HINCRBY for numeric fields in Redis - Step-by-Step Execution

Concept Flow - HINCRBY for numeric fields

Start with hash key and field

↓

Check if field exists in hash?

No→Initialize field to 0

Yes↓

Get current numeric value

↓

Add increment to current value

↓

Update field with new value

↓

Return new value

↓

End

This flow shows how HINCRBY checks a field in a hash, initializes it if missing, increments its numeric value, updates the field, and returns the new value.

Execution Sample

Redis

HSET user:1 age 30
HINCRBY user:1 age 5
HINCRBY user:1 visits 1

This code sets 'age' to 30, increments 'age' by 5, and increments a new field 'visits' by 1 in the 'user:1' hash.

Execution Table

Step	Command	Field Exists?	Current Value	Increment	New Value	Action
1	HSET user:1 age 30	N/A	N/A	N/A	30	Set 'age' to 30
2	HINCRBY user:1 age 5	Yes	30	5	35	Increment 'age' by 5
3	HINCRBY user:1 visits 1	No	0 (init)	1	1	Initialize 'visits' to 0 and increment by 1

💡 All commands executed; final hash fields: age=35, visits=1

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
user:1.age	null	30	35	35
user:1.visits	null	null	null	1

Key Moments - 2 Insights

What happens if the field does not exist before HINCRBY?

If the field does not exist (see step 3 in execution_table), Redis treats its value as 0 before adding the increment, then sets the field to the new value.

Can HINCRBY increment non-numeric fields?

No, HINCRBY only works on numeric fields. If the field contains a non-numeric string, Redis returns an error (not shown here).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'age' after step 2?

A 30

B 5

C 35

D null

💡 Hint

Check the 'New Value' column in row for step 2 in execution_table.

At which step does the 'visits' field get created and set?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

Look at the 'Field Exists?' and 'Action' columns for 'visits' in execution_table.

If we increment 'age' by -10 at step 2 instead of 5, what would be the new value?

A 20

B 40

C 35

D 30

💡 Hint

Subtract 10 from the initial 30 value shown in variable_tracker after step 1.

Concept Snapshot

HINCRBY key field increment
- Increments numeric field in a hash by increment
- If field missing, treats as 0 before increment
- Returns new value after increment
- Only works with numeric fields

Full Transcript

HINCRBY is a Redis command to increment a numeric field inside a hash. If the field does not exist, Redis treats it as zero before adding the increment. The command updates the field with the new value and returns it. For example, setting 'age' to 30, then incrementing by 5 results in 35. Incrementing a missing field like 'visits' by 1 initializes it to 1. This command only works on numeric fields; non-numeric fields cause errors.

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