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

Concept Flow - LTRIM for list capping

Start with Redis List

↓

Apply LTRIM command

↓

Specify start and stop indexes

↓

Redis keeps only elements in range

↓

Elements outside range are removed

↓

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.

Practice

1. What does the Redis command LTRIM mylist 0 9 do to the list mylist?

easy

A. Clears the entire list.

B. Removes the first 10 elements and keeps the rest.

C. Keeps only the last 10 elements of the list and removes the rest.

D. Keeps only the first 10 elements of the list and removes the rest.

Solution

1. Step 1: Understand LTRIM parameters

   The command LTRIM mylist 0 9 keeps elements from index 0 to 9 inclusive.

2. Step 2: Effect on list size

   This means only the first 10 elements remain; all others are removed.

3. Final Answer:

   Keeps only the first 10 elements of the list and removes the rest. -> Option D

4. Quick Check:

   LTRIM 0 9 keeps first 10 elements [OK]

Hint: LTRIM 0 9 means keep first 10 items only [OK]

Common Mistakes:

• Confusing start and stop indexes
• Thinking it removes first 10 instead of keeping them
• Assuming it keeps last 10 elements

2. Which of the following is the correct syntax to keep only the last 5 elements of a Redis list named tasks using LTRIM?

easy

A. LTRIM tasks -5 -1

B. LTRIM tasks 0 4

C. LTRIM tasks 5 -1

D. LTRIM tasks -1 -5

Solution

1. Step 1: Understand negative indexes in LTRIM

   Negative indexes count from the end: -1 is last element, -5 is fifth from last.

2. Step 2: Select correct range for last 5 elements

   Using LTRIM tasks -5 -1 keeps elements from fifth last to last, i.e., last 5 elements.

3. Final Answer:

   LTRIM tasks -5 -1 -> Option A

4. Quick Check:

   Negative indexes keep last elements [OK]

Hint: Use negative indexes to keep last elements [OK]

Common Mistakes:

• Using positive indexes to try to keep last elements
• Reversing negative indexes order
• Using invalid syntax with start > stop

3. Given a Redis list colors with elements ["red", "green", "blue", "yellow", "purple"], what will be the list content after executing LTRIM colors 1 3?

medium

A. ["green", "blue", "yellow"]

B. ["blue", "yellow", "purple"]

C. ["red", "green", "blue"]

D. ["yellow", "purple"]

Solution

1. Step 1: Identify indexes to keep

   LTRIM colors 1 3 keeps elements from index 1 to 3 inclusive.

2. Step 2: Extract elements at those indexes

   Indexes 1 to 3 are "green", "blue", "yellow".

3. Final Answer:

   ["green", "blue", "yellow"] -> Option A

4. Quick Check:

   LTRIM keeps specified index range [OK]

Hint: Indexes start at 0; keep elements between start and stop [OK]

Common Mistakes:

• Including element at index 0 by mistake
• Confusing index ranges
• Assuming LTRIM removes elements inside range

4. You run LTRIM mylist 5 2 on a Redis list. What will happen?

medium

A. The command will return an error due to invalid range.

B. The list will be emptied because start index is greater than stop index.

C. The list will be trimmed to elements from index 5 to 2, keeping those elements.

D. The list will remain unchanged.

Solution

1. Step 1: Understand LTRIM behavior with start > stop

   If start index is greater than stop index, Redis trims the list to empty.

2. Step 2: Apply to given command

   Since 5 > 2, LTRIM mylist 5 2 empties the list.

3. Final Answer:

   The list will be emptied because start index is greater than stop index. -> Option B

4. Quick Check:

   LTRIM with start > stop empties list [OK]

Hint: Start index must be ≤ stop; else list empties [OK]

Common Mistakes:

• Expecting an error instead of empty list
• Thinking list remains unchanged
• Assuming it keeps elements in reverse order

5. You want to keep only the most recent 20 messages in a Redis list chatlog. Which sequence of commands correctly caps the list size after adding a new message "Hello"?

hard

A. RPUSH chatlog "Hello"; LTRIM chatlog -20 0

B. RPUSH chatlog "Hello"; LTRIM chatlog 0 19

C. LPUSH chatlog "Hello"; LTRIM chatlog 0 19

D. LPUSH chatlog "Hello"; LTRIM chatlog -20 -1

Solution

1. Step 1: Understand list direction and adding new messages

   Using LPUSH adds new messages to the start (index 0), so recent messages are at the front.

2. Step 2: Use LTRIM to keep first 20 elements

   To keep the most recent 20 messages, trim to indexes 0 to 19, or equivalently LTRIM chatlog 0 19. Negative indexes also work but must match direction.

3. Step 3: Match push and trim commands

   Since LPUSH adds to front, LTRIM chatlog 0 19 keeps first 20 recent messages. LPUSH chatlog "Hello"; LTRIM chatlog -20 -1 uses LPUSH and LTRIM chatlog -20 -1, which keeps last 20 elements, but since new messages are at front, last 20 are oldest. So LPUSH chatlog "Hello"; LTRIM chatlog -20 -1 is incorrect.

4. Step 4: Correct option

   LPUSH chatlog "Hello"; LTRIM chatlog 0 19 uses LPUSH and LTRIM chatlog 0 19, which correctly keeps the 20 most recent messages at front.

5. Final Answer:

   LPUSH chatlog "Hello"; LTRIM chatlog 0 19 -> Option C

6. Quick Check:

   LPUSH + LTRIM 0 19 keeps recent 20 messages [OK]

Hint: Match push direction with LTRIM range to keep recent items [OK]

Common Mistakes:

• Using RPUSH but trimming front indexes
• Using negative indexes incorrectly with LPUSH
• Trimming wrong range causing oldest messages to stay