LINDEX for position access in Redis - Step-by-Step Execution

Concept Flow - LINDEX for position access

Start

↓

Receive LINDEX command with key and index

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Check if key exists

↓

Get list value at index

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Return value or nil

↓

End

LINDEX gets the element at a given position from a list stored in Redis. If the key or index is invalid, it returns nil.

Execution Sample

Redis

LINDEX mylist 0
LINDEX mylist 2
LINDEX mylist -1

Retrieve elements at positions 0, 2, and last (-1) from the list 'mylist'.

Execution Table

Step	Command	List State	Index	Action	Output
1	LINDEX mylist 0	["a", "b", "c", "d"]	0	Return element at index 0	"a"
2	LINDEX mylist 2	["a", "b", "c", "d"]	2	Return element at index 2	"c"
3	LINDEX mylist -1	["a", "b", "c", "d"]	-1	Return last element	"d"
4	LINDEX mylist 4	["a", "b", "c", "d"]	4	Index out of range, return nil	(nil)
5	LINDEX unknown 0	nil	0	Key does not exist, return nil	(nil)

💡 Commands stop after returning the requested element or nil if key/index invalid.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
mylist	["a", "b", "c", "d"]	["a", "b", "c", "d"]	["a", "b", "c", "d"]	["a", "b", "c", "d"]	["a", "b", "c", "d"]	["a", "b", "c", "d"]
index	undefined	0	2	-1	4	0
output	undefined	"a"	"c"	"d"	(nil)	(nil)

Key Moments - 3 Insights

Why does LINDEX return nil when the index is 4 but the list has only 4 elements?

Because list indexes start at 0, so valid indexes are 0 to 3. Index 4 is out of range, so LINDEX returns nil as shown in step 4 of the execution_table.

What happens if the key does not exist in Redis when using LINDEX?

LINDEX returns nil immediately because the key is missing, as shown in step 5 of the execution_table.

How does LINDEX handle negative indexes like -1?

Negative indexes count from the end of the list, so -1 returns the last element. This is shown in step 3 where index -1 returns "d".

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of LINDEX mylist 2?

A "c"

B "b"

C "d"

D nil

💡 Hint

Check row 2 in the execution_table under Output column.

At which step does LINDEX return nil because the key does not exist?

A Step 4

B Step 3

C Step 5

D Step 1

💡 Hint

Look for the step where List State is nil in the execution_table.

If the list had 5 elements, what would LINDEX mylist 4 return?

A nil

B The 5th element

C The 1st element

D Error

💡 Hint

Indexes start at 0, so index 4 is the 5th element if list length is 5.

Concept Snapshot

LINDEX key index
- Returns element at position 'index' in list stored at 'key'
- Index starts at 0; negative indexes count from end (-1 last)
- Returns nil if key missing or index out of range
- Useful for quick access to list elements by position

Full Transcript

LINDEX is a Redis command to get an element from a list by its position. You give it a key and an index. If the key exists and the index is valid, it returns the element at that position. Indexes start at zero, so 0 is the first element. Negative indexes count from the end, so -1 is the last element. If the key does not exist or the index is out of range, LINDEX returns nil. For example, with a list ["a", "b", "c", "d"], LINDEX mylist 0 returns "a", LINDEX mylist 2 returns "c", and LINDEX mylist -1 returns "d". If you ask for index 4, which is beyond the list length, it returns nil. If the key is missing, it also returns nil.

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