LRANGE for reading elements in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the first 3 elements from the list 'mylist'.

Redis

LRANGE mylist 0 [1]

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

A2

B3

C-1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as the end index returns 4 elements.

Using -1 returns the entire list.

The LRANGE command uses zero-based indexes. To get the first 3 elements, the end index should be 2.

2fill in blank

medium

Complete the code to get the last 4 elements from the list 'tasks'.

Redis

LRANGE tasks [1] -1

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

A4

B-4

C-3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 4 as start index skips first 4 elements instead of last 4.

Using -3 returns only last 3 elements.

Using a negative start index counts from the end. -4 means start from the 4th last element.

3fill in blank

hard

Fix the error in the code to get elements from index 2 to 5 in 'myqueue'.

Redis

LRANGE myqueue 2 [1]

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

A6

B-1

C5

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using 6 returns one extra element.

Using 4 returns one less element.

The end index is inclusive, so to get elements from 2 to 5, use 5 as the end index.

4fill in blank

hard

Fill both blanks to get elements from the 3rd to the 7th position in 'events'.

Redis

LRANGE events [1] [2]

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

A2

B3

C6

D7

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 (B) as start index gets from 4th position.

Using 7 (D) as end index gets up to 8th position.

Indexes start at 0, so the 3rd position (1-based) is index 2 (A), 7th position is index 6 (C). End index is inclusive.

5fill in blank

hard

Fill all three blanks to get elements from index 1 to 4 from 'queue' and store in variable 'result'.

Redis

result = redis.call('LRANGE', 'queue', [1], [2]) -- expecting [3] elements

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

A1

B4

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Expecting 3 elements instead of 4.

Using wrong start or end indexes.

Start=1 (A), end=4 (B) inclusive returns 4 elements (C) at indices 1,2,3,4. Number = end - start + 1 = 4.

Practice

1. What does the Redis command LRANGE mylist 0 -1 do?

easy

A. Returns the length of mylist.

B. Returns only the first element of mylist.

C. Returns all elements in the list mylist.

D. Deletes all elements in mylist.

Solution

1. Step 1: Understand LRANGE parameters

   The command LRANGE key start stop returns elements from index start to stop inclusive.

2. Step 2: Interpret 0 and -1 indexes

   Index 0 is the first element, and -1 means the last element, so it returns the whole list.

3. Final Answer:

   Returns all elements in the list mylist. -> Option C

4. Quick Check:

   LRANGE mylist 0 -1 = all elements [OK]

Hint: Use 0 and -1 to get the entire list quickly [OK]

Common Mistakes:

• Thinking -1 excludes the last element
• Confusing LRANGE with LLEN
• Assuming LRANGE deletes elements

2. Which of the following is the correct syntax to get the first 3 elements of a Redis list named tasks using LRANGE?

easy

A. LRANGE tasks 1 3

B. LRANGE tasks -3 -1

C. LRANGE tasks 0 3

D. LRANGE tasks 0 2

Solution

1. Step 1: Understand zero-based indexing

   Redis list indexes start at 0, so the first element is index 0.

2. Step 2: Calculate indexes for first 3 elements

   First 3 elements are at indexes 0, 1, and 2, so LRANGE should be 0 to 2.

3. Final Answer:

   LRANGE tasks 0 2 -> Option D

4. Quick Check:

   First 3 elements = indexes 0 to 2 [OK]

Hint: Remember list indexes start at 0, so first 3 are 0 to 2 [OK]

Common Mistakes:

• Starting at index 1 instead of 0
• Using stop index as 3 which returns 4 elements
• Using negative indexes incorrectly

3. Given a Redis list colors with elements ["red", "green", "blue", "yellow", "purple"], what is the result of LRANGE colors 1 3?

medium

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

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

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

D. ["yellow", "purple"]

Solution

1. Step 1: Identify elements by index

   Indexes: 0="red", 1="green", 2="blue", 3="yellow", 4="purple".

2. Step 2: Select elements from index 1 to 3

   Elements are "green", "blue", and "yellow".

3. Final Answer:

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

4. Quick Check:

   LRANGE colors 1 3 = green, blue, yellow [OK]

Hint: Count indexes starting at 0 to pick correct slice [OK]

Common Mistakes:

• Including index 0 when start is 1
• Using stop index as exclusive instead of inclusive
• Confusing element order

4. You run LRANGE mylist 2 0 on a Redis list. What is the problem with this command?

medium

A. The start index is greater than the stop index, so it returns an empty list.

B. Syntax error because indexes must be positive.

C. It returns the entire list ignoring indexes.

D. It deletes elements from index 0 to 2.

Solution

1. Step 1: Check index order in LRANGE

   LRANGE expects start index less than or equal to stop index to return elements.

2. Step 2: Analyze given indexes

   Start is 2, stop is 0, so start > stop, which returns an empty list.

3. Final Answer:

   The start index is greater than the stop index, so it returns an empty list. -> Option A

4. Quick Check:

   Start > stop means empty result [OK]

Hint: Start index must be ≤ stop index to get results [OK]

Common Mistakes:

• Assuming negative indexes are invalid syntax
• Expecting reversed range to return elements
• Confusing LRANGE with deletion commands

5. You have a Redis list logs with 11 elements. You want to get the last 5 elements using LRANGE. Which command is correct?

hard

A. LRANGE logs -5 -1

B. LRANGE logs 4 8

C. LRANGE logs 5 9

D. LRANGE logs -4 0

Solution

1. Step 1: Understand negative indexes in LRANGE

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

2. Step 2: Select last 5 elements using negative indexes

   LRANGE logs -5 -1 returns elements from fifth last to last, i.e., last 5 elements.

3. Step 3: Check other options

   LRANGE logs 5 9 uses positive indexes 5 to 9 which gets elements 5-9, but for 11 elements the last 5 are indexes 6-10, so incorrect. Negative indexes are safer if length changes.

4. Final Answer:

   LRANGE logs -5 -1 -> Option A

5. Quick Check:

   Negative indexes -5 to -1 = last 5 elements [OK]

Hint: Use negative indexes -5 to -1 for last 5 elements safely [OK]

Common Mistakes:

• Using wrong negative indexes like -4 to 0
• Confusing zero-based positive indexes with negative
• Assuming LRANGE excludes stop index