LPOP and RPOP for removal in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove and return the first element from the list stored at key.

Redis

redis.call('[1]', 'mylist')

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

ARPOP

BPOP

CLPOP

DREMOVE

Attempts:

3 left

💡 Hint

Common Mistakes

Using RPOP instead of LPOP removes from the right side.

Using POP or REMOVE commands which do not exist in Redis.

The LPOP command removes and returns the first element of the list.

2fill in blank

medium

Complete the code to remove and return the last element from the list stored at key.

Redis

redis.call('[1]', 'mylist')

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

ARPOP

BLPOP

CPOP

DDEL

Attempts:

3 left

💡 Hint

Common Mistakes

Using LPOP instead of RPOP removes from the left side.

Using POP or DEL commands which do not remove list elements.

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

3fill in blank

hard

Fix the error in the code to correctly remove the first element from the list 'tasks'.

Redis

redis.call('[1]', 'tasks')

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

ADEL

BPOP

CREMOVE

DLPOP

Attempts:

3 left

💡 Hint

Common Mistakes

Using POP or REMOVE which are invalid commands.

Using DEL which deletes the whole key, not an element.

Only LPOP correctly removes the first element from a Redis list.

4fill in blank

hard

Fill both blanks to remove the last element from 'queue' and the first element from 'stack'.

Redis

redis.call('[1]', 'queue')
redis.call('[2]', 'stack')

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

ARPOP

BLPOP

CDEL

DPOP

Attempts:

3 left

💡 Hint

Common Mistakes

Using DEL deletes the entire key, not an element.

Using POP which is not a Redis list command.

RPOP removes the last element, and LPOP removes the first element from Redis lists.

5fill in blank

hard

Fill all three blanks to remove the first element from 'list1', the last element from 'list2', and the first element from 'list3'.

Redis

redis.call('[1]', 'list1')
redis.call('[2]', 'list2')
redis.call('[3]', 'list3')

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

ARPOP

BLPOP

DDEL

Attempts:

3 left

💡 Hint

Common Mistakes

Using DEL instead of LPOP or RPOP.

Mixing up LPOP and RPOP commands.

Use LPOP to remove the first element and RPOP to remove the last element from Redis lists.

Practice

1. What does the Redis command LPOP do to a list?

easy

A. Removes and returns the first item from the list

B. Removes and returns the last item from the list

C. Adds an item to the start of the list

D. Adds an item to the end of the list

Solution

1. Step 1: Understand LPOP command

   LPOP removes the first (leftmost) item from a Redis list.

2. Step 2: Compare with other options

   Only Removes and returns the first item from the list correctly describes removal of the first item; others describe adding or removing last item.

3. Final Answer:

   Removes and returns the first item from the list -> Option A

4. Quick Check:

   LPOP = Remove first item [OK]

Hint: LPOP = Left POP (first item) [OK]

Common Mistakes:

• Confusing LPOP with RPOP
• Thinking LPOP adds items
• Assuming LPOP removes last item

2. Which of the following is the correct syntax to remove the last item from a Redis list named mylist?

easy

A. RPOP mylist

B. LPOP mylist

C. POP mylist

D. REMOVE mylist

Solution

1. Step 1: Identify command for last item removal

   RPOP removes the last (rightmost) item from a Redis list.

2. Step 2: Check syntax correctness

   RPOP mylist is the correct syntax; other options are invalid or remove first item.

3. Final Answer:

   RPOP mylist -> Option A

4. Quick Check:

   RPOP = Remove last item [OK]

Hint: RPOP = Right POP (last item) [OK]

Common Mistakes:

• Using LPOP instead of RPOP
• Using non-existent POP command
• Using REMOVE which is invalid

3. Given a Redis list colors with items ["red", "green", "blue"], what will be the result of LPOP colors followed by RPOP colors?

medium

A. "green" then "blue"

B. ["red", "blue"]

C. "red" then "blue"

D. ["green"]

Solution

1. Step 1: Apply LPOP on the list

   LPOP colors removes and returns the first item: "red". Remaining list: ["green", "blue"]

2. Step 2: Apply RPOP on the updated list

   RPOP colors removes and returns the last item: "blue". Remaining list: ["green"]

3. Final Answer:

   "red" then "blue" -> Option C

4. Quick Check:

   LPOP = "red", RPOP = "blue" [OK]

Hint: LPOP removes first, RPOP removes last [OK]

Common Mistakes:

• Thinking LPOP returns the whole list
• Confusing order of removal
• Expecting list output instead of removed items

4. You run the command LPOP mylist but get null as a result. What is the most likely reason?

medium

A. LPOP command syntax is incorrect

B. The list mylist does not exist or is empty

C. LPOP removes the last item, not the first

D. Redis server is down

Solution

1. Step 1: Understand LPOP return behavior

   If the list is empty or does not exist, LPOP returns null.

2. Step 2: Check other options

   Syntax is correct, LPOP removes first item, and server down would cause connection errors, not null.

3. Final Answer:

   The list mylist does not exist or is empty -> Option B

4. Quick Check:

   LPOP returns null if list empty [OK]

Hint: Null means empty or missing list [OK]

Common Mistakes:

• Assuming syntax error causes null
• Confusing LPOP with RPOP behavior
• Thinking server down returns null

5. You have a Redis list tasks with items ["task1", "task2", "task3", "task4"]. You want to remove the first and last tasks and get their names. Which sequence of commands will do this correctly?

hard

A. RPOP tasks; RPOP tasks

B. RPOP tasks; LPOP tasks

C. LPOP tasks; LPOP tasks

D. LPOP tasks; RPOP tasks

Solution

1. Step 1: Remove first task with LPOP

   LPOP tasks removes and returns "task1" (first item).

2. Step 2: Remove last task with RPOP

   RPOP tasks removes and returns "task4" (last item).

3. Final Answer:

   LPOP tasks; RPOP tasks -> Option D

4. Quick Check:

   First and last removed by LPOP then RPOP [OK]

Hint: LPOP first, RPOP last to remove both ends [OK]

Common Mistakes:

• Reversing command order
• Using same command twice
• Expecting one command to remove both ends