ZCARD for set size in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the number of elements in the sorted set named 'myzset'.

Redis

ZCARD [1]

Drag options to blanks, or click blank then click option'

Amyzset

Bmyset

Csortedset

Dset1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong key name.

Confusing sorted sets with regular sets.

The ZCARD command returns the number of elements in the sorted set specified by its key. Here, the key is myzset.

2fill in blank

medium

Complete the code to get the size of the sorted set stored in the variable 'keyName'.

Redis

ZCARD [1]

Drag options to blanks, or click blank then click option'

Akeyname

BkeyName

Ckey_name

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the case of the key name.

Using a different variable name.

The ZCARD command requires the exact key name. Here, the variable keyName holds the key, so it should be used as is.

3fill in blank

hard

Fix the error in the command to correctly get the size of the sorted set 'players'.

Redis

ZCARD [1]

Drag options to blanks, or click blank then click option'

Aplayers

Bplayer

Cplayerz

Dplayrs

Attempts:

3 left

💡 Hint

Common Mistakes

Misspelling the key name.

Using a singular form instead of plural.

The key name must exactly match the sorted set's name. Here, the correct key is players.

4fill in blank

hard

Fill both blanks to get the size of the sorted set stored in variable 'setKey' and store the result in variable 'count'.

Redis

count = [1] [2] setKey

Drag options to blanks, or click blank then click option'

AZCARD

B=

C==

DGET

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=' for assignment.

Using 'GET' which is not valid for sorted sets.

To get the size of a sorted set in Redis and assign it to a variable, use count = ZCARD setKey. The = assigns the result.

5fill in blank

hard

Fill all three blanks to write a Redis command that stores the size of the sorted set 'scores' into variable 'size', then prints it.

Redis

size [1] [2] scores
PRINT [3]

Drag options to blanks, or click blank then click option'

A=

BZCARD

Csize

DGET

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'GET' instead of 'ZCARD' to get size.

Printing the key name instead of the variable.

The command assigns the size of the sorted set 'scores' to size using = and ZCARD. Then it prints the variable size.

Practice

1. What does the Redis command ZCARD do when used on a sorted set?

easy

A. Deletes all elements in the sorted set

B. Returns the highest score in the sorted set

C. Returns the elements with the lowest score

D. Returns the number of elements in the sorted set

Solution

1. Step 1: Understand the purpose of ZCARD

   The ZCARD command is designed to count how many elements are in a sorted set.

2. Step 2: Compare with other options

   Other options describe different commands like fetching scores or deleting, which ZCARD does not do.

3. Final Answer:

   Returns the number of elements in the sorted set -> Option D

4. Quick Check:

   ZCARD = count of elements [OK]

Hint: ZCARD counts items in sorted set, not scores or values [OK]

Common Mistakes:

• Confusing ZCARD with ZRANGE or ZSCORE
• Thinking it deletes elements
• Assuming it returns scores instead of count

2. Which of the following is the correct syntax to get the size of a sorted set named myzset in Redis?

easy

A. ZCOUNT myzset

B. ZCARD myzset

C. ZSIZE myzset

D. ZLEN myzset

Solution

1. Step 1: Recall the correct command name

   The command to get the number of elements in a sorted set is ZCARD.

2. Step 2: Verify syntax correctness

   The syntax is ZCARD key, so ZCARD myzset is correct.

3. Final Answer:

   ZCARD myzset -> Option B

4. Quick Check:

   Correct command syntax = ZCARD myzset [OK]

Hint: Use ZCARD followed by set name to get size [OK]

Common Mistakes:

• Using ZCOUNT which counts by score range
• Using non-existent commands like ZSIZE or ZLEN
• Forgetting to include the key name

3. Given the following Redis commands executed in order:

ZADD fruits 1 apple 2 banana 3 cherry
ZADD fruits 4 date
ZREM fruits banana

What will ZCARD fruits return?

medium

A. 4

B. 1

C. 3

D. 2

Solution

1. Step 1: Count elements after additions

   After ZADD fruits 1 apple 2 banana 3 cherry, the set has 3 elements: apple, banana, cherry.

2. Step 2: Add one more element and remove one

   ZADD fruits 4 date adds date, total 4 elements. Then ZREM fruits banana removes banana, leaving 3 elements.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Elements after ops = 3 [OK]

Hint: Additions minus removals = final count [OK]

Common Mistakes:

• Counting removed elements
• Ignoring the last removal command
• Assuming ZADD replaces instead of adds

4. You run ZCARD myset but get an error: WRONGTYPE Operation against a key holding the wrong kind of value. What is the most likely cause?

medium

A. The key 'myset' is not a sorted set but another data type

B. The key 'myset' does not exist

C. The ZCARD command is misspelled

D. Redis server is down

Solution

1. Step 1: Understand the error message

   The error WRONGTYPE means the key exists but is not the expected data type.

2. Step 2: Match error to cause

   Since ZCARD works only on sorted sets, the key must be a different type like string or list.

3. Final Answer:

   The key 'myset' is not a sorted set but another data type -> Option A

4. Quick Check:

   WRONGTYPE = wrong data type key [OK]

Hint: WRONGTYPE means key is not a sorted set [OK]

Common Mistakes:

• Assuming key does not exist causes this error
• Thinking command spelling causes WRONGTYPE
• Blaming server status for this error

5. You have a sorted set scores with 1000 elements. You want to check if it is empty before processing. Which Redis command and check is the best and most efficient way?

hard

A. Use ZCARD scores and check if result is 0

B. Use ZRANGE scores 0 -1 and check if result is empty

C. Use ZCOUNT scores -inf +inf and check if result is 0

D. Use GET scores and check if result is null

Solution

1. Step 1: Understand command efficiency

   ZCARD returns the count quickly without fetching all elements, ideal for checking emptiness.

2. Step 2: Compare other options

   ZRANGE fetches all elements, costly for large sets. ZCOUNT counts by score range but is less direct. GET is for strings, not sorted sets.

3. Final Answer:

   Use ZCARD scores and check if result is 0 -> Option A

4. Quick Check:

   ZCARD = fast size check [OK]

Hint: ZCARD quickly returns size, best for emptiness check [OK]

Common Mistakes:

• Using ZRANGE wastes resources fetching all elements
• Using GET on a sorted set key causes errors
• Using ZCOUNT unnecessarily when ZCARD suffices