Contract structure and syntax in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a contract named MyContract.

Blockchain / Solidity

contract [1] {
}

Drag options to blanks, or click blank then click option'

AMyContract

Bmycontract

CContract

DmyContract

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or different casing for the contract name.

The contract name must start with a capital letter and match the declaration. 'MyContract' is the correct name.

2fill in blank

medium

Complete the code to declare a public state variable of type uint named count.

Blockchain / Solidity

contract Counter {
    [1] public count;
}

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Aint

Buint

Cstring

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using signed integer int or wrong types like string or bool.

In Solidity, uint is an unsigned integer type commonly used for counters.

3fill in blank

hard

Fix the error in the function declaration to make it a public function named increment.

Blockchain / Solidity

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

Drag options to blanks, or click blank then click option'

AIncrement

Binc

Cincrement

DincrementCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase first letter or different function names.

Function names in Solidity are case-sensitive. The function should be named exactly 'increment' as specified.

4fill in blank

hard

Complete the code to declare a constructor that sets count to zero.

Blockchain / Solidity

contract Counter {
    uint public count;
    constructor() { {
        count {BLANK_2}} 0;
    }
}

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A{

B=

C;

D}

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong symbols like ';' instead of '{' or '='.

The constructor body starts with '{' and the assignment uses '=' to set count to zero.

5fill in blank

hard

Fill both blanks to complete a function that returns the current count.

Blockchain / Solidity

contract Counter {
    uint public count;
    function getCount() public view returns({BLANK_1}}) { {
        return {{BLANK_2}};
    }
}

Drag options to blanks, or click blank then click option'

Auint256

B{

Ccount

Duint

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong return types, missing braces, or returning wrong variables.

The function returns a uint type, the body starts with '{', and it returns the variable count.

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