Data types (uint, int, bool, address, string) in Blockchain / Solidity - Mini Project: Build & Apply

Understanding Basic Blockchain Data Types

📖 Scenario: You are creating a simple smart contract to store and display different types of data commonly used in blockchain development.

🎯 Goal: Build a smart contract that declares variables of types uint, int, bool, address, and string, assigns values to them, and then outputs these values.

📋 What You'll Learn

Declare a uint variable named myUint with value 100

Declare an int variable named myInt with value -50

Declare a bool variable named myBool with value true

Declare an address variable named myAddress with value 0x1234567890123456789012345678901234567890

Declare a string variable named myString with value "Hello Blockchain"

Create a function getData that returns all these variables

💡 Why This Matters

🌍 Real World

Smart contracts use these basic data types to store and manage blockchain data like balances, flags, addresses, and messages.

💼 Career

Understanding these data types is essential for blockchain developers to write secure and efficient smart contracts.

1

Declare basic data type variables

Declare a uint variable called myUint and set it to 100. Declare an int variable called myInt and set it to -50 inside a Solidity contract named DataTypesDemo.

Blockchain / Solidity

contract DataTypesDemo {
    // Declare myUint and myInt variables here
    
}

Hint

Use uint for positive numbers and int for numbers that can be negative.

2

Add bool, address, and string variables

Inside the DataTypesDemo contract, declare a bool variable called myBool set to true, an address variable called myAddress set to 0x1234567890123456789012345678901234567890, and a string variable called myString set to "Hello Blockchain".

Blockchain / Solidity

contract DataTypesDemo {
    uint myUint = 100;
    int myInt = -50;
    // Declare myBool, myAddress, and myString variables here
}

Hint

Remember to use double quotes for strings and the exact address format for address.

3

Create a function to return all variables

Add a public view function called getData inside DataTypesDemo that returns myUint, myInt, myBool, myAddress, and myString in that order.

Blockchain / Solidity

contract DataTypesDemo {
    uint myUint = 100;
    int myInt = -50;
    bool myBool = true;
    address myAddress = 0x1234567890123456789012345678901234567890;
    string myString = "Hello Blockchain";

    // Add getData function here
}

Hint

Use public view and specify the return types in the function signature.

4

Output the stored data

Write a simple script or test code that calls the getData function and prints the returned values in the order: myUint, myInt, myBool, myAddress, myString. For this step, just write a console.log statement inside a JavaScript test function that logs the returned tuple.

Blockchain / Solidity

contract DataTypesDemo {
    uint myUint = 100;
    int myInt = -50;
    bool myBool = true;
    address myAddress = 0x1234567890123456789012345678901234567890;
    string myString = "Hello Blockchain";

    function getData() public view returns (uint, int, bool, address, string memory) {
        return (myUint, myInt, myBool, myAddress, myString);
    }
}

// JavaScript test code
async function test() {
    const dataTypesDemo = await DataTypesDemo.deployed();
    const data = await dataTypesDemo.getData();
    // Print the data below
    
}

Hint

Use console.log and convert numbers to strings if needed before printing.

Practice

1. Which data type should you use in Solidity to store a number that can only be zero or positive?

easy

A. bool

B. uint

C. int

D. address

Solution

1. Step 1: Understand the difference between uint and int

   uint stands for unsigned integer, meaning it can only hold zero or positive numbers. int can hold both positive and negative numbers.

2. Step 2: Identify the correct type for only zero or positive numbers

   Since the question asks for numbers that are zero or positive only, uint is the correct choice.

3. Final Answer:

   uint -> Option B

4. Quick Check:

   Positive numbers = uint [OK]

Hint: Use uint for non-negative numbers only [OK]

Common Mistakes:

• Choosing int for positive-only numbers
• Confusing bool with numeric types
• Using address for numbers

2. Which of the following is the correct way to declare a boolean variable named isActive in Solidity?

easy

A. bool isActive = 0;

B. bool isActive = 1;

C. bool isActive = "true";

D. bool isActive = true;

Solution

1. Step 1: Recall the correct boolean values in Solidity

   Boolean variables can only be assigned true or false, not numbers or strings.

2. Step 2: Check each option for correct boolean assignment

   bool isActive = true; assigns true correctly. Options B and D use numbers, and C uses a string, which are invalid for bool.

3. Final Answer:

   bool isActive = true; -> Option D

4. Quick Check:

   Boolean = true or false [OK]

Hint: Assign true or false to bool variables [OK]

Common Mistakes:

• Using 1 or 0 instead of true/false
• Assigning strings like "true"
• Missing semicolon at the end

3. What will be the output of this Solidity code snippet?

int x = -5;
uint y = 10;
bool flag = (x < 0);
string memory text = "Hello";
address addr = 0x1234567890123456789012345678901234567890;

// Output values
x
flag
text
addr

medium

A. Compilation error due to address format

B. 5, false, Hello, 0x1234567890123456789012345678901234567890

C. -5, true, Hello, 0x1234567890123456789012345678901234567890

D. -5, false, "Hello", 0x0

Solution

1. Step 1: Analyze each variable's value

   x is -5 (signed int), flag is true because x < 0, text is "Hello", and addr is a valid address.

2. Step 2: Confirm output matches variable values

   -5, true, Hello, 0x1234567890123456789012345678901234567890 correctly lists all values as expected. 5, false, Hello, 0x1234567890123456789012345678901234567890 incorrectly changes x and flag. -5, false, "Hello", 0x0 has wrong flag and addr. Compilation error due to address format is wrong because the address format is valid.

3. Final Answer:

   -5, true, Hello, 0x1234567890123456789012345678901234567890 -> Option C

4. Quick Check:

   Variable values printed as is [OK]

Hint: Check variable types and values carefully [OK]

Common Mistakes:

• Assuming uint can hold negative values
• Confusing string quotes in output
• Thinking address format is invalid

4. Identify the error in this Solidity variable declaration:

address myAddress = "0xAbC1234567890";

medium

A. Address must be assigned without quotes

B. Address must be declared as string

C. Address cannot start with 0x

D. Address must be declared as bool

Solution

1. Step 1: Understand address assignment rules

   In Solidity, addresses are assigned as hexadecimal literals without quotes, not as strings.

2. Step 2: Identify the error in the code

   The code uses quotes around the address, making it a string, which is invalid for address type.

3. Final Answer:

   Address must be assigned without quotes -> Option A

4. Quick Check:

   Address literals have no quotes [OK]

Hint: Assign addresses without quotes to avoid errors [OK]

Common Mistakes:

• Using quotes around addresses
• Declaring address as string
• Removing 0x prefix from address

5. You want to store user information in a Solidity contract with these fields: user ID (positive number), username (text), active status (true/false), and wallet address. Which data types should you use for these fields?

hard

A. uint for user ID, string for username, bool for active status, address for wallet

B. int for user ID, string for username, uint for active status, address for wallet

C. uint for user ID, address for username, bool for active status, string for wallet

D. string for user ID, string for username, bool for active status, address for wallet

Solution

1. Step 1: Match each field to the correct data type

   User ID is a positive number, so uint fits. Username is text, so string. Active status is true/false, so bool. Wallet address is a blockchain address, so address.

2. Step 2: Verify options against these matches

   uint for user ID, string for username, bool for active status, address for wallet matches all fields correctly. Other options mix types incorrectly, like using int for user ID or swapping string and address.

3. Final Answer:

   uint for user ID, string for username, bool for active status, address for wallet -> Option A

4. Quick Check:

   Match field types carefully [OK]

Hint: Match data types to field meaning exactly [OK]

Common Mistakes:

• Using int for positive-only IDs
• Confusing string and address
• Using uint for boolean values