HINCRBY for numeric fields in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this HINCRBY command?

Given a Redis hash key user:1000 with field score set to 10, what will be the value of score after running HINCRBY user:1000 score 5?

Redis

HSET user:1000 score 10
HINCRBY user:1000 score 5
HGET user:1000 score

A 15

B 5

C 10

D Error: field is not an integer

💡 Hint

HINCRBY adds the increment to the current integer value stored in the field.

HINCRBY increments the numeric value stored in the specified field by the given amount. Starting from 10, adding 5 results in 15.

❓ query_result

intermediate

What happens if the field does not exist?

If the Redis hash user:1001 does not have the field points, what will be the result of HINCRBY user:1001 points 7?

Redis

HINCRBY user:1001 points 7
HGET user:1001 points

A 0

B 7

C Error: field does not exist

D null

💡 Hint

HINCRBY treats missing fields as zero before incrementing.

If the field does not exist, Redis treats its value as 0 and increments by the given amount, so the new value is 7.

📝 Syntax

advanced

Which HINCRBY command is syntactically correct?

Choose the correct syntax to increment the field level by 3 in the hash game:player1.

A HINCRBY level game:player1 3

B HINCRBY game:player1 level

C HINCRBY game:player1 3 level

D HINCRBY game:player1 level 3

💡 Hint

The syntax is HINCRBY key field increment.

The correct syntax is HINCRBY followed by the key, then the field, then the increment number.

❓ query_result

advanced

What error occurs if the field contains a non-integer value?

If the field score in hash user:1002 contains the string ten, what happens when running HINCRBY user:1002 score 1?

Redis

HSET user:1002 score ten
HINCRBY user:1002 score 1

A Error: hash value is not an integer

B score becomes 1

C score becomes 'ten1'

D score becomes 11

💡 Hint

HINCRBY requires the field value to be an integer string.

If the field value is not an integer string, HINCRBY returns an error.

🧠 Conceptual

expert

Why is HINCRBY preferred over GET and SET for incrementing numeric fields?

Consider incrementing a numeric field in a Redis hash. Why is using HINCRBY better than using HGET to get the value, incrementing it in the client, and then HSET to update it?

A GET and SET commands do not work on hash fields.

B HINCRBY is slower but more readable than GET and SET.

C HINCRBY is atomic, preventing race conditions in concurrent environments.

D HINCRBY automatically converts strings to integers.

💡 Hint

Think about what happens if multiple clients increment at the same time.

HINCRBY performs the increment atomically inside Redis, avoiding race conditions that can happen if you GET, increment, and SET separately.

Practice

1. What does the Redis command HINCRBY do when used on a hash field?

easy

A. It increments or decrements the numeric value stored in the field by a given amount.

B. It deletes the field from the hash.

C. It sets the field to a new string value.

D. It returns the current value of the field without changing it.

Solution

1. Step 1: Understand the purpose of HINCRBY

   HINCRBY is designed to change numeric values inside a hash by adding or subtracting a specified number.

2. Step 2: Identify the effect on the hash field

   It modifies the existing numeric value by the given increment or decrement amount.

3. Final Answer:

   It increments or decrements the numeric value stored in the field by a given amount. -> Option A

4. Quick Check:

   HINCRBY changes numbers inside a hash = B [OK]

Hint: HINCRBY always changes numbers by adding or subtracting [OK]

Common Mistakes:

• Thinking HINCRBY deletes or resets fields
• Using HINCRBY on non-numeric fields without error
• Confusing HINCRBY with commands that just read values

2. Which of the following is the correct syntax to increment the field score by 5 in a Redis hash named player1?

easy

A. HINCRBY 5 player1 score

B. HINCRBY score player1 5

C. HINCRBY player1 5 score

D. HINCRBY player1 score 5

Solution

1. Step 1: Recall the correct order of HINCRBY arguments

   The syntax is: HINCRBY key field increment.

2. Step 2: Match the syntax to the given options

   HINCRBY player1 score 5 follows the correct order: key = player1, field = score, increment = 5.

3. Final Answer:

   HINCRBY player1 score 5 -> Option D

4. Quick Check:

   HINCRBY key field increment = A [OK]

Hint: Remember: HINCRBY key field increment [OK]

Common Mistakes:

• Swapping field and key positions
• Placing increment before field or key
• Using wrong argument order causing errors

3. Given the following Redis commands executed in order:
HSET player1 score 10
HINCRBY player1 score 7
HINCRBY player1 score -3
What is the final value of score in player1?

medium

A. 10

B. 17

C. 14

D. 7

Solution

1. Step 1: Set initial value of score

   HSET sets score to 10.

2. Step 2: Apply first increment

   HINCRBY adds 7, so score becomes 10 + 7 = 17.

3. Step 3: Apply second increment (decrement)

   HINCRBY subtracts 3, so score becomes 17 - 3 = 14.

4. Final Answer:

   14 -> Option C

5. Quick Check:

   10 + 7 - 3 = 14 [OK]

Hint: Add increments step-by-step to find final value [OK]

Common Mistakes:

• Ignoring negative increments
• Adding increments incorrectly
• Assuming initial value is zero

4. You run the command HINCRBY player1 level 1 but get an error. The field level currently holds the string value beginner. What is the cause of the error?

medium

A. The increment value must be a string, not a number.

B. HINCRBY cannot increment a field with a non-numeric value.

C. The key player1 does not exist.

D. HINCRBY requires the field to be empty before incrementing.

Solution

1. Step 1: Check the field value type

   The field level contains a non-numeric string "beginner".

2. Step 2: Understand HINCRBY requirements

   HINCRBY only works on fields with numeric values; it cannot increment strings.

3. Final Answer:

   HINCRBY cannot increment a field with a non-numeric value. -> Option B

4. Quick Check:

   Non-numeric field causes HINCRBY error = C [OK]

Hint: HINCRBY only works on numeric fields, not strings [OK]

Common Mistakes:

• Assuming HINCRBY creates or converts strings automatically
• Thinking key absence causes this error
• Believing increment must be string type

5. You have a Redis hash inventory with fields apples = 10 and oranges = 5. You want to add 3 apples and subtract 2 oranges using HINCRBY commands. Which sequence of commands correctly updates the inventory?

hard

A. HINCRBY inventory apples 3HINCRBY inventory oranges -2

B. HINCRBY inventory apples -3HINCRBY inventory oranges 2

C. HINCRBY inventory apples 3HINCRBY inventory oranges 2

D. HINCRBY inventory apples -3HINCRBY inventory oranges -2

Solution

1. Step 1: Identify the required changes

   We want to add 3 to apples and subtract 2 from oranges.

2. Step 2: Match increments with commands

   Adding 3 means increment by 3; subtracting 2 means increment by -2.

3. Step 3: Verify command sequence

   HINCRBY inventory apples 3
   HINCRBY inventory oranges -2 correctly uses HINCRBY inventory apples 3 and HINCRBY inventory oranges -2.

4. Final Answer:

   HINCRBY inventory apples 3
   HINCRBY inventory oranges -2 -> Option A

5. Quick Check:

   Add positive, subtract negative with HINCRBY = A [OK]

Hint: Use positive to add, negative to subtract with HINCRBY [OK]

Common Mistakes:

• Using negative to add or positive to subtract incorrectly
• Mixing up fields and increments
• Assuming HINCRBY can subtract by positive number