LTRIM for list capping in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to trim the list 'mylist' to keep only the first 100 elements.

Redis

LTRIM mylist 0 [1]

Drag options to blanks, or click blank then click option'

A101

B100

C-1

D99

Attempts:

3 left

💡 Hint

Common Mistakes

Using 100 as the end index which keeps 101 elements.

Using negative indexes incorrectly.

The LTRIM command keeps elements from the start index to the end index inclusive. To keep the first 100 elements, the end index should be 99 (since indexing starts at 0).

2fill in blank

medium

Complete the code to keep only the last 50 elements of the list 'events'.

Redis

LTRIM events [1] -1

Drag options to blanks, or click blank then click option'

A-49

B0

C-50

D49

Attempts:

3 left

💡 Hint

Common Mistakes

Using -50 as start index which keeps 51 elements.

Using positive indexes which may not keep last elements.

Using negative indexes, -1 is the last element, so to keep last 50 elements, start at -49.

3fill in blank

hard

Fix the error in the command to keep only the first 10 elements of 'tasks'.

Redis

LTRIM tasks 0 [1]

Drag options to blanks, or click blank then click option'

A10

B-10

C9

D-9

Attempts:

3 left

💡 Hint

Common Mistakes

Using 10 as end index which keeps 11 elements.

Using negative indexes incorrectly.

The end index is inclusive and zero-based, so to keep 10 elements, the end index must be 9.

4fill in blank

hard

Fill both blanks to trim 'logs' list to keep elements from index 5 to 15.

Redis

LTRIM logs [1] [2]

Drag options to blanks, or click blank then click option'

A5

B15

C10

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping start and end indexes.

Using indexes outside the list range.

The command keeps elements starting at index 5 and ending at index 15 inclusive.

5fill in blank

hard

Fill all three blanks to trim 'queue' list to keep elements from index 2 to 12.

Redis

LTRIM [1] [2] [3]

Drag options to blanks, or click blank then click option'

Aqueue

B2

C12

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong list name.

Incorrect start or end indexes.

The command trims the list named 'queue' from index 2 to 12 inclusive.

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