LREM for element removal in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove all occurrences of 'apple' from the list 'fruits'.

Redis

LREM fruits 0 [1]

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

A"apple"

Bapple

C'apple'

Dapple"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the element.

Using mismatched or missing quotes.

The LREM command requires the element to be a string enclosed in quotes. Using single quotes around 'apple' is correct.

2fill in blank

medium

Complete the code to remove only the first occurrence of 'banana' from the list 'fruits'.

Redis

LREM fruits [1] "banana"

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

A-1

B0

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 removes all occurrences, not just the first.

Using negative numbers removes from tail to head.

Using 1 as the count removes the first occurrence of the element.

3fill in blank

hard

Fix the error in the code to remove the last occurrence of 'orange' from the list 'fruits'.

Redis

LREM fruits [1] "orange"

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

A-1

B0

C1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 removes the first occurrence, not the last.

Using 0 removes all occurrences.

Using -1 as the count removes the last occurrence of the element from the list.

4fill in blank

hard

Fill both blanks to remove two occurrences of 'grape' starting from the tail in the list 'fruits'.

Redis

LREM fruits [1] [2]

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

A-2

B2

C"grape"

D"apple"

Attempts:

3 left

💡 Hint

Common Mistakes

Using positive count removes from head, not tail.

Using wrong element name.

Count -2 removes two occurrences from tail to head. The element to remove is "grape".

5fill in blank

hard

Fill all three blanks to remove three occurrences of 'melon' starting from the head in the list 'fruits'.

Redis

LREM [1] [2] [3]

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

Afruits

B3

C"melon"

D"banana"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong list name.

Using negative count to remove from tail.

Using wrong element name.

The command removes 3 occurrences of "melon" from the list named fruits starting from the head.

Practice

1. What does the Redis command LREM mylist 2 "apple" do?

easy

A. Removes up to 2 occurrences of "apple" from the head of the list.

B. Removes all occurrences of "apple" from the list.

C. Removes 2 elements from the tail of the list regardless of value.

D. Removes the first 2 elements of the list.

Solution

1. Step 1: Understand the LREM command parameters

   LREM takes a key, a count, and a value. The count 2 means remove up to 2 matching elements.

2. Step 2: Interpret the count sign and value

   Positive count removes from head to tail. So it removes up to 2 "apple" elements starting from the front.

3. Final Answer:

   Removes up to 2 occurrences of "apple" from the head of the list. -> Option A

4. Quick Check:

   LREM with positive count removes from head [OK]

Hint: Positive count removes from head; negative from tail [OK]

Common Mistakes:

• Thinking count removes elements regardless of value
• Confusing positive and negative count directions
• Assuming count removes all matches always

2. Which of the following is the correct syntax to remove exactly 3 occurrences of "banana" from the tail of a Redis list named fruits?

easy

A. LREM fruits 3 "banana"

B. LREM fruits -3 "banana"

C. LREM fruits 0 "banana"

D. LREM fruits "banana" 3

Solution

1. Step 1: Recall LREM syntax

   The syntax is LREM key count value. Negative count removes from tail.

2. Step 2: Identify correct count for tail removal

   To remove 3 occurrences from tail, count must be -3.

3. Final Answer:

   LREM fruits -3 "banana" -> Option B

4. Quick Check:

   Negative count removes from tail [OK]

Hint: Use negative count to remove from tail [OK]

Common Mistakes:

• Using positive count to remove from tail
• Swapping value and count positions
• Using zero count to remove specific elements

3. Given a Redis list colors with elements ["red", "blue", "red", "green", "red"], what will be the list after executing LREM colors 2 "red"?

medium

A. ["blue", "red", "green", "red"]

B. ["red", "blue", "green"]

C. ["blue", "green", "red"]

D. ["red", "blue", "red", "green"]

Solution

1. Step 1: Identify elements and count

   List is ["red", "blue", "red", "green", "red"]. LREM with count 2 removes up to 2 "red" from head.

2. Step 2: Remove first 2 "red" from head

   First "red" at index 0 removed, second "red" at index 2 removed. Remaining elements: ["blue", "green", "red"]

3. Final Answer:

   ["blue", "green", "red"] -> Option C

4. Quick Check:

   Removed first 2 "red" from head [OK]

Hint: Count 2 removes first 2 matches from head [OK]

Common Mistakes:

• Removing all "red" elements instead of 2
• Removing from tail instead of head
• Confusing list order after removal

4. You run LREM mylist -1 "orange" but it does not remove any element. What could be the reason?

medium

A. The list mylist does not contain "orange".

B. Negative count cannot be used with LREM.

C. The command syntax is incorrect; value must come before count.

D. LREM only removes elements from the head, not the tail.

Solution

1. Step 1: Check command syntax and usage

   LREM syntax is correct and negative count removes from tail.

2. Step 2: Consider why no removal happened

   If no elements removed, likely the value "orange" is not in the list.

3. Final Answer:

   The list mylist does not contain "orange". -> Option A

4. Quick Check:

   No matching element means no removal [OK]

Hint: No matching value means LREM removes nothing [OK]

Common Mistakes:

• Thinking negative count is invalid
• Assuming syntax order is different
• Believing LREM only removes from head

5. You have a Redis list tasks with elements ["email", "call", "email", "meeting", "email", "call"]. You want to remove all occurrences of "email" except the last one. Which sequence of commands achieves this?

hard

A. LREM tasks -2 "email"

B. LREM tasks 0 "email"

C. LREM tasks -5 "email"

D. LREM tasks 2 "email"

Solution

1. Step 1: Count total "email" elements

   There are 3 "email" elements at positions 0, 2, 4.

2. Step 2: Removal strategy

   Positive count removes matches from head-to-tail. Count 2 removes the first two "email"s, leaving the last one.

3. Step 3: Simulate execution

   Remove index 0, list shifts to ["call", "email", "meeting", "email", "call"], then remove next match at new index 1 (orig 2). Result: ["call", "meeting", "email", "call"].

4. Final Answer:

   LREM tasks 2 "email" -> Option D

5. Quick Check:

   Positive count removes first matches from head [OK]

Hint: Positive count removes early matches from head [OK]

Common Mistakes:

• Using negative count removes later matches from tail
• Using zero count removes all occurrences
• Using large negative count removes all if exceeds occurrences