SISMEMBER for membership check in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command SISMEMBER do?

It checks if a given element is a member of a specified set and returns 1 if true, 0 if false.

beginner

How do you use SISMEMBER to check if 'apple' is in the set 'fruits'?

Use the command: SISMEMBER fruits apple. It returns 1 if 'apple' is in 'fruits', otherwise 0.

beginner

What type of data structure does SISMEMBER work with in Redis?

SISMEMBER works only with sets, which are unordered collections of unique elements.

beginner

What is the return value of SISMEMBER if the element is not found in the set?

It returns 0, indicating the element is not a member of the set.

intermediate

Can SISMEMBER be used on Redis data types other than sets?

No, SISMEMBER only works with sets. Using it on other types will cause an error.

What does the Redis command SISMEMBER myset value return if 'value' is in 'myset'?

A 0

B The value itself

C An error

D 1

SISMEMBER returns 1 if the element is a member of the set.

Which Redis data type is required for SISMEMBER to work?

A Set

B Hash

C String

D List

SISMEMBER only works with sets.

What does SISMEMBER return if the element is not in the set?

A Null

B 1

C 0

D An error

It returns 0 to indicate the element is not a member.

If you run SISMEMBER on a key that does not exist, what is the result?

A 0

B An error

C 1

D Null

If the set does not exist, Redis treats it as empty and returns 0.

Which of the following commands checks membership in a Redis set?

A GET

B SISMEMBER

C HGET

D LPUSH

SISMEMBER is the command to check membership in a set.

Explain how the Redis command SISMEMBER works and what it returns.

Think about checking if an item is in a group.

Describe a real-life example where you might use SISMEMBER in Redis.

Imagine checking if a friend is in your contact list.

Practice

1. What does the Redis command SISMEMBER do?

easy

A. Adds a value to a set

B. Removes a value from a set

C. Checks if a value is a member of a set and returns 1 or 0

D. Returns all members of a set

Solution

1. Step 1: Understand SISMEMBER purpose

   The SISMEMBER command checks membership of a value in a Redis set.

2. Step 2: Identify return values

   It returns 1 if the value is in the set, otherwise 0.

3. Final Answer:

   Checks if a value is a member of a set and returns 1 or 0 -> Option C

4. Quick Check:

   SISMEMBER returns 1 or 0 [OK]

Hint: SISMEMBER returns 1 if present, else 0 [OK]

Common Mistakes:

• Confusing SISMEMBER with SADD or SREM
• Expecting SISMEMBER to return the value itself
• Thinking SISMEMBER returns all set members

2. Which of the following is the correct syntax to check if 'apple' is in the set 'fruits' using Redis?

easy

A. SISMEMBER(fruits, apple)

B. SISMEMBER 'fruits' 'apple'

C. SISMEMBER fruits, apple

D. SISMEMBER fruits apple

Solution

1. Step 1: Recall Redis command syntax

   Redis commands use space-separated arguments without commas or parentheses.

2. Step 2: Match correct syntax

   Correct syntax is SISMEMBER key member, so SISMEMBER fruits apple is correct.

3. Final Answer:

   SISMEMBER fruits apple -> Option D

4. Quick Check:

   Redis commands use spaces, no commas or quotes needed [OK]

Hint: Use space-separated arguments without quotes or commas [OK]

Common Mistakes:

• Adding commas between arguments
• Using parentheses like a function call
• Adding quotes around arguments unnecessarily

3. Given the Redis set 'colors' contains {"red", "blue", "green"}, what is the output of SISMEMBER colors yellow?

medium

A. 0

B. 1

C. red

D. Error

Solution

1. Step 1: Check if 'yellow' is in the set 'colors'

   The set contains "red", "blue", and "green" but not "yellow".

2. Step 2: Understand SISMEMBER return value

   SISMEMBER returns 1 if member exists, else 0. Since "yellow" is not in the set, it returns 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Member absent = 0 [OK]

Hint: If value not in set, SISMEMBER returns 0 [OK]

Common Mistakes:

• Assuming it returns the value itself
• Confusing 0 with an error
• Thinking it returns the whole set

4. You run the command SISMEMBER colors without specifying a member. What will happen?

medium

A. Syntax error

B. Returns 1

C. Returns 0

D. Returns all members

Solution

1. Step 1: Check command argument requirements

   SISMEMBER requires exactly two arguments: the set key and the member to check.

2. Step 2: Analyze missing argument effect

   Omitting the member argument causes a syntax error in Redis.

3. Final Answer:

   Syntax error -> Option A

4. Quick Check:

   Missing arguments cause syntax error [OK]

Hint: SISMEMBER needs two arguments; missing one causes error [OK]

Common Mistakes:

• Expecting default member to be checked
• Thinking it returns 0 if member missing
• Assuming it returns all members

5. You have a Redis set 'users' with members {"alice", "bob", "carol"}. You want to check if 'dave' is a member and add him only if he is not already present. Which sequence of commands correctly does this?

hard

A. SISMEMBER users dave
SREM users dave

B. SISMEMBER users dave
SADD users dave

C. SADD users dave
SISMEMBER users dave

D. SREM users dave
SADD users dave

Solution

1. Step 1: Check membership before adding

   Use SISMEMBER to see if 'dave' is already in 'users'.

2. Step 2: Add 'dave' only if not present

   If SISMEMBER returns 0, then use SADD to add 'dave'.

3. Final Answer:

   SISMEMBER users dave SADD users dave -> Option B

4. Quick Check:

   Check membership then add if absent [OK]

Hint: Check with SISMEMBER before SADD to avoid duplicates [OK]

Common Mistakes:

• Adding before checking membership
• Removing member instead of adding
• Checking membership after adding