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

Concept Flow - LREM for element removal

Start with Redis List

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Call LREM key count value

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Scan list from head or tail based on count

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Compare each element with value

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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.