LINDEX for position access in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command LINDEX do?

The LINDEX command returns the element at a specific position (index) in a Redis list.

beginner

How does Redis count positions in a list when using LINDEX?

Positions start at 0 for the first element. Negative numbers count from the end, with -1 being the last element.

beginner

What will LINDEX mylist 2 return if mylist contains ["a", "b", "c", "d"]?

It will return "c", the element at index 2 (third element).

beginner

What happens if you use LINDEX with an index that is out of range?

Redis returns null because there is no element at that position.

beginner

Can LINDEX be used to modify elements in a Redis list?

No, LINDEX only reads an element. To modify, use LSET.

What index does LINDEX use to get the first element of a list?

A -2

B 1

C -1

D 0

Redis lists start at index 0 for the first element.

What does LINDEX mylist -1 return?

A The first element

B The last element

C The second element

D An error

Negative indexes count from the end; -1 is the last element.

If mylist has 3 elements, what does LINDEX mylist 5 return?

A null

B The last element

C The 6th element

D An error message

Index 5 is out of range, so Redis returns null.

Which command would you use to change an element at a specific position in a Redis list?

A LINDEX

B LPUSH

C LSET

D LRANGE

LSET modifies an element at a given index.

What type of data structure does LINDEX work with in Redis?

A Lists

B Hashes

C Strings

D Sets

LINDEX accesses elements in Redis lists.

Explain how the LINDEX command works in Redis and how indexing is handled.

Think about how you count positions in a list from front and back.

Describe what happens when you use LINDEX with an index that does not exist in the list.

What does Redis do when you ask for something that is not there?

Practice

1. What does the Redis command LINDEX mylist 0 do?

easy

A. Deletes the first element of the list 'mylist'.

B. Returns the first element of the list 'mylist'.

C. Returns the length of the list 'mylist'.

D. Adds an element at the start of the list 'mylist'.

Solution

1. Step 1: Understand the LINDEX command

   LINDEX returns the element at the specified index in a Redis list without modifying it.

2. Step 2: Interpret the index 0

   Index 0 means the first element in the list.

3. Final Answer:

   Returns the first element of the list 'mylist'. -> Option B

4. Quick Check:

   LINDEX with 0 returns first element [OK]

Hint: LINDEX with 0 gets first item, -1 gets last [OK]

Common Mistakes:

• Thinking LINDEX deletes or modifies the list
• Confusing LINDEX with LLEN (length)
• Using negative index incorrectly

2. Which of the following is the correct syntax to get the last element of a Redis list named tasks using LINDEX?

easy

A. LINDEX tasks -1

B. LINDEX tasks 1

C. LINDEX tasks 0

D. LINDEX tasks length

Solution

1. Step 1: Recall how to access last element

   In Redis, negative index -1 means the last element in a list.

2. Step 2: Check the options

   Only LINDEX tasks -1 uses -1 correctly to get the last element.

3. Final Answer:

   LINDEX tasks -1 -> Option A

4. Quick Check:

   Negative index -1 returns last element [OK]

Hint: Use -1 index to get last list item with LINDEX [OK]

Common Mistakes:

• Using positive index 1 for last element
• Using 'length' as index (invalid)
• Confusing 0 with last element

3. Given a Redis list colors with elements ["red", "green", "blue", "yellow"], what is the result of LINDEX colors 2?

medium

A. "blue"

B. "yellow"

C. nil

D. "green"

Solution

1. Step 1: Identify list elements and indexes

   List elements: 0:"red", 1:"green", 2:"blue", 3:"yellow".

2. Step 2: Find element at index 2

   Index 2 corresponds to "blue".

3. Final Answer:

   "blue" -> Option A

4. Quick Check:

   Index 2 in list is "blue" [OK]

Hint: Count from 0 to find element at index [OK]

Common Mistakes:

• Counting from 1 instead of 0
• Confusing index 2 with last element
• Expecting nil for valid index

4. You run LINDEX mylist 5 but get (nil) as a result. What is the most likely reason?

medium

A. The index 5 is negative and invalid.

B. LINDEX command is misspelled.

C. The list 'mylist' is empty but has 6 elements.

D. The list 'mylist' has fewer than 6 elements.

Solution

1. Step 1: Understand LINDEX behavior for out-of-range index

   If the index is outside the list range, LINDEX returns nil.

2. Step 2: Analyze index 5

   Index 5 means the 6th element (counting from 0). If list has fewer than 6 elements, result is nil.

3. Final Answer:

   The list 'mylist' has fewer than 6 elements. -> Option D

4. Quick Check:

   Out-of-range index returns nil [OK]

Hint: Index too big returns nil, check list length [OK]

Common Mistakes:

• Assuming nil means syntax error
• Thinking negative index 5 is invalid
• Confusing empty list with list length

5. You have a Redis list fruits with elements ["apple", "banana", "cherry", "date"]. You want to get the second last element using LINDEX. Which command should you use?

hard

A. LINDEX fruits -1

B. LINDEX fruits 2

C. LINDEX fruits -2

D. LINDEX fruits 1

Solution

1. Step 1: Understand negative indexes in LINDEX

   Negative indexes count from the end: -1 is last, -2 is second last.

2. Step 2: Identify second last element index

   Second last element is at index -2.

3. Final Answer:

   LINDEX fruits -2 -> Option C

4. Quick Check:

   Use -2 to get second last element [OK]

Hint: Use -2 index for second last element [OK]

Common Mistakes:

• Using positive index 2 (third element)
• Using -1 which is last element
• Confusing index 1 with second last