LPUSH and RPUSH for insertion in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the resulting list after LPUSH?

You start with an empty Redis list named 'fruits'. You run the command:
LPUSH fruits apple banana cherry
What is the order of elements in the list 'fruits' after this command?

Redis

LPUSH fruits apple banana cherry

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

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

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

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

💡 Hint

LPUSH inserts elements to the start of the list, from left to right.

LPUSH adds elements to the head of the list in the order they are given, so the last element ends up at the head.

❓ query_result

intermediate

What is the resulting list after RPUSH?

You have an empty Redis list named 'colors'. You run the command:
RPUSH colors red green blue
What is the order of elements in the list 'colors' after this command?

Redis

RPUSH colors red green blue

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

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

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

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

💡 Hint

RPUSH adds elements to the end of the list in the order they appear.

RPUSH appends elements to the tail of the list in the order given, so the list keeps the same order as the arguments.

📝 Syntax

advanced

Which LPUSH command is syntactically correct?

Which of the following LPUSH commands will run without syntax errors in Redis?

A LPUSH mylist "one" "two" "three"

B LPUSH mylist (one, two, three)

C LPUSH mylist [one two three]

D LPUSH mylist one; two; three

💡 Hint

Redis commands take arguments separated by spaces, no brackets or commas.

Only option A uses the correct Redis command syntax with space-separated arguments and optional quotes for strings.

❓ query_result

advanced

What is the list after mixed LPUSH and RPUSH?

You start with an empty Redis list named 'numbers'. You run these commands in order:
1. RPUSH numbers 1 2 3
2. LPUSH numbers 0
3. RPUSH numbers 4
What is the final order of elements in the list 'numbers'?

Redis

RPUSH numbers 1 2 3
LPUSH numbers 0
RPUSH numbers 4

A [0, 4, 1, 2, 3]

B [1, 2, 3, 0, 4]

C [0, 1, 2, 3, 4]

D [4, 3, 2, 1, 0]

💡 Hint

LPUSH adds to the front, RPUSH adds to the end.

RPUSH adds 1,2,3 to the end, LPUSH adds 0 to the front, then RPUSH adds 4 to the end, resulting in [0,1,2,3,4].

🧠 Conceptual

expert

Why choose LPUSH over RPUSH in Redis list insertion?

Which reason best explains when LPUSH is preferred over RPUSH for inserting elements into a Redis list?

A When you want to add elements to the start of the list quickly, like a stack (LIFO) behavior.

B When you want to add elements to the end of the list to maintain insertion order (FIFO).

C When you want to sort the list automatically after insertion.

D When you want to remove elements from the list instead of adding.

💡 Hint

LPUSH adds elements at the head, useful for last-in-first-out order.

LPUSH inserts at the front, making it ideal for stack-like behavior where the last added element is first to be removed.

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.