GETRANGE and SETRANGE in Redis

Introduction

GETRANGE and SETRANGE let you read or change parts of a text stored in Redis without touching the whole text.

You want to read a small part of a long message stored in Redis.

You need to update just a few characters inside a stored string without replacing it all.

You want to check a substring inside a stored value quickly.

You want to save bandwidth by only sending or changing parts of a string.

You want to fix a typo inside a stored string without rewriting the entire string.

Syntax

Redis

GETRANGE key start end
SETRANGE key offset value

GETRANGE returns the substring from start to end (inclusive).

SETRANGE overwrites part of the string starting at offset with value. If the string is shorter, it is extended with zero-bytes.

Examples

Gets the first 5 characters (positions 0 to 4) of the string stored at mykey.

Redis

GETRANGE mykey 0 4

Replaces the string starting at position 6 with "world" inside the value stored at mykey.

Redis

SETRANGE mykey 6 "world"

Gets the last 5 characters of the string stored at mykey. Negative indexes count from the end.

Redis

GETRANGE mykey -5 -1

Sample Program

This example first sets the string "Hello Redis!" to mykey. Then it gets the first 5 characters, which is "Hello". Next, it replaces the substring starting at position 6 with "World", changing the string to "Hello World!". Finally, it gets the entire updated string.

Redis

SET mykey "Hello Redis!"
GETRANGE mykey 0 4
SETRANGE mykey 6 "World"
GETRANGE mykey 0 -1

Important Notes

Indexes in GETRANGE are inclusive and zero-based.

SETRANGE returns the length of the string after the change.

If SETRANGE extends the string, the new bytes between old end and offset are zero-bytes (\x00).

Summary

GETRANGE reads a part of a string by specifying start and end positions.

SETRANGE replaces part of a string starting at a given offset with new text.

Both commands help work with substrings without rewriting the whole string.

Practice

1. What does the Redis command GETRANGE key 0 4 do?

easy

A. Returns the substring of the value stored at key from index 0 to 4 inclusive.

B. Deletes the first 5 characters of the string stored at key.

C. Replaces the first 5 characters of the string stored at key with zeros.

D. Returns the length of the string stored at key.

Solution

1. Step 1: Understand GETRANGE parameters

   The command GETRANGE key start end returns the substring from the start index to the end index inclusive.

2. Step 2: Apply to given indices

   With start=0 and end=4, it returns characters at positions 0,1,2,3,4 from the string stored at key.

3. Final Answer:

   Returns substring from index 0 to 4 inclusive -> Option A

4. Quick Check:

   GETRANGE extracts substring = A [OK]

Hint: GETRANGE extracts substring by start and end indexes [OK]

Common Mistakes:

• Thinking GETRANGE deletes or modifies the string
• Confusing GETRANGE with SETRANGE
• Assuming end index is exclusive

2. Which of the following is the correct syntax to replace part of a string value in Redis using SETRANGE?

easy

A. SETRANGE key value offset

B. SETRANGE key offset value

C. SET key offset value

D. GETRANGE key offset value

Solution

1. Step 1: Recall SETRANGE syntax

   The correct syntax is SETRANGE key offset value, where offset is the start position to replace and value is the new substring.

2. Step 2: Check options

   Only SETRANGE key offset value matches this syntax exactly.

3. Final Answer:

   SETRANGE key offset value -> Option B

4. Quick Check:

   Correct SETRANGE syntax = A [OK]

Hint: Remember SETRANGE order: key, offset, then value [OK]

Common Mistakes:

• Swapping offset and value positions
• Using GETRANGE instead of SETRANGE
• Using SET instead of SETRANGE

3. Given the Redis string value at key mykey is "HelloWorld", what will be the result of GETRANGE mykey 5 7?

medium

A. "World"

B. "Hello"

C. "loW"

D. "Wor"

Solution

1. Step 1: Identify substring indices

   The string is "HelloWorld" with indices: H(0), e(1), l(2), l(3), o(4), W(5), o(6), r(7), l(8), d(9).

2. Step 2: Extract substring from index 5 to 7

   Characters at 5,6,7 are 'W', 'o', 'r', so the substring is "Wor".

3. Final Answer:

   "Wor" -> Option D

4. Quick Check:

   GETRANGE returns substring = B [OK]

Hint: Count string indexes from 0 to get substring [OK]

Common Mistakes:

• Counting indexes starting at 1
• Including characters outside the range
• Confusing GETRANGE with SETRANGE

4. You run SETRANGE mykey 3 "abc" on a Redis string "abcdef". The result is unexpected. What is the likely mistake?

medium

A. Using quotes around the substring value in the command.

B. Offset 3 is out of range for the string length.

C. SETRANGE cannot modify existing strings.

D. The key mykey does not exist.

Solution

1. Step 1: Understand SETRANGE usage

   SETRANGE expects the substring value without quotes; quotes are interpreted literally.

2. Step 2: Identify the mistake

   Using quotes like "abc" inserts the quotes as characters, causing unexpected results.

3. Final Answer:

   Using quotes around substring causes wrong replacement -> Option A

4. Quick Check:

   Quotes in SETRANGE value cause errors = C [OK]

Hint: Do not include quotes around substring in SETRANGE [OK]

Common Mistakes:

• Including quotes around substring value
• Assuming offset must be within string length
• Thinking SETRANGE creates new keys automatically

5. If the Redis key text holds the string "abcdefghij", what will be the final string after executing SETRANGE text 5 "XYZ" followed by GETRANGE text 3 7?

hard

A. "defgh"

B. "fghij"

C. "deXYZ"

D. "defXY"

Solution

1. Step 1: Apply SETRANGE at offset 5

   Original string: "abcdefghij". SETRANGE replaces characters at positions 5,6,7 with "XYZ". Positions: a(0), b(1), c(2), d(3), e(4), f(5), g(6), h(7), i(8), j(9). After replacement: "abcdeXYZij".

2. Step 2: Apply GETRANGE from 3 to 7

   Characters at positions 3 to 7 are: d(3), e(4), X(5), Y(6), Z(7). So substring is "deXYZ".

3. Final Answer:

   "deXYZ" -> Option C

4. Quick Check:

   SETRANGE modifies substring, GETRANGE extracts it = D [OK]

Hint: Apply SETRANGE first, then GETRANGE on updated string [OK]

Common Mistakes:

• Not updating string before GETRANGE
• Miscounting offset positions
• Assuming GETRANGE returns original string