LPOP and RPOP for removal in Redis - Step-by-Step Execution

Concept Flow - LPOP and RPOP for removal

Start with a list

↓

Choose removal command

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LPOP

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Return removed element

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List updated without removed element

This flow shows how LPOP removes the first element and RPOP removes the last element from a Redis list, returning the removed element and updating the list.

Execution Sample

Redis

LPUSH mylist "a" "b" "c"
LPOP mylist
RPOP mylist

Add elements 'a', 'b', 'c' to list 'mylist', then remove first element with LPOP and last element with RPOP.

Execution Table

Step	Command	List Before	Action	Removed Element	List After
1	LPUSH mylist "a" "b" "c"	[]	Add 'c', 'b', 'a' to front	N/A	["c", "b", "a"]
2	LPOP mylist	["c", "b", "a"]	Remove first element	"c"	["b", "a"]
3	RPOP mylist	["b", "a"]	Remove last element	"a"	["b"]
4	LPOP mylist	["b"]	Remove first element	"b"	[]
5	LPOP mylist	[]	List empty, nothing to remove	nil	[]

💡 Step 5 stops because the list is empty and LPOP returns nil.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
mylist	[]	["c", "b", "a"]	["b", "a"]	["b"]	[]	[]

Key Moments - 2 Insights

Why does LPOP remove the element 'c' even though it was pushed last?

Because LPUSH adds elements to the front, the list order after LPUSH is ['c', 'b', 'a']. LPOP removes the first element, which is 'c' as shown in Step 2 of the execution_table.

What happens when you try to LPOP from an empty list?

As shown in Step 5, LPOP returns nil and the list remains empty because there is no element to remove.

Visual Quiz - 3 Questions

Look at the execution_table, what is the list after Step 3?

A ["b"]

B ["b", "a"]

C ["c", "b", "a"]

D []

💡 Hint

Check the 'List After' column in Step 3 of the execution_table.

At which step does LPOP return nil?

A Step 2

B Step 5

C Step 3

D Step 4

💡 Hint

Look at the 'Removed Element' column for nil value in the execution_table.

If we replaced LPOP with RPOP in Step 2, what would be the removed element?

A "c"

B "b"

C "a"

D nil

💡 Hint

Consider the list order after LPUSH and what RPOP removes (last element).

Concept Snapshot

LPOP removes and returns the first element of a Redis list.
RPOP removes and returns the last element.
If the list is empty, both return nil.
LPUSH adds elements to the front, so LPOP removes the most recently pushed element.
Use these commands to treat lists like queues or stacks.

Full Transcript

This visual execution trace shows how Redis commands LPOP and RPOP remove elements from a list. Starting with an empty list, LPUSH adds elements 'a', 'b', 'c' to the front, resulting in ['c', 'b', 'a']. LPOP removes the first element 'c', updating the list to ['b', 'a']. RPOP then removes the last element 'a', leaving ['b']. Another LPOP removes 'b', emptying the list. Attempting LPOP on an empty list returns nil. This demonstrates how LPOP and RPOP work to remove elements from the start or end of a list respectively.