Enums in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an enum in blockchain programming?

An enum is a special type that lets you define a set of named values. It helps you group related options together, like days of the week or transaction states.

beginner

Why use enums instead of plain numbers or strings?

Enums make code easier to read and less error-prone because you use meaningful names instead of numbers or strings. This helps prevent mistakes and makes your code clearer.

beginner

How do you define an enum for transaction status with values: Pending, Completed, Failed?

You write something like:

enum TransactionStatus { Pending, Completed, Failed }

This groups the three states under one type.

intermediate

Can enums have underlying numeric values in blockchain languages?

Yes, enums often have underlying numbers starting from zero by default. You can also assign specific numbers if needed, which helps when storing or comparing values.

intermediate

How do enums improve smart contract security?

Enums limit possible values to a fixed set, reducing bugs from invalid inputs. This helps smart contracts behave predictably and safely.

What does an enum represent in blockchain programming?

A A function to send transactions

B A variable that changes randomly

C A fixed set of named values

D A type of cryptocurrency

Enums define a fixed set of named values to represent related options clearly.

Which of these is a correct enum definition for colors?

A enum Colors { Red, Green, Blue }

B enum Colors = (Red, Green, Blue)

C enum Colors: Red, Green, Blue

D enum Colors -> Red, Green, Blue

The correct syntax uses curly braces and commas to list enum values.

What is the default underlying value of the first enum member?

A 0

B 1

C Undefined

D Depends on the blockchain

By default, the first enum member has the numeric value 0.

How do enums help prevent bugs in smart contracts?

A By speeding up transactions

B By allowing any value to be used

C By encrypting data automatically

D By limiting values to a fixed set

Enums restrict values to a fixed set, reducing invalid inputs and bugs.

Can you assign specific numbers to enum members?

A No, enums only use default numbering

B Yes, you can assign specific numeric values

C Only strings can be assigned

D Only in off-chain code

You can assign specific numbers to enum members if you want custom values.

Explain what an enum is and why it is useful in blockchain programming.

Think about how enums help organize options like transaction states.

Describe how enums can improve the security and reliability of smart contracts.

Consider how limiting choices helps avoid mistakes.

Practice

1. What is the main purpose of using enum in blockchain programming?

easy

A. To execute smart contracts faster

B. To create random numbers for cryptography

C. To store large amounts of data efficiently

D. To group related named values together for clarity and safety

Solution

1. Step 1: Understand enum purpose

   Enums group related named values, making code easier to read and safer.

2. Step 2: Compare options

   Only To group related named values together for clarity and safety correctly describes enums' main purpose in blockchain code.

3. Final Answer:

   To group related named values together for clarity and safety -> Option D

4. Quick Check:

   Enums group named values = A [OK]

Hint: Enums group fixed options with names, not data or speed [OK]

Common Mistakes:

• Confusing enums with random number generators
• Thinking enums store large data
• Believing enums speed up contract execution

2. Which of the following is the correct syntax to declare an enum named Status with values Pending, Active, and Closed in Solidity?

easy

A. enum Status: Pending, Active, Closed

B. enum Status = (Pending, Active, Closed);

C. enum Status { Pending, Active, Closed }

D. enum Status { 'Pending', 'Active', 'Closed' }

Solution

1. Step 1: Recall Solidity enum syntax

   In Solidity, enums are declared with enum Name { Value1, Value2, ... } without equals or colons.

2. Step 2: Check each option

   enum Status { Pending, Active, Closed } matches correct syntax; others use invalid symbols or quotes.

3. Final Answer:

   enum Status { Pending, Active, Closed } -> Option C

4. Quick Check:

   Solidity enum syntax uses braces and commas = C [OK]

Hint: Enums use braces and commas, no equals or quotes [OK]

Common Mistakes:

• Using equals sign instead of braces
• Adding quotes around enum values
• Using colons instead of braces

3. Given the Solidity enum declaration:

enum Role { User, Admin, Guest }

and the code:

Role userRole = Role.Admin;
if (userRole == Role.Guest) {
    return "Guest Access";
} else {
    return "Not Guest";
}

What will be the output?

medium

A. "Not Guest"

B. "Guest Access"

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand enum values and assignment

   Role.Admin is assigned to userRole, which is not equal to Role.Guest.

2. Step 2: Evaluate the if condition

   The condition userRole == Role.Guest is false, so the else branch runs returning "Not Guest".

3. Final Answer:

   "Not Guest" -> Option A

4. Quick Check:

   Role.Admin != Role.Guest, so output = "Not Guest" [OK]

Hint: Compare enum values directly; Admin ≠ Guest means else runs [OK]

Common Mistakes:

• Assuming Admin equals Guest
• Expecting compilation or runtime errors
• Confusing enum names with strings

4. Identify the error in this Solidity enum usage:

enum State { Created, Locked, Inactive }
State public state;
function setState(uint _state) public {
    state = _state;
}

medium

A. Enum declaration syntax is wrong

B. Cannot assign uint directly to enum variable

C. Function setState should be private

D. Missing return type in function

Solution

1. Step 1: Analyze enum assignment

   Enums require assignment from enum values, not raw uint integers.

2. Step 2: Check function assignment

   Assigning _state (uint) directly to state (enum) causes a type mismatch error.

3. Final Answer:

   Cannot assign uint directly to enum variable -> Option B

4. Quick Check:

   Enum variables need enum values, not uints [OK]

Hint: Assign enum values, not integers, to enum variables [OK]

Common Mistakes:

• Assigning integers directly to enums
• Thinking enum syntax is wrong
• Confusing function visibility with errors

5. You want to represent user roles in a blockchain contract using enums. You need to check if a user has either Admin or Moderator role to allow special access. Given:

enum Role { User, Admin, Moderator, Guest }

Which of the following Solidity code snippets correctly checks for this condition?

hard

A. if (userRole == Role.Admin || userRole == Role.Moderator) { /* allow access */ }

B. if (userRole == Role.Admin && userRole == Role.Moderator) { /* allow access */ }

C. if (userRole in [Role.Admin, Role.Moderator]) { /* allow access */ }

D. if (userRole.equals(Role.Admin) || userRole.equals(Role.Moderator)) { /* allow access */ }

Solution

1. Step 1: Understand logical conditions for enums

   To allow access if userRole is Admin OR Moderator, use logical OR (||) between equality checks.

2. Step 2: Evaluate each option

   if (userRole == Role.Admin || userRole == Role.Moderator) { /* allow access */ } uses correct OR condition; B uses AND which is impossible; C uses invalid syntax; A uses non-existent method equals.

3. Final Answer:

   if (userRole == Role.Admin || userRole == Role.Moderator) { /* allow access */ } -> Option A

4. Quick Check:

   Use || for OR between enum comparisons [OK]

Hint: Use || to check multiple enum values, not && or invalid syntax [OK]

Common Mistakes:

• Using && instead of || for OR condition
• Trying to use 'in' keyword for enums
• Calling non-existent methods on enums