Contract structure and syntax in Blockchain / Solidity - Mini Project: Build & Apply

Basic Smart Contract Structure and Syntax

📖 Scenario: You want to create a simple smart contract on the blockchain that stores a number and allows anyone to read it.

🎯 Goal: Build a basic Solidity contract with a state variable and a function to read it.

📋 What You'll Learn

Create a contract named SimpleStorage

Declare a public unsigned integer variable called storedNumber initialized to 0

Write a function called getNumber that returns the value of storedNumber

Use correct Solidity syntax for contract, variable, and function declarations

💡 Why This Matters

🌍 Real World

Smart contracts are programs that run on blockchains to automate agreements without middlemen.

💼 Career

Understanding contract structure and syntax is essential for blockchain developers building decentralized applications.

1

Create the contract and state variable

Write a Solidity contract named SimpleStorage and declare a public unsigned integer variable called storedNumber initialized to 0.

Blockchain / Solidity

// Your code here

Hint

Start with contract SimpleStorage { } and inside it declare uint public storedNumber = 0;

2

Add a function to read the stored number

Inside the SimpleStorage contract, write a public view function called getNumber that returns a uint and returns the value of storedNumber.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleStorage {
    uint public storedNumber = 0;

    // Your code here
}

Hint

Define function getNumber() public view returns (uint) { return storedNumber; } inside the contract.

3

Add a function to update the stored number

Add a public function called setNumber that takes a uint parameter named newNumber and updates storedNumber with it.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleStorage {
    uint public storedNumber = 0;

    function getNumber() public view returns (uint) {
        return storedNumber;
    }

    // Your code here
}

Hint

Write function setNumber(uint newNumber) public { storedNumber = newNumber; } inside the contract.

4

Test the contract output

Write a comment that shows the expected output when calling getNumber() after deploying the contract and before calling setNumber. The output should be 0.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleStorage {
    uint public storedNumber = 0;

    function getNumber() public view returns (uint) {
        return storedNumber;
    }

    function setNumber(uint newNumber) public {
        storedNumber = newNumber;
    }
}

// Your code here

Hint

Write a comment like // Expected output of getNumber() after deployment: 0 below the contract.

Practice

1.

What is the correct way to start defining a contract named SimpleStorage in Solidity?

easy

A. contract SimpleStorage { }

B. class SimpleStorage { }

C. function SimpleStorage() { }

D. module SimpleStorage { }

Solution

1. Step 1: Understand Solidity contract declaration

   In Solidity, contracts are declared using the keyword contract followed by the contract name and curly braces.

2. Step 2: Match the syntax with the options

   Only contract SimpleStorage { } uses contract SimpleStorage { }, which is the correct syntax to start a contract.

3. Final Answer:

   contract SimpleStorage { } -> Option A

4. Quick Check:

   Contract declaration = contract SimpleStorage { } [OK]

Hint: Look for 'contract' keyword to start a contract [OK]

Common Mistakes:

• Using 'class' instead of 'contract'
• Using 'function' to define contract
• Using 'module' which is not valid in Solidity

2.

Which of the following is the correct syntax to declare a state variable count of type uint inside a contract?

easy

A. uint count = 0

B. var count = uint;

C. uint count;

D. count uint;

Solution

1. Step 1: Recall Solidity variable declaration syntax

   State variables are declared by specifying the type followed by the variable name and a semicolon.

2. Step 2: Check each option

   uint count; correctly declares uint count;. uint count = 0 misses the semicolon. Options B and D use incorrect syntax.

3. Final Answer:

   uint count; -> Option C

4. Quick Check:

   Variable declaration = type name; [OK]

Hint: Remember: type then variable name, end with semicolon [OK]

Common Mistakes:

• Omitting semicolon at the end
• Using 'var' keyword which is not valid in Solidity
• Reversing type and variable name

3.

What will be the output of the following Solidity function when called?

contract Test {
    uint number = 5;
    function getNumber() public view returns (uint) {
        return number + 3;
    }
}

medium

A. 5

B. 8

C. 3

D. Compilation error

Solution

1. Step 1: Understand the variable and function

   The state variable number is set to 5. The function returns number + 3.

2. Step 2: Calculate the return value

   5 + 3 equals 8, so the function returns 8.

3. Final Answer:

   8 -> Option B

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add variable value and constant inside return [OK]

Common Mistakes:

• Confusing variable value with function output
• Assuming function returns variable directly without addition
• Thinking there is a syntax error

4.

Identify the error in the following Solidity contract snippet:

contract Counter {
    uint count
    function increment() public {
        count += 1;
    }
}

medium

A. Contract name should be lowercase

B. Function increment should be private

C. Variable count should be declared inside the function

D. Missing semicolon after uint count

Solution

1. Step 1: Check variable declaration syntax

   The line uint count is missing a semicolon at the end, which is required in Solidity.

2. Step 2: Verify other parts

   Function visibility as public is valid. Variables are correctly declared outside functions. Contract names can be uppercase.

3. Final Answer:

   Missing semicolon after uint count -> Option D

4. Quick Check:

   Semicolon missing = syntax error [OK]

Hint: Check for missing semicolons after variable declarations [OK]

Common Mistakes:

• Forgetting semicolon after variable declaration
• Thinking function visibility must be private
• Assuming contract names must be lowercase

5.

You want to create a contract that stores a person's name and age, and has a function to update the age. Which of the following contract structures correctly implements this?

contract Person {
    string name;
    uint age;

    function setAge(uint newAge) public {
        age = newAge;
    }
}

hard

A. Correct: variables declared and function updates age properly

B. Incorrect: missing visibility for variables

C. Incorrect: function should return the new age

D. Incorrect: variables should be declared inside the function

Solution

1. Step 1: Check variable declarations

   Variables name and age are declared at contract level, which is correct for storing data.

2. Step 2: Check function to update age

   The function setAge takes a parameter and assigns it to age, correctly updating the state variable.

3. Final Answer:

   Correct: variables declared and function updates age properly -> Option A

4. Quick Check:

   State variables + update function = correct contract [OK]

Hint: State variables outside, update functions inside contract [OK]

Common Mistakes:

• Declaring variables inside functions instead of contract
• Expecting update functions to return values unnecessarily
• Forgetting to declare variables at contract level