Arrays (fixed and dynamic) in Blockchain / Solidity - Mini Project: Build & Apply

Working with Fixed and Dynamic Arrays in Solidity

📖 Scenario: You are building a simple smart contract to manage a list of favorite numbers. Some numbers are fixed and cannot be changed, while others can be added dynamically by users.

🎯 Goal: Create a Solidity contract that uses both fixed-size and dynamic arrays to store numbers. You will initialize a fixed array, set a limit for adding numbers dynamically, add numbers to the dynamic array, and finally display all stored numbers.

📋 What You'll Learn

Create a fixed-size array called fixedNumbers with exactly 3 numbers: 10, 20, and 30

Create a dynamic array called dynamicNumbers to store unsigned integers

Create a variable called maxDynamicNumbers set to 5 to limit additions to the dynamic array

Write a function addNumber(uint number) that adds a number to dynamicNumbers only if it has less than maxDynamicNumbers elements

Write a function getAllNumbers() that returns both arrays combined as a single array

💡 Why This Matters

🌍 Real World

Smart contracts often need to store fixed and changing lists of data, like token balances or user inputs.

💼 Career

Understanding arrays in Solidity is essential for blockchain developers building decentralized applications.

1

Create a fixed-size array with 3 numbers

Create a fixed-size array called fixedNumbers of type uint[3] and initialize it with the numbers 10, 20, and 30 inside the contract.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumbers {
    // Create fixed-size array fixedNumbers with 10, 20, 30
    // Your code here
}

Hint

Use uint[3] fixedNumbers = [10, 20, 30]; inside the contract to create the fixed array.

2

Add a dynamic array and a max limit variable

Add a dynamic array called dynamicNumbers of type uint[] and a variable called maxDynamicNumbers set to 5 inside the contract.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumbers {
    uint[3] fixedNumbers = [10, 20, 30];
    // Add dynamic array dynamicNumbers and maxDynamicNumbers = 5
    // Your code here
}

Hint

Use uint[] dynamicNumbers; for the dynamic array and uint maxDynamicNumbers = 5; for the limit.

3

Write a function to add numbers to the dynamic array with a limit

Write a public function called addNumber that takes a uint number as input. Inside the function, add the number to dynamicNumbers only if the length of dynamicNumbers is less than maxDynamicNumbers.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumbers {
    uint[3] fixedNumbers = [10, 20, 30];
    uint[] dynamicNumbers;
    uint maxDynamicNumbers = 5;

    // Write addNumber function to add number if dynamicNumbers length < maxDynamicNumbers
    // Your code here
}

Hint

Use dynamicNumbers.length to check the size and dynamicNumbers.push(number); to add.

4

Create a function to return all numbers combined

Write a public view function called getAllNumbers that returns a uint[] memory array containing all numbers from fixedNumbers followed by all numbers from dynamicNumbers. Combine both arrays inside the function.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumbers {
    uint[3] fixedNumbers = [10, 20, 30];
    uint[] dynamicNumbers;
    uint maxDynamicNumbers = 5;

    function addNumber(uint number) public {
        if(dynamicNumbers.length < maxDynamicNumbers) {
            dynamicNumbers.push(number);
        }
    }

    // Write getAllNumbers function to return combined fixedNumbers and dynamicNumbers
    // Your code here
}

Hint

Create a new array with size equal to the sum of both arrays, copy elements from each, then return it.

Practice

1.

What is the main difference between a fixed array and a dynamic array in blockchain smart contracts?

easy

A. Fixed arrays have a set size, dynamic arrays can change size during execution.

B. Fixed arrays can only store numbers, dynamic arrays can store any data type.

C. Fixed arrays are faster to access, dynamic arrays cannot be accessed directly.

D. Fixed arrays are stored off-chain, dynamic arrays are stored on-chain.

Solution

1. Step 1: Understand fixed array size

   Fixed arrays have a size defined when declared and cannot change during contract execution.

2. Step 2: Understand dynamic array size

   Dynamic arrays can grow or shrink by adding or removing elements during contract execution.

3. Final Answer:

   Fixed arrays have a set size, dynamic arrays can change size during execution. -> Option A

4. Quick Check:

   Fixed vs dynamic size = A [OK]

Hint: Remember fixed size means no change, dynamic means flexible size [OK]

Common Mistakes:

• Thinking fixed arrays can change size
• Confusing data types with array size
• Assuming dynamic arrays are slower to access

2.

Which of the following is the correct syntax to declare a dynamic array of uint in Solidity?

?

easy

A. uint numbers[];

B. uint[] numbers;

C. uint numbers();

D. uint[] numbers = new uint[];

Solution

1. Step 1: Recall Solidity array syntax

   Dynamic arrays are declared with the type followed by empty square brackets: uint[].

2. Step 2: Check each option

   uint[] numbers; uses correct syntax: uint[] numbers;. uint numbers[]; places brackets after variable name, which is invalid. uint numbers(); uses parentheses, which is wrong. uint[] numbers = new uint[]; tries to initialize without size, which is invalid syntax.

3. Final Answer:

   uint[] numbers; -> Option B

4. Quick Check:

   Dynamic array syntax = A [OK]

Hint: Type[] name; is the correct dynamic array syntax [OK]

Common Mistakes:

• Placing brackets after variable name
• Using parentheses instead of brackets
• Trying to initialize without size incorrectly

3.

What will be the output of the following Solidity code snippet?

pragma solidity ^0.8.0;
contract Test {
    uint[] public numbers;
    function addNumbers() public {
        numbers.push(10);
        numbers.push(20);
        numbers.push(30);
    }
    function getSecond() public view returns (uint) {
        return numbers[1];
    }
}

Assuming addNumbers() is called once before getSecond(), what does getSecond() return?

medium

A. 10

B. Index out of range error

C. 30

D. 20

Solution

1. Step 1: Understand array after push calls

   After calling addNumbers(), the array numbers contains [10, 20, 30].

2. Step 2: Access the element at index 1

   Arrays in Solidity are zero-indexed, so numbers[1] is the second element, which is 20.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   numbers[1] = 20 [OK]

Hint: Remember arrays start at index 0 in Solidity [OK]

Common Mistakes:

• Confusing index 1 with first element
• Expecting an error for valid index
• Mixing up push order

4.

Identify the error in this Solidity code snippet that tries to remove the last element from a dynamic array:

pragma solidity ^0.8.0;
contract RemoveLast {
    uint[] public data = [1, 2, 3];
    function removeLast() public {
        data.pop(1);
    }
}

medium

A. pop() removes the first element, not the last.

B. Array cannot be initialized with values like [1, 2, 3].

C. pop() does not take any arguments; calling pop(1) causes an error.

D. pop() is not available for dynamic arrays.

Solution

1. Step 1: Check pop() method usage

   In Solidity, pop() removes the last element and takes no arguments.

2. Step 2: Identify incorrect call

   The code calls data.pop(1); which is invalid syntax and causes a compilation error.

3. Final Answer:

   pop() does not take any arguments; calling pop(1) causes an error. -> Option C

4. Quick Check:

   pop() no args = C [OK]

Hint: pop() never takes parameters in Solidity [OK]

Common Mistakes:

• Passing arguments to pop()
• Thinking pop() removes first element
• Believing pop() is unavailable for dynamic arrays

5.

Given a fixed array uint[3] fixedArr = [5, 10, 15]; and a dynamic array uint[] dynamicArr;, which code snippet correctly copies all elements from fixedArr to dynamicArr in Solidity?

hard

A. for (uint i = 0; i < fixedArr.length; i++) { dynamicArr.push(fixedArr[i]); }

B. dynamicArr = fixedArr;

C. dynamicArr = new uint[](fixedArr.length); dynamicArr = fixedArr;

D. dynamicArr.push(fixedArr);

Solution

1. Step 1: Understand array assignment rules

   In Solidity, fixed-size arrays cannot be directly assigned to dynamic arrays.

2. Step 2: Copy elements using a loop

   Using a for loop to push each element from fixedArr into dynamicArr is the correct way.

3. Final Answer:

   for (uint i = 0; i < fixedArr.length; i++) { dynamicArr.push(fixedArr[i]); } -> Option A

4. Quick Check:

   Copy fixed to dynamic with loop = D [OK]

Hint: Use a loop to push fixed array elements into dynamic array [OK]

Common Mistakes:

• Trying to assign fixed array directly to dynamic array
• Using push with whole array instead of elements
• Reinitializing dynamic array incorrectly