LINDEX for position access in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of LINDEX for a valid positive index?

Given a Redis list mylist with elements ["apple", "banana", "cherry"], what is the output of LINDEX mylist 1?

Redis

LINDEX mylist 1

A "banana"

B "apple"

C "cherry"

D nil

💡 Hint

Remember that Redis lists are zero-indexed, so index 1 is the second element.

LINDEX returns the element at the specified zero-based index. Index 1 corresponds to the second element, which is "banana".

❓ query_result

intermediate

What does LINDEX return for a negative index?

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

Redis

LINDEX colors -2

A "blue"

B "yellow"

C "green"

D nil

💡 Hint

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

Index -1 is the last element "yellow", so -2 is the second last element "blue".

📝 Syntax

advanced

Which LINDEX command syntax is correct?

Which of the following Redis commands correctly uses LINDEX to get the first element of list tasks?

A LINDEX tasks

B LINDEX tasks 0

C LINDEX 0 tasks

D LINDEX tasks -0

💡 Hint

LINDEX requires the key first, then the index as an integer.

The correct syntax is LINDEX key index. Option B follows this format with index 0 for the first element.

❓ query_result

advanced

What is the output when LINDEX index is out of range?

Given a Redis list numbers with elements ["one", "two", "three"], what is the output of LINDEX numbers 5?

Redis

LINDEX numbers 5

A Error: index out of range

B "three"

C nil

D "five"

💡 Hint

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

Since index 5 is beyond the last element (index 2), Redis returns nil indicating no element found.

🧠 Conceptual

expert

How does LINDEX handle concurrent modifications?

If a Redis list queue is modified (elements added or removed) by other clients while a client runs LINDEX queue 2, what can be said about the returned value?

A LINDEX returns a random element from the list if concurrent modifications happen.

B LINDEX always returns the element that was at index 2 when the list was first created, ignoring changes.

C LINDEX blocks until no other clients modify the list, then returns the element at index 2.

D LINDEX returns the element at index 2 at the exact time of the command execution, reflecting any concurrent changes.

💡 Hint

Think about how Redis commands are atomic and reflect the current state at execution.

Redis commands are atomic and see the list state at the moment of execution, so LINDEX returns the element at index 2 at that time, including any concurrent changes.

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