LRANGE for reading elements in Redis - Step-by-Step Execution

Concept Flow - LRANGE for reading elements

Start with a Redis list

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Receive LRANGE command with start and stop

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Calculate actual start and stop indexes

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Extract elements from list within indexes

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Return extracted elements as result

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End

LRANGE reads a range of elements from a Redis list by calculating start and stop indexes and returning the elements in that range.

Execution Sample

Redis

LRANGE mylist 1 3

Reads elements from index 1 to 3 (inclusive) from the list named 'mylist'.

Execution Table

Step	Command	Start Index	Stop Index	Extracted Elements	Result
1	LRANGE mylist 1 3	1	3	["element2", "element3", "element4"]	Returns elements at indexes 1, 2, 3
2	LRANGE mylist 0 -1	0	-1	["element1", "element2", "element3", "element4", "element5"]	Returns all elements (0 to last)
3	LRANGE mylist 2 10	2	10	["element3", "element4", "element5"]	Returns elements from 2 to end (stop > list length)
4	LRANGE mylist -3 -1	-3	-1	["element3", "element4", "element5"]	Negative indexes count from end
5	LRANGE mylist 4 2	4	2	[]	Empty list because start > stop

💡 Execution stops after extracting elements or returning empty list if indexes invalid.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
start_index	N/A	1	0	2	2	4
stop_index	N/A	3	-1	10	4	2
extracted_elements	[]	["element2", "element3", "element4"]	["element1", "element2", "element3", "element4", "element5"]	["element3", "element4", "element5"]	["element3", "element4", "element5"]	[]

Key Moments - 3 Insights

Why does LRANGE return an empty list when start index is greater than stop index?

Because LRANGE extracts elements from start to stop inclusive, if start > stop, there are no elements in that range, so it returns an empty list as shown in row 5 of the execution_table.

How do negative indexes work in LRANGE?

Negative indexes count from the end of the list backwards. For example, -1 is the last element, -2 is second last, etc. This is shown in row 4 where start=-3 and stop=-1 extracts last three elements.

What happens if the stop index is larger than the list length?

LRANGE returns elements from start index up to the end of the list. It does not error. This is shown in row 3 where stop=10 but list length is 5, so it returns elements from index 2 to 4.

Visual Quiz - 3 Questions

Look at the execution_table row 1. What elements does LRANGE return for command 'LRANGE mylist 1 3'?

A ["element1", "element2", "element3"]

B ["element3", "element4", "element5"]

C ["element2", "element3", "element4"]

D []

💡 Hint

Check the Extracted Elements column in row 1 of execution_table.

At which execution_table row does LRANGE use negative indexes to extract elements?

A Row 2

B Row 4

C Row 3

D Row 5

💡 Hint

Look for negative start and stop indexes in the Start Index and Stop Index columns.

If the start index is 4 and stop index is 2, what will LRANGE return according to the execution_table?

A []

B ["element3", "element4"]

C ["element5"]

D ["element4", "element5"]

💡 Hint

Check row 5 where start > stop and see the Result column.

Concept Snapshot

LRANGE key start stop
- Reads elements from a Redis list between start and stop indexes inclusive.
- Indexes can be negative to count from the end (-1 last element).
- If stop > list length, returns up to end.
- If start > stop, returns empty list.
- Returns elements as an array.

Full Transcript

LRANGE is a Redis command to read elements from a list by specifying start and stop indexes. It calculates the actual indexes, supports negative indexes counting from the end, and returns the elements in that range. If the start index is greater than the stop index, it returns an empty list. If the stop index is beyond the list length, it returns elements up to the end. This visual execution shows step-by-step how LRANGE extracts elements and what results it returns for different index values.

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