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

Concept Flow - LINDEX for position access

Start

↓

Receive LINDEX command with key and index

↓

Check if key exists

↓

Get list value at index

↓

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.