ZREM for removal in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of removing members with ZREM?

Given a sorted set myzset with members and scores:

ZADD myzset 1 one 2 two 3 three

What will be the output of the command:

ZREM myzset two four

and what will be the new members in myzset?

Redis

ZADD myzset 1 one 2 two 3 three
ZREM myzset two four
ZRANGE myzset 0 -1 WITHSCORES

A Output of ZREM: 0; New members: one (1), two (2), three (3)

B Output of ZREM: 2; New members: one (1), three (3)

C Output of ZREM: 1; New members: one (1), three (3)

D Output of ZREM: 1; New members: one (1), two (2), three (3)

💡 Hint

ZREM returns the number of members actually removed. Only existing members count.

The command ZREM myzset two four tries to remove 'two' and 'four'. 'two' exists and is removed, 'four' does not exist. So the output is 1. The remaining members are 'one' and 'three'.

🧠 Conceptual

intermediate

Understanding ZREM behavior with non-existing members

If you run ZREM myzset a b c on a sorted set myzset that contains only x and y, what will be the output?

A 0

B 3

C Error

D 1

💡 Hint

ZREM counts only members that were actually removed.

Since none of 'a', 'b', or 'c' exist in the set, no members are removed, so the output is 0.

📝 Syntax

advanced

Which ZREM command syntax is correct?

Which of the following commands correctly removes members 'a' and 'b' from the sorted set 'myzset'?

A ZREM myzset a b

B ZREM myzset [a, b]

C ZREM myzset {a, b}

D ZREM myzset 'a b'

💡 Hint

ZREM takes the key followed by one or more members separated by spaces.

The correct syntax is to list members separated by spaces without brackets or quotes. Options A, B, and C are invalid syntax.

❓ optimization

advanced

Efficient removal of multiple members with ZREM

You want to remove 100 members from a sorted set in Redis. Which approach is the most efficient?

A Call ZREM 100 times, each time with one member

B Call ZREM once with all 100 members listed

C Call ZREM twice with 50 members each time

D Call ZREM once with a comma-separated string of members

💡 Hint

Fewer calls to Redis server reduce overhead.

Calling ZREM once with all members reduces network overhead and is more efficient than multiple calls.

🔧 Debug

expert

Why does this ZREM command fail?

You run the command:

ZREM myzset 1 2 3

but it does not remove any members, even though you expect members '1', '2', and '3' to be removed. What is the most likely reason?

A ZREM only removes members with scores matching the arguments

B ZREM requires the scores to be specified along with members

C myzset does not exist

D The members are stored as strings, but you are passing numbers without quotes

💡 Hint

Redis stores members as strings exactly as added.

ZREM removes members by exact string match. If members were added as strings '1', '2', '3', passing numbers without quotes may cause no match.

Practice

1. What does the Redis command ZREM do?

easy

A. Removes specified members from a sorted set

B. Adds new members to a sorted set

C. Returns the score of a member in a sorted set

D. Deletes the entire sorted set

Solution

1. Step 1: Understand the purpose of ZREM

   The command ZREM is used to remove one or more members from a sorted set in Redis.

2. Step 2: Compare with other commands

   Adding members is done by ZADD, getting scores by ZSCORE, and deleting the whole set by DEL, so these are not correct.

3. Final Answer:

   Removes specified members from a sorted set -> Option A

4. Quick Check:

   ZREM removes members = B [OK]

Hint: ZREM always removes members from sorted sets [OK]

Common Mistakes:

• Confusing ZREM with ZADD
• Thinking ZREM deletes the whole set
• Assuming ZREM returns member scores

2. Which of the following is the correct syntax to remove members 'a' and 'b' from a sorted set named 'myzset' using ZREM?

easy

A. ZREM myzset (a,b)

B. ZREM myzset {a,b}

C. ZREM myzset ['a','b']

D. ZREM myzset a b

Solution

1. Step 1: Recall ZREM syntax

   ZREM command syntax is: ZREM key member [member ...]. Members are listed separated by spaces.

2. Step 2: Analyze options

   ZREM myzset a b uses space-separated members correctly. Options B, C, and D use invalid syntax with braces, brackets, or parentheses which Redis does not accept.

3. Final Answer:

   ZREM myzset a b -> Option D

4. Quick Check:

   Correct ZREM syntax = A [OK]

Hint: List members separated by spaces after key in ZREM [OK]

Common Mistakes:

• Using brackets or braces around members
• Separating members with commas
• Passing members as a single string

3. Given a sorted set 'myzset' with members and scores: {"a":1, "b":2, "c":3}, what will be the result of ZREM myzset b d followed by ZCARD myzset?

medium

A. 1

B. 2

C. 3

D. 0

Solution

1. Step 1: Understand ZREM effect

   ZREM removes specified members if they exist. 'b' exists and will be removed. 'd' does not exist and is ignored without error.

2. Step 2: Calculate remaining members

   Initially 3 members: a, b, c. After removal of 'b', members left are 'a' and 'c', so 2 remain.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Removed 1 member, 2 remain = C [OK]

Hint: ZREM ignores missing members, count remaining with ZCARD [OK]

Common Mistakes:

• Assuming error on removing non-existent member
• Counting removed non-existent member
• Confusing ZREM with ZADD effect

4. You run the command ZREM myzset without specifying any members. What will happen?

medium

A. Syntax error due to missing members

B. Returns 0 and does nothing

C. Removes the first member by default

D. Removes all members from 'myzset'

Solution

1. Step 1: Check ZREM command requirements

   ZREM requires at least one member argument to remove. Omitting members is invalid syntax.

2. Step 2: Result of missing members

   Running ZREM without members causes a syntax error, Redis will not execute the command.

3. Final Answer:

   Syntax error due to missing members -> Option A

4. Quick Check:

   ZREM needs members, missing causes syntax error = D [OK]

Hint: Always specify members to remove with ZREM [OK]

Common Mistakes:

• Assuming ZREM clears entire set without members
• Expecting default removal of first member
• Thinking it returns 0 silently

5. You have a sorted set 'myzset' with members: {"x":10, "y":20, "z":30}. You want to remove all members with scores less than 25 using ZREM. Which approach correctly achieves this?

hard

A. Use ZREM myzset x y

B. Use ZREM myzset with a score range argument

C. Use ZREMRANGEBYSCORE myzset -inf 24

D. Use ZREM myzset z

Solution

1. Step 1: Understand ZREM limitations

   ZREM removes specified members by name, it cannot remove by score range directly.

2. Step 2: Identify members to remove

   Members x (score 10) and y (score 20) are less than 25, so to remove them by score range, use ZREMRANGEBYSCORE myzset -inf 24.

3. Step 3: Analyze options

   A correctly uses ZREMRANGEBYSCORE to remove by score range. B is invalid syntax. C removes members by name but requires manual identification. D removes wrong member.

4. Final Answer:

   Use ZREMRANGEBYSCORE myzset -inf 24 -> Option C

5. Quick Check:

   Removing by score range requires ZREMRANGEBYSCORE = A [OK]

Hint: Use ZREMRANGEBYSCORE to remove members by score range [OK]

Common Mistakes:

• Trying to remove by score with ZREM
• Using invalid syntax with ZREM
• Removing wrong members by name