HINCRBY for numeric fields in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command HINCRBY do?

HINCRBY increments the integer value of a field in a hash by a given number. If the field does not exist, it is set to 0 before the increment.

beginner

What happens if you use HINCRBY on a field that does not exist in the hash?

The field is created with the value 0, then incremented by the specified amount.

beginner

Can HINCRBY be used to decrement a numeric field in a Redis hash?

Yes, by passing a negative number as the increment, HINCRBY can decrement the field's value.

intermediate

What error occurs if you try to use HINCRBY on a field that contains a non-integer value?

Redis returns an error because HINCRBY only works with integer values.

beginner

Example: What is the result of HINCRBY user:1 visits 3 if visits was 5 before?

The new value of visits will be 8 (5 + 3).

What does HINCRBY myhash field 10 do if field does not exist?

A Does nothing

B Returns an error

C Creates <code>field</code> with value 10

D Creates <code>field</code> with value 0

If the field does not exist, Redis sets it to 0 then adds 10, so the final value is 10.

Which data type must the field value be for HINCRBY to work?

A Set

B String

C List

D Integer

HINCRBY works only on integer values stored in hash fields.

What happens if you run HINCRBY myhash field -2?

A Increments <code>field</code> by 2

B Decrements <code>field</code> by 2

C Deletes <code>field</code>

D Returns an error

Passing a negative number decrements the field's value by that amount.

If field contains the string "hello", what happens when you run HINCRBY myhash field 1?

A Redis returns an error

B Field becomes 1

C Field becomes "hello1"

D Field is deleted

Redis returns an error because the field value is not an integer.

What is the return value of HINCRBY after incrementing a field?

A The new value after increment

B The increment amount

C Always 0

D The old value before increment

HINCRBY returns the new value of the field after the increment.

Explain how the Redis command HINCRBY works with numeric fields in a hash.

Think about how you add or subtract numbers in a table of values.

What errors or limitations should you be aware of when using HINCRBY?

Consider what happens if you try to add numbers to words.

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