LRANGE for reading elements in Redis - Time & Space Complexity

Time Complexity: LRANGE for reading elements

O(k)

Understanding Time Complexity

When using Redis to read parts of a list, it's important to know how the time it takes grows as the list gets bigger.

We want to understand how the LRANGE command's work changes with the size of the list and the requested range.

Scenario Under Consideration

Analyze the time complexity of the following Redis command.

LRANGE mylist 0 99
    

This command reads the first 100 elements from the list named 'mylist'.

Identify Repeating Operations

Look at what repeats when LRANGE runs.

• Primary operation: Reading each element in the requested range from the list.
• How many times: Once for each element in the range (here, 100 times).

How Execution Grows With Input

The time to run LRANGE grows with how many elements you ask for, not the total list size.

Input Size (n)	Approx. Operations
10	10 reads
100	100 reads
1000	1000 reads

Pattern observation: The work grows directly with the number of elements requested.

Final Time Complexity

Time Complexity: O(k)

This means the time depends on how many elements you want to read, not the whole list size.

Common Mistake

[X] Wrong: "LRANGE always takes time based on the total list size."

[OK] Correct: Actually, LRANGE only reads the elements you ask for, so the time depends on the range size, not the full list.

Interview Connect

Understanding how LRANGE scales helps you explain how Redis handles list reads efficiently, a useful skill for real projects and interviews.

Self-Check

What if we changed LRANGE to read the entire list instead of a part? How would the time complexity change?

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