LREM for element removal in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this LREM command?

Given a Redis list mylist with elements ["apple", "banana", "apple", "cherry", "apple"], what will be the list after running LREM mylist 2 "apple"?

Redis

LREM mylist 2 "apple"

A ["apple", "banana", "cherry"]

B ["banana", "apple", "cherry", "apple"]

C ["banana", "cherry"]

D ["banana", "cherry", "apple"]

💡 Hint

LREM removes elements equal to the value, starting from the head, up to the count specified.

LREM with count 2 removes the first two occurrences of "apple" from the list. The original list has three "apple" elements. Removing the first two leaves one "apple" at the end.

❓ query_result

intermediate

What happens when LREM count is negative?

Given a Redis list mylist with elements ["x", "y", "x", "z", "x"], what will be the list after running LREM mylist -1 "x"?

Redis

LREM mylist -1 "x"

A ["x", "y", "x", "z"]

B ["y", "x", "z", "x"]

C ["x", "y", "z", "x"]

D ["y", "x", "z"]

💡 Hint

Negative count removes elements from the tail of the list.

LREM with count -1 removes one occurrence of "x" starting from the tail (right side). The last "x" is removed, leaving the rest intact.

📝 Syntax

advanced

Which LREM command syntax is correct?

Select the syntactically correct Redis LREM command to remove 3 occurrences of "orange" from list fruits:

A LREM fruits "3" "orange"

B LREM fruits 3 orange

C LREM fruits 3 "orange"

D LREM fruits "orange" 3

💡 Hint

The count must be a number, and the value a string argument.

The correct syntax requires the count as a number and the value as a string. Option C correctly quotes the string value and uses a numeric count.

❓ optimization

advanced

How to efficiently remove all occurrences of an element from a large Redis list?

You want to remove all occurrences of "blue" from a very large Redis list colors. Which approach is most efficient?

A Use LREM colors 0 "blue" to remove all occurrences in one command

B Use LREM colors 1 "blue" repeatedly until no more removals

C Use LPOP repeatedly and reinsert elements except "blue"

D Use RPOP repeatedly and reinsert elements except "blue"

💡 Hint

LREM with count 0 removes all matching elements at once.

LREM with count 0 removes all occurrences of the value in a single command, which is more efficient than multiple calls or popping and reinserting.

🔧 Debug

expert

Why does this LREM command not remove any elements?

Given a Redis list pets with elements ["dog", "cat", "dog", "bird"], the command LREM pets 1 "Dog" returns 0 and the list remains unchanged. Why?

Redis

LREM pets 1 "Dog"

A The count parameter must be negative to remove elements

B Redis LREM is case-sensitive; "Dog" does not match "dog"

C The list key "pets" does not exist

D LREM only removes elements from the tail when count is positive

💡 Hint

Check the exact spelling and case of the element to remove.

Redis string matching is case-sensitive. "Dog" is different from "dog", so no elements are removed.