LTRIM for list capping in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command LTRIM do?

The LTRIM command trims a list to only keep the specified range of elements, removing all others outside that range.

beginner

How can LTRIM be used to keep only the latest 100 items in a Redis list?

Use LTRIM key 0 99 to keep only the first 100 elements (index 0 to 99) and remove older elements beyond that.

beginner

What happens if you use LTRIM key 0 -1 on a Redis list?

It keeps the entire list unchanged because -1 means the last element, so the range covers the whole list.

intermediate

Why is LTRIM useful for list capping in Redis?

It helps limit the size of a list to a fixed number of recent items, saving memory and keeping data manageable.

intermediate

If you want to keep the last 50 elements of a list, which LTRIM command should you use?

Use LTRIM key -50 -1 to keep the last 50 elements, where -50 is the 50th element from the end.

What does LTRIM mylist 0 9 do?

A Keeps the first 10 elements of 'mylist' and removes the rest

B Removes the first 10 elements of 'mylist'

C Keeps the last 10 elements of 'mylist'

D Deletes the entire 'mylist'

LTRIM with 0 9 keeps elements from index 0 to 9, which is the first 10 elements.

How do you keep only the last 20 elements of a Redis list named 'events'?

A LTRIM events 0 19

B LTRIM events -20 -1

C LTRIM events 20 -1

D LTRIM events -1 -20

Using negative indexes, -20 is the 20th element from the end, so LTRIM events -20 -1 keeps the last 20 elements.

What happens if LTRIM is called with a start index greater than the end index?

A The list remains unchanged

B An error is returned

C The list is cleared (emptied)

D The list is reversed

If start > end, LTRIM removes all elements, resulting in an empty list.

Which Redis command is commonly used together with LTRIM to implement a capped list?

A LPUSH

B GET

C DEL

D SET

LPUSH adds new elements to the head of the list, and LTRIM trims it to a fixed size, together implementing a capped list.

If you want to keep the entire list unchanged, which LTRIM command would you use?

A LTRIM key 0 0

B LTRIM key -1 0

C LTRIM key 1 0

D LTRIM key 0 -1

LTRIM key 0 -1 keeps the whole list because -1 means the last element.

Explain how the Redis command LTRIM helps in managing the size of a list.

Think about keeping only a fixed number of recent entries.

Describe the difference between using positive and negative indexes in the LTRIM command.

Consider how you count positions from the front or back of the list.

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