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

Concept Flow - LPUSH and RPUSH for insertion

Start with empty list

↓

LPUSH value

↓

Value added to LEFT (front)

↓

RPUSH value

↓

Value added to RIGHT (end)

↓

List updated with new elements

↓

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.

Practice

1. What does the Redis command LPUSH mylist "apple" do?

easy

A. Adds "apple" to the start (left) of the list named mylist.

B. Adds "apple" to the end (right) of the list named mylist.

C. Removes "apple" from the list named mylist.

D. Creates a new list named mylist without adding any elements.

Solution

1. Step 1: Understand LPUSH command behavior

   The LPUSH command inserts the given element(s) at the start (left) of the list.

2. Step 2: Apply to the example

   LPUSH mylist "apple" adds "apple" to the left side of the list named mylist.

3. Final Answer:

   Adds "apple" to the start (left) of the list named mylist. -> Option A

4. Quick Check:

   LPUSH = insert left [OK]

Hint: LPUSH inserts at the start (left) of the list [OK]

Common Mistakes:

• Confusing LPUSH with RPUSH and thinking it adds to the end.
• Assuming LPUSH removes elements instead of adding.
• Believing LPUSH creates a list without adding elements.

2. Which of the following is the correct syntax to add "banana" to the end of a Redis list named fruits?

easy

A. RPUSH fruits "banana"

B. LPUSH fruits "banana"

C. PUSH fruits "banana"

D. APPEND fruits "banana"

Solution

1. Step 1: Identify command to add at the end

   RPUSH adds elements to the end (right) of a Redis list.

2. Step 2: Check syntax correctness

   RPUSH fruits "banana" correctly adds "banana" to the end of the list named fruits.

3. Final Answer:

   RPUSH fruits "banana" -> Option A

4. Quick Check:

   RPUSH = insert right [OK]

Hint: RPUSH adds to the end (right) of the list [OK]

Common Mistakes:

• Using LPUSH instead of RPUSH to add at the end.
• Using non-existent commands like PUSH or APPEND.
• Incorrect command order or missing quotes.

3. Given an empty Redis list colors, what will be the result of these commands executed in order?
LPUSH colors "red"
RPUSH colors "blue"
LPUSH colors "green"
What is the content of the list colors after these commands?

medium

A. ["red", "blue", "green"]

B. ["red", "green", "blue"]

C. ["blue", "red", "green"]

D. ["green", "red", "blue"]

Solution

1. Step 1: Apply LPUSH colors "red" on empty list

   List becomes ["red"] because LPUSH adds "red" to the start.

2. Step 2: Apply RPUSH colors "blue"

   List becomes ["red", "blue"] because RPUSH adds "blue" to the end.

3. Step 3: Apply LPUSH colors "green"

   List becomes ["green", "red", "blue"] because LPUSH adds "green" to the start.

4. Final Answer:

   ["green", "red", "blue"] -> Option D

5. Quick Check:

   LPUSH adds left, RPUSH adds right [OK]

Hint: Track list from left (LPUSH) and right (RPUSH) insertions [OK]

Common Mistakes:

• Mixing order of elements after LPUSH and RPUSH.
• Assuming RPUSH adds to the start.
• Ignoring the order of commands executed.

4. You run the command LPUSH mylist "orange" "lemon" but get a "WRONGTYPE Operation against a key holding the wrong kind of value" error. What is the likely cause?

medium

A. LPUSH does not accept multiple values in one command.

B. The key mylist exists but is not a list.

C. LPUSH requires each value to be added with separate commands.

D. The correct syntax requires the key to be last.

Solution

1. Step 1: Check LPUSH command syntax

   LPUSH supports multiple values in one command and creates a list if the key does not exist.

2. Step 2: Identify the WRONGTYPE error cause

   The error occurs because the key 'mylist' already exists but holds a different data type (not a list).

3. Final Answer:

   The key mylist exists but is not a list. -> Option B

4. Quick Check:

   LPUSH + WRONGTYPE = key exists wrong type [OK]

Hint: LPUSH fails with WRONGTYPE if key exists as non-list [OK]

Common Mistakes:

• Assuming LPUSH cannot add multiple values at once.
• Thinking list must exist before LPUSH.
• Misordering command arguments.

5. You want to create a Redis list tasks with the elements ["task1", "task2", "task3"] in that exact order, where task1 is the first element and task3 is the last. Which sequence of commands will achieve this?

hard

A. LPUSH tasks "task1"; LPUSH tasks "task2"; LPUSH tasks "task3"

B. RPUSH tasks "task3"; RPUSH tasks "task2"; RPUSH tasks "task1"

C. RPUSH tasks "task1"; RPUSH tasks "task2"; RPUSH tasks "task3"

D. LPUSH tasks "task1"; RPUSH tasks "task2"; LPUSH tasks "task3"

Solution

1. Step 1: Understand desired order

   We want the list as ["task1", "task2", "task3"] with task1 first and task3 last.

2. Step 2: Choose commands to add elements in order

   RPUSH adds elements to the end, so adding task1, then task2, then task3 with RPUSH keeps the order correct.

3. Step 3: Analyze other options

   LPUSH adds to start, so LPUSH tasks "task1"; LPUSH tasks "task2"; LPUSH tasks "task3" reverses order; LPUSH tasks "task1"; RPUSH tasks "task2"; LPUSH tasks "task3" mixes commands causing wrong order; RPUSH tasks "task3"; RPUSH tasks "task2"; RPUSH tasks "task1" reverses order.

4. Final Answer:

   RPUSH tasks "task1"; RPUSH tasks "task2"; RPUSH tasks "task3" -> Option C

5. Quick Check:

   RPUSH in order = correct list order [OK]

Hint: Use RPUSH in order to keep elements in desired sequence [OK]

Common Mistakes:

• Using LPUSH in order, which reverses the list.
• Mixing LPUSH and RPUSH causing unexpected order.
• Adding elements in reverse order.