Data types (uint, int, bool, address, string) in Blockchain / Solidity

Introduction

Data types tell the computer what kind of information we want to store. They help keep data organized and safe.

When you want to store a number that can only be positive, like a token balance.

When you need to store a number that can be positive or negative, like a temperature change.

When you want to store true or false answers, like if a user is verified.

When you need to store a blockchain address to send or receive tokens.

When you want to store text, like a name or message.

Syntax

Blockchain / Solidity

uint number;  // unsigned integer (only positive)
int number;   // signed integer (positive or negative)
bool flag;    // true or false
address addr; // blockchain address
string text;  // text string

uint means the number cannot be negative.

address stores a blockchain wallet or contract location.

Examples

This stores a positive number 30 in age.

Blockchain / Solidity

uint age = 30;

This stores a number that can be negative, here -5 degrees.

Blockchain / Solidity

int temperature = -5;

This stores a true/false value, here true means active.

Blockchain / Solidity

bool isActive = true;

This stores a blockchain address in owner.

Blockchain / Solidity

address owner = 0x1234567890abcdef1234567890abcdef12345678;

This stores text, here the name "Alice".

Blockchain / Solidity

string name = "Alice";

Sample Program

This smart contract shows different data types. It stores a positive number, a negative number, a true/false value, an address, and some text. The owner is set to the person who creates the contract.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract DataTypesExample {
    uint public tokenBalance = 1000;
    int public temperature = -10;
    bool public isVerified = false;
    address public owner;
    string public greeting = "Hello, Blockchain!";

    constructor() {
        owner = msg.sender;
    }
}

Important Notes

Use uint when you know the number will never be negative.

address is special for blockchain wallets and contracts.

string stores text but can use more gas (cost) on blockchain.

Summary

uint stores positive numbers only.

int stores numbers that can be positive or negative.

bool stores true or false values.

address stores blockchain wallet or contract addresses.

string stores text messages.

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