LREM for element removal in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the LREM command do in Redis?

LREM removes elements from a list that match a given value. You specify how many matching elements to remove and the value to remove.

intermediate

How does the count parameter affect the LREM command?

The count parameter controls how many matching elements to remove: positive count removes from head to tail, negative count removes from tail to head, zero removes all matching elements.

beginner

Syntax of the LREM command?

LREM key count value Example: LREM mylist 2 "apple" removes up to 2 occurrences of "apple" from the list named mylist.

beginner

What happens if count is zero in LREM?

If count is zero, LREM removes all elements equal to the given value from the list.

beginner

Can LREM remove elements that do not exist in the list?

No, LREM only removes elements that match the value. If no elements match, the list remains unchanged and the command returns 0.

What does LREM mylist 3 "orange" do?

A Removes up to 3 occurrences of "orange" from the head of mylist

B Removes all occurrences of "orange" from mylist

C Removes 3 elements from the tail of mylist regardless of value

D Adds 3 "orange" elements to mylist

LREM with positive count removes up to that many matching elements from head to tail.

What does LREM mylist -2 "banana" do?

A Removes all "banana" elements from mylist

B Removes 2 "banana" elements from the head of mylist

C Removes 2 "banana" elements from the tail of mylist

D Removes 2 elements from mylist regardless of value

Negative count removes matching elements starting from the tail.

If count is zero in LREM, what happens?

A The list is cleared

B Only the first matching element is removed

C No elements are removed

D All matching elements are removed

Count zero means remove all matching elements.

What does LREM return?

A The removed elements themselves

B The number of elements removed

C The new length of the list

D Always 0

LREM returns how many elements it removed.

If the value does not exist in the list, what does LREM do?

A Does nothing and returns 0

B Removes the first element anyway

C Removes all elements

D Throws an error

If no matching elements exist, LREM does nothing and returns 0.

Explain how the count parameter affects the behavior of the LREM command in Redis.

Think about direction and how many elements are removed.

Describe a real-life example where you might use LREM to remove elements from a Redis list.

Imagine a list of things you want to update by removing some items.

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