instead of RPOP to remove the last element.

Using the list name as the element to add.

RPUSH adds an element to the end of the list. RPOP removes and returns the last element.

5fill in blank

hard

Fill all three blanks to add an element to the start, get the first element, and remove it from a Redis list.

Redis

LPUSH mylist [1]
[2] mylist 0 0
[3] mylist

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

Aelement

BLRANGE

CLPOP

DRPUSH

Attempts:

3 left

💡 Hint

Common Mistakes

Using RPUSH instead of LPUSH to add to the start.

Using RPOP instead of LPOP to remove the first element.

Using LRANGE with wrong indexes.

LPUSH adds an element to the start. LRANGE mylist 0 0 gets the first element. LPOP removes the first element.

Practice

1. Why are Redis lists useful for storing ordered sequences?

easy

A. Because they only store unique items

B. Because they keep items in the order you add them

C. Because they store data without any order

D. Because they automatically sort items alphabetically

Solution

1. Step 1: Understand list behavior in Redis

   Redis lists keep the order of elements as they are added, preserving sequence.

2. Step 2: Compare with other data types

   Unlike sets or hashes, lists maintain insertion order, which is key for ordered sequences.

3. Final Answer:

   Because they keep items in the order you add them -> Option B

4. Quick Check:

   Lists preserve order = C [OK]

Hint: Remember: lists keep insertion order intact [OK]

Common Mistakes:

• Thinking lists sort items automatically
• Confusing lists with sets that have no order
• Assuming lists only store unique items

2. Which Redis command adds an element to the end of a list?

easy

A. LPUSH

B. RPOP

C. LPOP

D. RPUSH

Solution

1. Step 1: Identify commands for list insertion

   LPUSH adds to the start (left), RPUSH adds to the end (right) of the list.

2. Step 2: Match command to adding at the end

   RPUSH is the command that appends an element to the end of the list.

3. Final Answer:

   RPUSH -> Option D

4. Quick Check:

   RPUSH adds to end = D [OK]

Hint: RPUSH = Right push (end), LPUSH = Left push (start) [OK]

Common Mistakes:

• Confusing LPUSH and RPUSH commands
• Using LPOP or RPOP which remove elements
• Assuming LPUSH adds to the end

3. Given the commands:
RPUSH mylist A
LPUSH mylist B
RPUSH mylist C
What will LRANGE mylist 0 -1 return?

medium

A. ["A", "B", "C"]

B. ["C", "A", "B"]

C. ["B", "A", "C"]

D. ["B", "C", "A"]

Solution

1. Step 1: Trace list after each command

   RPUSH mylist A adds 'A' at end: [A]
   LPUSH mylist B adds 'B' at start: [B, A]
   RPUSH mylist C adds 'C' at end: [B, A, C]

2. Step 2: Understand LRANGE 0 -1

   LRANGE 0 -1 returns all elements from start to end in order.

3. Final Answer:

   ["B", "A", "C"] -> Option C

4. Quick Check:

   List order after pushes = B [OK]

Hint: LPUSH adds left, RPUSH adds right; order matters [OK]

Common Mistakes:

• Assuming RPUSH always adds first
• Mixing up LPUSH and RPUSH effects
• Misreading LRANGE indexes

4. You run these commands:
LPUSH mylist X
RPUSH mylist Y
LPOP mylist
But the list is empty after. What is the likely mistake?

medium

A. LPOP removed the only element, leaving list empty

B. RPUSH does not add elements to the list

C. LPUSH adds elements to the end, so LPOP removes nothing

D. You must use LRANGE to add elements

Solution

1. Step 1: Understand commands effect

   LPUSH mylist X adds 'X' at start: [X]
   RPUSH mylist Y adds 'Y' at end: [X, Y]
   LPOP mylist removes first element 'X': [Y]

2. Step 2: Check why list might be empty

   If list is empty after LPOP, likely you ran LPOP twice or list was empty before commands.

3. Final Answer:

   LPOP removed the only element, leaving list empty -> Option A

4. Quick Check:

   LPOP removes first item = A [OK]

Hint: LPOP removes first item; check list size after pops [OK]

Common Mistakes:

• Thinking RPUSH doesn't add elements
• Confusing LPUSH adds to end, not start
• Believing LRANGE adds elements

5. You want to keep a chat message history in order. Which Redis list commands let you add new messages at the end and read all messages in order?

hard

A. Use RPUSH to add messages and LRANGE 0 -1 to read all

B. Use LPUSH to add messages and LRANGE 0 -1 to read all

C. Use RPUSH to add messages and LPOP to read all

D. Use LPUSH to add messages and RPOP to read all

Solution

1. Step 1: Choose command to add messages in order

   RPUSH adds messages to the end, preserving chronological order.

2. Step 2: Choose command to read all messages in order

   LRANGE 0 -1 reads all list elements from start to end in order.

3. Final Answer:

   Use RPUSH to add messages and LRANGE 0 -1 to read all -> Option A

4. Quick Check:

   RPUSH + LRANGE = A [OK]

Hint: Add with RPUSH, read with LRANGE 0 -1 for order [OK]

Common Mistakes:

• Using LPUSH adds messages in reverse order
• Using LPOP or RPOP to read all messages
• Assuming RPOP reads messages in order