LRANGE for reading elements in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the LRANGE command do in Redis?

LRANGE returns a specified range of elements from a list stored at a given key. It reads elements between the start and stop indexes.

beginner

How do you specify the range of elements to read with LRANGE?

You provide two numbers: the start index and the stop index. Both are zero-based and inclusive.

intermediate

What happens if the stop index in LRANGE is larger than the list length?

LRANGE will return elements up to the end of the list without error.

intermediate

How does LRANGE handle negative indexes?

Negative indexes count from the end of the list. For example, -1 is the last element, -2 is the second last, and so on.

beginner

Example: What does LRANGE mylist 0 2 return if mylist = ["a", "b", "c", "d"]?

It returns ["a", "b", "c"] — the first three elements from index 0 to 2 inclusive.

What does LRANGE mylist 1 3 return if mylist = ["x", "y", "z", "w"]?

A ["z", "w"]

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

C ["y", "z", "w"]

D ["y", "z"]

Indexes 1 to 3 include elements at positions 1, 2, and 3: "y", "z", and "w".

If you want to get the last element of a list using LRANGE, which indexes do you use?

A -1 -1

B 0 0

C 0 -1

D -2 -1

Using -1 for both start and stop returns only the last element.

What will LRANGE return if the start index is greater than the stop index?

A An error

B An empty list

C The whole list

D The first element only

LRANGE returns an empty list if start > stop.

How does LRANGE treat the stop index?

A Ignored

B Exclusive (stop element not included)

C Always zero

D Inclusive (stop element included)

The stop index is inclusive, so the element at that index is included in the result.

If a list has 5 elements, what does LRANGE mylist 0 10 return?

A The first 5 elements

B An empty list

C An error because 10 is out of range

D Only the first element

LRANGE returns all elements up to the end if stop index is beyond list length.

Explain how to use LRANGE to read elements from a Redis list, including how indexes work.

Think about how you pick a slice of a list using start and stop positions.

Describe what happens when LRANGE is used with indexes outside the list's length or with negative indexes.

Consider how LRANGE handles boundaries and negative numbers.

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