SRANDMEMBER for random elements in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does SRANDMEMBER return when count is positive?

Given a Redis set myset containing {"a", "b", "c", "d"}, what is the expected output of SRANDMEMBER myset 2?

Redis

SADD myset a b c d
SRANDMEMBER myset 2

A An error because count must be negative or zero

B An array of 2 random elements from the set, possibly with duplicates, e.g., ["a", "a"]

C A single random element as a string, e.g., "c"

D An array of 2 unique random elements from the set, e.g., ["b", "d"]

💡 Hint

When count is positive, SRANDMEMBER returns unique elements without repetition.

SRANDMEMBER with a positive count returns an array of unique random elements from the set, up to the count specified, without duplicates.

❓ query_result

intermediate

What happens when SRANDMEMBER count is negative?

Given a Redis set colors with elements {"red", "green", "blue"}, what is the output of SRANDMEMBER colors -4?

Redis

SADD colors red green blue
SRANDMEMBER colors -4

A An array of 4 random elements from the set, possibly with duplicates, e.g., ["red", "red", "blue", "green"]

B An array of 3 unique elements because the set only has 3 members

C A single random element as a string

D An error because count cannot be negative

💡 Hint

Negative count allows duplicates in the returned elements.

When count is negative, SRANDMEMBER returns the absolute value of count random elements, allowing duplicates.

📝 Syntax

advanced

Which SRANDMEMBER command syntax is invalid?

Identify the invalid SRANDMEMBER command syntax from the options below.

A SRANDMEMBER myset -2

B SRANDMEMBER myset 3

C SRANDMEMBER myset 0

D SRANDMEMBER myset

💡 Hint

Count of zero is not a valid argument for SRANDMEMBER.

SRANDMEMBER with count=0 is invalid and causes an error; count must be omitted or a non-zero integer.

❓ optimization

advanced

How to efficiently get 5 random unique elements from a large set?

You have a Redis set with 1 million elements. Which command is the most efficient to get 5 unique random elements?

A Use SPOP mylargeSet 5 and then add them back

B SRANDMEMBER mylargeSet 5

C Use SMEMBERS mylargeSet and pick 5 random elements in client code

D SRANDMEMBER mylargeSet -5

💡 Hint

SRANDMEMBER with positive count returns unique elements efficiently.

SRANDMEMBER with a positive count returns unique random elements efficiently without modifying the set.

🧠 Conceptual

expert

Why does SRANDMEMBER with negative count allow duplicates?

Explain the reason behind SRANDMEMBER returning duplicates when the count argument is negative.

A Because negative count tells Redis to sample with replacement, allowing repeated elements.

B Because Redis ignores the sign and always returns unique elements.

C Because negative count causes Redis to return an error and no elements.

D Because Redis converts negative count to zero and returns an empty array.

💡 Hint

Think about sampling with or without replacement.

Negative count means Redis samples with replacement, so elements can appear multiple times in the result.