LTRIM for list capping in Redis - Step-by-Step Execution

Concept Flow - LTRIM for list capping

Start with Redis List

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Apply LTRIM command

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Specify start and stop indexes

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Redis keeps only elements in range

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Elements outside range are removed

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List is now capped to specified size

LTRIM keeps only the specified range of elements in a Redis list, removing all others to cap its size.

Execution Sample

Redis

RPUSH mylist a b c d e
LTRIM mylist 0 2
LRANGE mylist 0 -1

Add 5 elements to list, then trim to keep only first 3 elements, then read all elements.

Execution Table

Step	Command	List Before	Action	List After	Notes
1	RPUSH mylist a b c d e	[]	Add elements a,b,c,d,e to end	[a, b, c, d, e]	List created with 5 elements
2	LTRIM mylist 0 2	[a, b, c, d, e]	Keep elements from index 0 to 2	[a, b, c]	Elements d,e removed, list capped to 3
3	LRANGE mylist 0 -1	[a, b, c]	Read all elements	[a, b, c]	Final list content after trim

💡 LTRIM stops after removing elements outside the specified range, list capped to size 3

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
mylist	[]	[a, b, c, d, e]	[a, b, c]	[a, b, c]

Key Moments - 3 Insights

Why does LTRIM keep elements from start to stop indexes inclusive?

LTRIM uses inclusive indexes, so elements at both start and stop positions remain. See execution_table row 2 where indexes 0 to 2 keep elements a,b,c.

What happens if stop index is larger than list length?

LTRIM treats stop index beyond list length as end of list, so it keeps elements up to the list's end without error. This is why specifying 0 2 works even if list is shorter.

Does LTRIM modify the list in place or create a new list?

LTRIM modifies the existing list in place by removing elements outside the range, as shown in variable_tracker where mylist changes after step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the content of mylist after step 2?

A [a, b, c]

B [a, b, c, d]

C [b, c, d]

D [a, b, c, d, e]

💡 Hint

Check the 'List After' column in row 2 of execution_table

At which step does the list get capped to size 3?

A Step 1

B Step 3

C Step 2

D No capping occurs

💡 Hint

Look at the 'Action' and 'List After' columns in execution_table row 2

If LTRIM was called with 1 3 instead of 0 2, what would be the list after trimming?

A [a, b, c]

B [b, c, d]

C [c, d, e]

D [a, b]

💡 Hint

LTRIM keeps elements from start to stop indexes inclusive; indexes 1 to 3 correspond to b,c,d

Concept Snapshot

LTRIM key start stop
- Keeps only elements from start to stop indexes (inclusive)
- Removes elements outside this range
- Used to cap list size efficiently
- Indexes start at 0
- Modifies list in place

Full Transcript

LTRIM is a Redis command that trims a list to keep only elements within a specified range of indexes. The command takes a key and two indexes: start and stop. It removes all elements outside this range, effectively capping the list size. For example, if a list has elements a,b,c,d,e and we run LTRIM with start 0 and stop 2, only a,b,c remain. The command modifies the list in place. If the stop index is beyond the list length, it treats it as the end of the list. This is useful to keep a list from growing too large by trimming old elements.