INCRBY and DECRBY in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of INCRBY on a key with an integer value?

Given a Redis key counter with value 10, what will be the value of counter after running INCRBY counter 5?

Redis

SET counter 10
INCRBY counter 5
GET counter

A 15

B 5

C 10

D Error: value is not an integer

💡 Hint

INCRBY adds the given number to the current integer value stored at the key.

The INCRBY command increases the integer value stored at the key by the specified amount. Since the initial value is 10, adding 5 results in 15.

❓ query_result

intermediate

What happens when DECRBY is used on a non-existing key?

What will be the value of key score after running DECRBY score 3 if score does not exist before?

Redis

DEL score
DECRBY score 3
GET score

A 3

B 0

C -3

D Error: key does not exist

💡 Hint

Redis treats non-existing keys as 0 for increment/decrement commands.

If the key does not exist, Redis treats its value as 0. Then DECRBY score 3 subtracts 3, resulting in -3.

📝 Syntax

advanced

Which command syntax is correct for incrementing a key by 10?

Choose the correct Redis command to increase the value of key visits by 10.

A INCRBY visits 10

B INCR visits 10

C INCRBY 10 visits

D INCRBY(visits, 10)

💡 Hint

The syntax is INCRBY key increment.

The correct syntax for incrementing a key by a specific number is INCRBY key increment. Options B, C, and D are invalid syntax.

❓ query_result

advanced

What error occurs when INCRBY is used on a key holding a string?

If key name holds the string value "Alice", what happens when running INCRBY name 1?

Redis

SET name "Alice"
INCRBY name 1

A Value becomes 0

B Value becomes "Alice1"

C Value becomes 1

D Error: value is not an integer or out of range

💡 Hint

INCRBY only works on keys holding integer values.

Redis throws an error if you try to increment a key holding a non-integer string value.

🧠 Conceptual

expert

How does Redis handle concurrent INCRBY and DECRBY commands on the same key?

Consider multiple clients running INCRBY counter 2 and DECRBY counter 1 concurrently on the same key counter. What guarantees does Redis provide about the final value?

A Redis queues commands but applies only increments or only decrements, ignoring the other.

B Redis guarantees atomicity; all increments and decrements are applied without race conditions, so the final value is consistent.

C Redis may lose some increments or decrements due to race conditions, so the final value is unpredictable.

D Redis locks the key permanently after the first command, blocking others indefinitely.

💡 Hint

Redis commands like INCRBY and DECRBY are atomic operations.

Redis processes commands sequentially and atomically, so concurrent INCRBY and DECRBY commands on the same key do not cause race conditions. The final value reflects all operations applied in some order.

Practice

1. What does the Redis command INCRBY key 5 do?

easy

A. Sets the value of key to 5.

B. Subtracts 5 from the current integer value stored at key.

C. Deletes the key from Redis.

D. Adds 5 to the current integer value stored at key.

Solution

1. Step 1: Understand the INCRBY command

   The INCRBY command increases the integer value stored at the specified key by the given amount.

2. Step 2: Apply the command to the example

   Using INCRBY key 5 adds 5 to the current value of key.

3. Final Answer:

   Adds 5 to the current integer value stored at key. -> Option D

4. Quick Check:

   INCRBY adds number = Adds 5 [OK]

Hint: INCRBY always adds the number to the key's value [OK]

Common Mistakes:

• Confusing INCRBY with DECRBY
• Thinking INCRBY sets the value instead of adding
• Assuming it deletes the key

2. Which of the following is the correct syntax to subtract 3 from a Redis key named counter?

easy

A. DECR counter 3

B. INCR counter -3

C. DECRBY counter 3

D. SUBTRACT counter 3

Solution

1. Step 1: Identify the correct command for subtraction

   The Redis command to subtract a number from a key is DECRBY key number.

2. Step 2: Check the syntax options

   DECRBY counter 3 correctly subtracts 3 from counter. INCR counter -3 is invalid (INCR takes only one argument). DECR counter 3 and SUBTRACT counter 3 are invalid.

3. Final Answer:

   DECRBY counter 3 -> Option C

4. Quick Check:

   DECRBY subtracts number = DECRBY counter 3 [OK]

Hint: Use DECRBY key number to subtract correctly [OK]

Common Mistakes:

• Using DECR with two arguments (only one allowed)
• Using INCR with an increment argument (invalid syntax)
• Using non-existent commands like SUBTRACT

3. Given the following Redis commands executed in order:
SET score 10
INCRBY score 7
DECRBY score 4
What is the final value of score?

medium

A. 10

B. 13

C. 3

D. 17

Solution

1. Step 1: Set initial value of score

   SET score 10 sets the value of score to 10.

2. Step 2: Apply INCRBY and DECRBY commands

   INCRBY score 7 adds 7, making it 17. Then DECRBY score 4 subtracts 4, resulting in 13.

3. Final Answer:

   13 -> Option B

4. Quick Check:

   10 + 7 - 4 = 13 [OK]

Hint: Add then subtract values stepwise to find final number [OK]

Common Mistakes:

• Ignoring the order of commands
• Subtracting before adding
• Confusing initial value with final

4. You run the command DECRBY visits 2 but get an error. Which is the most likely cause?

medium

A. The key visits is not an integer.

B. DECRBY requires three arguments, but only two were given.

C. DECRBY cannot subtract numbers, only add.

D. The Redis server is down.

Solution

1. Step 1: Understand DECRBY requirements

   DECRBY subtracts a number from an integer stored at the key. If the key does not exist, Redis treats it as 0. But if the key holds a non-integer value, an error occurs.

2. Step 2: Analyze the error cause

   The error likely means visits exists but is not an integer, causing DECRBY to fail.

3. Final Answer:

   The key visits is not an integer. -> Option A

4. Quick Check:

   Non-integer key causes DECRBY error [OK]

Hint: DECRBY needs integer key value; else error occurs [OK]

Common Mistakes:

• Thinking DECRBY needs 3 arguments
• Believing DECRBY only adds
• Assuming server is down without checking

5. You want to track page views in Redis using a key page_views. Initially, it does not exist. You run:
INCRBY page_views 10
DECRBY page_views 3
INCRBY page_views 5
What is the final value of page_views and why?

hard

A. 12, because Redis treats missing keys as 0 and updates accordingly.

B. 15, because DECRBY does not affect the value.

C. Error, because the key did not exist initially.

D. 10, because only the first INCRBY counts.

Solution

1. Step 1: Understand Redis behavior with missing keys

   If a key does not exist, Redis treats its value as 0 for INCRBY and DECRBY commands.

2. Step 2: Calculate the final value step-by-step

   Starting at 0, INCRBY page_views 10 sets it to 10. Then DECRBY page_views 3 subtracts 3, making 7. Finally, INCRBY page_views 5 adds 5, resulting in 12.

3. Final Answer:

   12, because Redis treats missing keys as 0 and updates accordingly. -> Option A

4. Quick Check:

   0 + 10 - 3 + 5 = 12 [OK]

Hint: Missing keys start at 0 for INCRBY/DECRBY [OK]

Common Mistakes:

• Expecting error on missing key
• Ignoring DECRBY effect
• Assuming only first command counts