LREM for element removal in Redis - Step-by-Step Execution

Concept Flow - LREM for element removal

Start with Redis List

↓

Call LREM key count value

↓

Scan list from head or tail based on count

↓

Compare each element with value

↓

If match and count not reached, remove element

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Continue until count removals done or list ends

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Return number of removed elements

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End

LREM scans the list and removes elements matching the value up to the count specified, then returns how many were removed.

Execution Sample

Redis

LREM mylist 2 "apple"

Removes up to 2 occurrences of "apple" from the list named mylist.

Execution Table

Step	List State	Count Remaining	Element Checked	Match?	Action	Removed Count
1	["apple", "banana", "apple", "cherry", "apple"]	2	"apple"	Yes	Remove element	1
2	["banana", "apple", "cherry", "apple"]	1	"banana"	No	Keep element	1
3	["banana", "apple", "cherry", "apple"]	1	"apple"	Yes	Remove element	2
4	["banana", "cherry", "apple"]	0	Stop scanning	-	Stop removal	2

💡 Count reached 0 after removing 2 elements, so scanning stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
List State	["apple", "banana", "apple", "cherry", "apple"]	["banana", "apple", "cherry", "apple"]	["banana", "cherry", "apple"]	["banana", "cherry", "apple"]	["banana", "cherry", "apple"]
Count Remaining	2	1	1	0	0
Removed Count	0	1	1	2	2

Key Moments - 2 Insights

Why does LREM stop scanning the list before reaching the end?

Because the count of removals specified (2) was reached at step 3, so it stops scanning further as shown in execution_table row 4.

What happens if an element does not match the value to remove?

The element is kept in the list and the count of removals does not decrease, as seen in step 2 where "banana" is checked and kept.

Visual Quiz - 3 Questions

Look at the execution table, what is the list state after the first removal?

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

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

C ["banana", "apple", "cherry", "apple"]

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

💡 Hint

Check the 'List State' column at step 2 in the execution_table.

At which step does the count of removals reach zero?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Count Remaining' column in the execution_table.

If the count was 0 (remove all), how would the removal process change?

A It would remove all matching elements in the list.

B It would remove only the first matching element.

C It would remove no elements.

D It would remove elements only from the tail.

💡 Hint

Recall that count = 0 means remove all matching elements, not limited by count.

Concept Snapshot

LREM key count value
- Removes elements equal to value from list key
- count > 0: remove from head up to count
- count < 0: remove from tail up to abs(count)
- count = 0: remove all matching elements
- Returns number of removed elements

Full Transcript

The LREM command in Redis removes elements from a list that match a given value. It scans the list from the head or tail depending on the count parameter. If count is positive, it removes up to count matching elements from the head. If negative, from the tail. If zero, it removes all matching elements. The command returns how many elements were removed. In the example, LREM mylist 2 "apple" removes up to two "apple" elements from the list. The execution table shows each step checking elements, removing matches, updating the list and count, and stopping when the count reaches zero.

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