LPUSH and RPUSH for insertion in Redis - Step-by-Step Execution

Concept Flow - LPUSH and RPUSH for insertion

Start with empty list

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LPUSH value

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Value added to LEFT (front)

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RPUSH value

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Value added to RIGHT (end)

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List updated with new elements

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Repeat as needed or END

LPUSH adds elements to the front of the list, RPUSH adds elements to the end, updating the list each time.

Execution Sample

Redis

LPUSH mylist "a"
RPUSH mylist "b"
LPUSH mylist "c"

This code inserts 'a' at the front, then 'b' at the end, then 'c' at the front of the list.

Execution Table

Step	Command	List Before	Action	List After
1	LPUSH mylist "a"	[]	Add 'a' to front	["a"]
2	RPUSH mylist "b"	["a"]	Add 'b' to end	["a", "b"]
3	LPUSH mylist "c"	["a", "b"]	Add 'c' to front	["c", "a", "b"]
4	END	["c", "a", "b"]	No more commands	["c", "a", "b"]

💡 No more commands to execute, final list is ["c", "a", "b"]

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
mylist	[]	["a"]	["a", "b"]	["c", "a", "b"]	["c", "a", "b"]

Key Moments - 2 Insights

Why does LPUSH add elements to the front, not the end?

LPUSH always inserts at the start (left) of the list, as shown in execution_table rows 1 and 3 where new elements appear before existing ones.

What happens if we use RPUSH on an empty list?

RPUSH adds the element to the end, which is also the start if the list is empty, similar to LPUSH on empty list (see execution_table row 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the list after step 2?

A ["b", "a"]

B ["a", "b"]

C ["a"]

D ["b"]

💡 Hint

Check the 'List After' column in row 2 of the execution_table.

At which step does the list become ["c", "a", "b"]?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'List After' column for each step in the execution_table.

If we replaced the last LPUSH with RPUSH "c", what would the final list be?

A ["a", "b", "c"]

B ["c", "a", "b"]

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

D ["b", "a", "c"]

💡 Hint

RPUSH adds to the end, so 'c' would be after 'b' (see concept_flow).

Concept Snapshot

LPUSH key value - inserts value at the front (left) of the list
RPUSH key value - inserts value at the end (right) of the list
If list doesn't exist, it is created
Order matters: LPUSH adds before existing elements, RPUSH adds after
Use to control insertion side in Redis lists

Full Transcript

This visual execution shows how LPUSH and RPUSH commands add elements to a Redis list. Starting with an empty list, LPUSH adds elements to the front, RPUSH adds to the end. The execution table traces each command, showing the list before and after insertion. The variable tracker follows the list's state changes. Key moments clarify why LPUSH adds to the front and what happens with empty lists. The quiz tests understanding of list states after each step and effects of changing commands.