HINCRBY for numeric fields in Redis

Introduction

HINCRBY helps you add or subtract numbers stored inside a small group of values called a hash. It makes changing numbers easy and fast.

You want to count how many times something happens, like page views for a website.

You need to update scores in a game for different players quickly.

You want to keep track of inventory amounts for products in a store.

You want to add or subtract points from a user's profile without rewriting all data.

Syntax

Redis

HINCRBY key field increment

key is the name of the hash where numbers are stored.

field is the specific number inside the hash you want to change.

increment is how much you add (positive) or subtract (negative).

Examples

Adds 1 to the 'visits' number inside the 'user:1000' hash.

Redis

HINCRBY user:1000 visits 1

Subtracts 3 from the 'stock' number inside the 'product:200' hash.

Redis

HINCRBY product:200 stock -3

Adds 10 points to 'player1' score inside the 'game:score' hash.

Redis

HINCRBY game:score player1 10

Sample Program

This example first sets the points to 10 for user:1, then adds 5 points, subtracts 3 points, and finally gets the current points.

Redis

HSET user:1 points 10
HINCRBY user:1 points 5
HINCRBY user:1 points -3
HGET user:1 points

Important Notes

If the field does not exist, HINCRBY creates it and sets it to the increment value.

Only works with integer numbers stored as strings inside the hash.

Using a non-numeric field will cause an error.

Summary

HINCRBY changes numbers inside a hash by adding or subtracting.

It is fast and useful for counters, scores, and inventory.

Remember to use it only on numeric fields.

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