HLEN for field count in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of HLEN on a hash with three fields?

Consider a Redis hash stored with key user:1000 having fields name, age, and email. What will the command HLEN user:1000 return?

Redis

HMSET user:1000 name "Alice" age "30" email "[e-mail]"
HLEN user:1000

A 3

B Error: key does not exist

C 0

D 1

💡 Hint

HLEN returns the number of fields in the hash stored at the key.

HLEN returns the count of fields in the hash. Since there are three fields, the output is 3.

❓ query_result

intermediate

What does HLEN return for a non-existing key?

If you run HLEN on a key that does not exist in Redis, what is the output?

Redis

HLEN non_existing_key

A 0

B Error: key does not exist

C nil

D -1

💡 Hint

Redis treats non-existing hashes as empty.

HLEN returns 0 for keys that do not exist because it treats them as empty hashes.

📝 Syntax

advanced

Which HLEN command syntax is correct?

Identify the correct syntax to get the number of fields in a Redis hash named session:123.

A HLEN(session:123)

B HLEN {session:123}

C HLEN session:123

D HLEN 'session:123'

💡 Hint

Redis commands do not use parentheses or braces.

The correct syntax is just the command followed by the key name without quotes or parentheses.

🧠 Conceptual

advanced

What happens if HLEN is run on a key holding a string?

If a Redis key mykey holds a string value, what will happen when you run HLEN mykey?

Redis

SET mykey "hello"
HLEN mykey

A 1

B Error: WRONGTYPE Operation against a key holding the wrong kind of value

C 0

D nil

💡 Hint

HLEN only works on hashes, not strings.

HLEN on a key holding a string returns a WRONGTYPE error because the command expects a hash.

❓ optimization

expert

Efficiently counting fields in a large hash

You have a very large Redis hash with millions of fields. You want to get the number of fields quickly. Which approach is best?

A Store the field count separately and update it on each modification

B Use HGETALL and count the fields in the client

C Use HSCAN to iterate and count fields manually

D Use HLEN command directly on the hash key

💡 Hint

HLEN is optimized for counting fields in a hash.

HLEN is a fast command that returns the number of fields without transferring all data. Other methods are slower or require extra work.

Practice

1. What does the Redis command HLEN do when used on a hash key?

easy

A. It deletes the hash key from the database.

B. It returns the number of fields in the hash.

C. It returns the total number of keys in the database.

D. It returns the value of the first field in the hash.

Solution

1. Step 1: Understand the purpose of HLEN

   The HLEN command is designed to count how many fields exist inside a given hash key in Redis.

2. Step 2: Compare with other Redis commands

   Unlike commands that count keys or delete data, HLEN specifically returns the count of fields inside a hash.

3. Final Answer:

   It returns the number of fields in the hash. -> Option B

4. Quick Check:

   HLEN = field count [OK]

Hint: HLEN counts fields inside a hash, not keys or values [OK]

Common Mistakes:

• Confusing HLEN with commands that count keys
• Thinking HLEN returns field values
• Assuming HLEN deletes data

2. Which of the following is the correct syntax to get the number of fields in a Redis hash named user:1000?

easy

A. HLEN['user:1000']

B. HLEN(user:1000)

C. HLEN user:1000

D. HLEN user:1000 fields

Solution

1. Step 1: Recall Redis command syntax

   Redis commands use the format COMMAND key without parentheses or brackets.

2. Step 2: Identify correct syntax for HLEN

   The correct syntax is simply HLEN key, so HLEN user:1000 is valid.

3. Final Answer:

   HLEN user:1000 -> Option C

4. Quick Check:

   HLEN key [OK]

Hint: Use HLEN followed by the key name without extra symbols [OK]

Common Mistakes:

• Adding parentheses or brackets
• Appending extra words after the key
• Using incorrect command format

3. Given the following Redis commands executed in order:

HSET product:1 name "Laptop" price "1200" stock "30"
HLEN product:1

What will be the output of the HLEN product:1 command?

medium

A. 3

B. 1

C. 0

D. Error

Solution

1. Step 1: Count fields added by HSET

   The HSET command adds three fields: name, price, and stock to the hash product:1.

2. Step 2: Use HLEN to count fields

   HLEN returns the number of fields in the hash, which is 3 in this case.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   HLEN counts fields = 3 [OK]

Hint: Count fields set by HSET, HLEN returns that count [OK]

Common Mistakes:

• Confusing field count with values
• Expecting HLEN to return 1 for one HSET command
• Assuming HLEN returns 0 if fields exist

4. You run the command HLEN session:123 but get 0. What is the most likely reason?

medium

A. Redis server is down.

B. HLEN command syntax is incorrect.

C. HLEN cannot be used on hashes.

D. The key session:123 does not exist in Redis.

Solution

1. Step 1: Understand HLEN behavior on missing keys

   If the hash key does not exist or is empty, HLEN returns 0.

2. Step 2: Check cause

   A result of 0 when expecting fields most likely means the key session:123 does not exist in Redis.

3. Final Answer:

   The key session:123 does not exist in Redis. -> Option D

4. Quick Check:

   HLEN missing key = 0 [OK]

Hint: Check if key exists before using HLEN [OK]

Common Mistakes:

• Assuming syntax error causes this
• Thinking HLEN can't be used on hashes
• Ignoring key existence

5. You have a Redis hash cart:567 with fields: item1, item2, and item3. You want to check if the cart is empty or not using HLEN. Which Redis command sequence correctly checks and prints "Empty" if no items, or "Has items" if there are fields?

hard

A. HLEN cart:567
IF result == 0 THEN print "Empty" ELSE print "Has items"

B. HLEN cart:567
IF result > 3 THEN print "Empty" ELSE print "Has items"

C. HLEN cart:567
IF result == -1 THEN print "Empty" ELSE print "Has items"

D. HLEN cart:567
IF result == 1 THEN print "Empty" ELSE print "Has items"

Solution

1. Step 1: Understand HLEN output

   HLEN returns the number of fields in the hash; 0 means empty, any positive number means fields exist.

2. Step 2: Use conditional check on HLEN result

   If HLEN result is 0, print "Empty"; otherwise, print "Has items".

3. Final Answer:

   HLEN cart:567 IF result == 0 THEN print "Empty" ELSE print "Has items" -> Option A

4. Quick Check:

   HLEN 0 means empty [OK]

Hint: Check if HLEN result equals zero to detect empty hash [OK]

Common Mistakes:

• Checking for wrong values like -1 or 1
• Using greater than 3 instead of zero
• Assuming HLEN returns boolean