INCR and DECR for counters in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the value after incrementing a counter?

Given a Redis key counter with initial value 5, what will be the value after running INCR counter once?

Redis

SET counter 5
INCR counter
GET counter

A 5

B 6

C 7

D Error: key is not an integer

💡 Hint

INCR adds 1 to the current integer value stored at the key.

The INCR command increases the integer value stored at the key by 1. Starting from 5, it becomes 6.

❓ query_result

intermediate

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

If the Redis key visits does not exist, what will be the result of DECR visits?

Redis

DEL visits
DECR visits
GET visits

A -1

B 0

C Error: key does not exist

D nil

💡 Hint

INCR and DECR treat missing keys as zero before operation.

When DECR is called on a non-existing key, Redis treats it as 0 and subtracts 1, resulting in -1.

📝 Syntax

advanced

Which command sequence correctly increments a counter only if it exists?

You want to increment the Redis key score only if it already exists. Which sequence achieves this without creating the key if missing?

A

EXISTS score
INCR score

B INCRBY score 1

C INCR score

D

WATCH score
INCR score
EXEC

💡 Hint

Check if the key exists before incrementing.

Option A checks if the key exists before incrementing, preventing creation if missing. INCR alone creates the key with value 1 if missing.

❓ query_result

advanced

What is the final value after multiple increments and decrements?

Starting with counter set to 10, what is the value after these commands? INCR counter DECR counter DECR counter INCR counter

Redis

SET counter 10
INCR counter
DECR counter
DECR counter
INCR counter
GET counter

A 11

B 9

C 10

D 8

💡 Hint

Count how many increments and decrements happen.

Starting at 10: +1 → 11, -1 → 10, -1 → 9, +1 → 10 final value.

🧠 Conceptual

expert

Why might using INCR on a key storing a non-integer value cause an error?

Consider a Redis key user:count storing the string value hello. What happens if you run INCR user:count and why?

Redis

SET user:count hello
INCR user:count

A Redis converts 'hello' to 0 and increments to 1

B The value becomes 'hello1' by appending 1

C The value resets to 1 ignoring the old value

D Redis returns an error because the value is not an integer

💡 Hint

INCR expects the key to hold an integer value or be missing.

INCR fails with an error if the key holds a non-integer string because it cannot convert it to a number.