LRANGE for reading elements in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this LRANGE command return?

Given a Redis list mylist with elements ["a", "b", "c", "d", "e"], what is the output of the command LRANGE mylist 1 3?

Redis

LRANGE mylist 1 3

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

B ["b", "c", "d"]

C ["c", "d", "e"]

D ["b", "c", "d", "e"]

💡 Hint

Remember that LRANGE uses zero-based indexing and includes both start and end positions.

LRANGE returns elements from the start index to the end index inclusive. Index 1 is 'b', index 3 is 'd', so the result is ['b', 'c', 'd'].

❓ query_result

intermediate

What is the output of LRANGE with negative indices?

If mylist contains ["x", "y", "z", "w"], what does LRANGE mylist -3 -1 return?

Redis

LRANGE mylist -3 -1

A ["y", "z", "w"]

B ["x", "y", "z"]

C ["w"]

D ["z", "w"]

💡 Hint

Negative indices count from the end of the list, starting at -1 for the last element.

Index -3 corresponds to 'y', and -1 corresponds to 'w'. LRANGE returns elements from 'y' to 'w' inclusive.

📝 Syntax

advanced

Which LRANGE command is syntactically correct?

Identify the syntactically valid LRANGE command to get elements from index 2 to 4 from list mylist.

A LRANGE mylist 2-4

B LRANGE mylist 2,4

C LRANGE mylist (2 4)

D LRANGE mylist 2 4

💡 Hint

LRANGE syntax requires the key followed by two separate integer arguments for start and stop.

Only option D uses the correct syntax: LRANGE key start stop with spaces separating arguments.

❓ optimization

advanced

Efficiently reading the last 5 elements of a large list

You have a Redis list biglist with 1 million elements. Which LRANGE command efficiently retrieves the last 5 elements?

A LRANGE biglist 999995 999999

B LRANGE biglist 0 4

C LRANGE biglist -5 -1

D LRANGE biglist 999995 -1

💡 Hint

Negative indices are a convenient way to access elements from the end without calculating exact positions.

Using negative indices like -5 to -1 directly accesses the last 5 elements without needing to know the list length.

🧠 Conceptual

expert

What happens if LRANGE start index is greater than end index?

Consider a Redis list mylist with elements ["p", "q", "r"]. What is the result of LRANGE mylist 2 1?

A An empty list []

B A syntax error

C Returns elements from index 1 to 2

D Returns all elements

💡 Hint

LRANGE returns elements from start to end index inclusive, but start must be less or equal to end.

If start index is greater than end index, LRANGE returns an empty list without error.

Practice

1. What does the Redis command LRANGE mylist 0 -1 do?

easy

A. Returns the length of mylist.

B. Returns only the first element of mylist.

C. Returns all elements in the list mylist.

D. Deletes all elements in mylist.

Solution

1. Step 1: Understand LRANGE parameters

   The command LRANGE key start stop returns elements from index start to stop inclusive.

2. Step 2: Interpret 0 and -1 indexes

   Index 0 is the first element, and -1 means the last element, so it returns the whole list.

3. Final Answer:

   Returns all elements in the list mylist. -> Option C

4. Quick Check:

   LRANGE mylist 0 -1 = all elements [OK]

Hint: Use 0 and -1 to get the entire list quickly [OK]

Common Mistakes:

• Thinking -1 excludes the last element
• Confusing LRANGE with LLEN
• Assuming LRANGE deletes elements

2. Which of the following is the correct syntax to get the first 3 elements of a Redis list named tasks using LRANGE?

easy

A. LRANGE tasks 1 3

B. LRANGE tasks -3 -1

C. LRANGE tasks 0 3

D. LRANGE tasks 0 2

Solution

1. Step 1: Understand zero-based indexing

   Redis list indexes start at 0, so the first element is index 0.

2. Step 2: Calculate indexes for first 3 elements

   First 3 elements are at indexes 0, 1, and 2, so LRANGE should be 0 to 2.

3. Final Answer:

   LRANGE tasks 0 2 -> Option D

4. Quick Check:

   First 3 elements = indexes 0 to 2 [OK]

Hint: Remember list indexes start at 0, so first 3 are 0 to 2 [OK]

Common Mistakes:

• Starting at index 1 instead of 0
• Using stop index as 3 which returns 4 elements
• Using negative indexes incorrectly

3. Given a Redis list colors with elements ["red", "green", "blue", "yellow", "purple"], what is the result of LRANGE colors 1 3?

medium

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

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

C. ["blue", "yellow", "purple"]

D. ["yellow", "purple"]

Solution

1. Step 1: Identify elements by index

   Indexes: 0="red", 1="green", 2="blue", 3="yellow", 4="purple".

2. Step 2: Select elements from index 1 to 3

   Elements are "green", "blue", and "yellow".

3. Final Answer:

   ["green", "blue", "yellow"] -> Option B

4. Quick Check:

   LRANGE colors 1 3 = green, blue, yellow [OK]

Hint: Count indexes starting at 0 to pick correct slice [OK]

Common Mistakes:

• Including index 0 when start is 1
• Using stop index as exclusive instead of inclusive
• Confusing element order

4. You run LRANGE mylist 2 0 on a Redis list. What is the problem with this command?

medium

A. The start index is greater than the stop index, so it returns an empty list.

B. Syntax error because indexes must be positive.

C. It returns the entire list ignoring indexes.

D. It deletes elements from index 0 to 2.

Solution

1. Step 1: Check index order in LRANGE

   LRANGE expects start index less than or equal to stop index to return elements.

2. Step 2: Analyze given indexes

   Start is 2, stop is 0, so start > stop, which returns an empty list.

3. Final Answer:

   The start index is greater than the stop index, so it returns an empty list. -> Option A

4. Quick Check:

   Start > stop means empty result [OK]

Hint: Start index must be ≤ stop index to get results [OK]

Common Mistakes:

• Assuming negative indexes are invalid syntax
• Expecting reversed range to return elements
• Confusing LRANGE with deletion commands

5. You have a Redis list logs with 11 elements. You want to get the last 5 elements using LRANGE. Which command is correct?

hard

A. LRANGE logs -5 -1

B. LRANGE logs 4 8

C. LRANGE logs 5 9

D. LRANGE logs -4 0

Solution

1. Step 1: Understand negative indexes in LRANGE

   Negative indexes count from the end: -1 is last element, -5 is fifth from last.

2. Step 2: Select last 5 elements using negative indexes

   LRANGE logs -5 -1 returns elements from fifth last to last, i.e., last 5 elements.

3. Step 3: Check other options

   LRANGE logs 5 9 uses positive indexes 5 to 9 which gets elements 5-9, but for 11 elements the last 5 are indexes 6-10, so incorrect. Negative indexes are safer if length changes.

4. Final Answer:

   LRANGE logs -5 -1 -> Option A

5. Quick Check:

   Negative indexes -5 to -1 = last 5 elements [OK]

Hint: Use negative indexes -5 to -1 for last 5 elements safely [OK]

Common Mistakes:

• Using wrong negative indexes like -4 to 0
• Confusing zero-based positive indexes with negative
• Assuming LRANGE excludes stop index