APPEND for string concatenation in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of APPEND command?

Given a Redis key greeting with value "Hello", what will be the value of greeting after executing APPEND greeting " World"?

Redis

SET greeting "Hello"
APPEND greeting " World"
GET greeting

A "Hello"

B "HelloWorld"

C " WorldHello"

D "Hello World"

💡 Hint

APPEND adds the new string to the end of the existing string.

The APPEND command adds the specified string to the end of the existing string value stored at the key. So " World" is appended to "Hello", resulting in "Hello World".

❓ query_result

intermediate

What does APPEND return?

If the key name has value "Redis", what does the command APPEND name "DB" return?

Redis

SET name "Redis"
APPEND name "DB"

A 7

B 5

C RedisDB

D Error

💡 Hint

APPEND returns the length of the new string after appending.

APPEND returns the length of the string after the append operation. "Redis" has length 5, appending "DB" adds 2 characters, total length 7.

📝 Syntax

advanced

Identify the syntax error in APPEND usage

Which of the following APPEND commands will cause a syntax error in Redis?

A APPEND key1 "text"

B APPEND key1

C APPEND "key1" "text"

D APPEND key1 text

💡 Hint

APPEND requires exactly two arguments: key and value.

Option B is missing the second argument (the string to append), causing a syntax error. Options A, C, and D are valid syntax (quotes optional for keys and values in Redis CLI).

🧠 Conceptual

advanced

What happens if APPEND is used on a non-string key?

If the key counter holds an integer value (stored as a string) and you run APPEND counter "1", what will happen?

Redis

SET counter "100"
APPEND counter "1"
GET counter

A The value becomes "1001" as a string

B Redis throws a type error

C The value becomes 101 (integer addition)

D The value remains "100" unchanged

💡 Hint

Redis stores all strings as strings, even numbers are strings.

Redis treats all values as strings. APPEND concatenates the string "1" to "100", resulting in "1001". No type error occurs.

❓ optimization

expert

Efficiently concatenate multiple strings using APPEND

You want to concatenate the strings "a", "b", "c", "d" to an empty Redis key letters. Which sequence of commands is the most efficient to get the final value "abcd"?

A

APPEND letters "a"
APPEND letters "b"
APPEND letters "c"
APPEND letters "d"

B

SET letters "a"
APPEND letters "b"
APPEND letters "c"
APPEND letters "d"

C SET letters "abcd"

D

SET letters "a"
SET letters "b"
SET letters "c"
SET letters "d"

💡 Hint

APPEND adds to existing string; SET overwrites.

Option C sets the entire string at once, which is the most efficient. Option C works because APPEND on a non-existing key treats it as empty, so first APPEND "a" creates "a" but involves multiple commands. Option C initializes the key with "a" and then appends "b", "c", "d" sequentially, resulting in "abcd" but with more commands. Option C overwrites the key each time, ending with "d" only.