Arrays (fixed and dynamic) in Blockchain / Solidity

Introduction

Arrays help you store many values in one place. Fixed arrays have a set size, dynamic arrays can grow or shrink.

When you want to keep a list of items like user addresses or token balances.

When you know exactly how many items you need to store (fixed array).

When the number of items can change over time, like a list of transactions (dynamic array).

When you want to loop through all stored values easily.

When you want to organize data in a simple, ordered way.

Syntax

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ArrayExample {
    // Fixed size array of 3 unsigned integers
    uint[3] fixedArray;

    // Dynamic array of unsigned integers
    uint[] dynamicArray;

    // Function to add an element to dynamic array
    function addElement(uint element) public {
        dynamicArray.push(element);
    }

    // Function to get element from fixed array
    function getFixedElement(uint index) public view returns (uint) {
        return fixedArray[index];
    }

    // Function to set element in fixed array
    function setFixedElement(uint index, uint value) public {
        fixedArray[index] = value;
    }
}

Fixed arrays have a size set when declared and cannot change.

Dynamic arrays can change size using push() and pop() functions.

Examples

This fixed array holds exactly two numbers. You cannot add or remove elements.

Blockchain / Solidity

uint[2] fixedArray = [10, 20];
// fixedArray has exactly 2 elements: 10 and 20

This dynamic array starts empty and grows as we add elements.

Blockchain / Solidity

uint[] dynamicArray;
dynamicArray.push(5);
dynamicArray.push(15);
// dynamicArray now has 2 elements: 5 and 15

An empty dynamic array has no elements initially.

Blockchain / Solidity

uint[] emptyArray;
// emptyArray is empty and has length 0

A fixed array with one element can store exactly one value.

Blockchain / Solidity

uint[1] singleElementArray;
singleElementArray[0] = 42;
// fixed array with one element set to 42

Sample Program

This contract shows a fixed array with 3 numbers set in the constructor. It also has a dynamic array where you can add numbers anytime. You can get both arrays as comma-separated strings.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ArrayDemo {
    uint[3] fixedArray;
    uint[] dynamicArray;

    // Initialize fixed array with values
    constructor() {
        fixedArray[0] = 100;
        fixedArray[1] = 200;
        fixedArray[2] = 300;
    }

    // Add element to dynamic array
    function addToDynamicArray(uint value) public {
        dynamicArray.push(value);
    }

    // Get fixed array elements as a string
    function getFixedArray() public view returns (string memory) {
        return string(abi.encodePacked(
            uintToString(fixedArray[0]), ", ",
            uintToString(fixedArray[1]), ", ",
            uintToString(fixedArray[2])
        ));
    }

    // Get dynamic array elements as a string
    function getDynamicArray() public view returns (string memory) {
        if(dynamicArray.length == 0) {
            return "Empty";
        }
        string memory result = uintToString(dynamicArray[0]);
        for(uint i = 1; i < dynamicArray.length; i++) {
            result = string(abi.encodePacked(result, ", ", uintToString(dynamicArray[i])));
        }
        return result;
    }

    // Helper function to convert uint to string
    function uintToString(uint v) internal pure returns (string memory) {
        if (v == 0) {
            return "0";
        }
        uint j = v;
        uint len;
        while (j != 0) {
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint k = len;
        while (v != 0) {
            k = k-1;
            uint8 temp = uint8(48 + v % 10);
            bstr[k] = bytes1(temp);
            v /= 10;
        }
        return string(bstr);
    }
}

Important Notes

Accessing an index outside the fixed array size causes an error.

Dynamic arrays use more gas when growing, so use fixed arrays if size is known.

Common mistake: forgetting that fixed arrays cannot change size after declaration.

Summary

Fixed arrays have a set size and cannot grow or shrink.

Dynamic arrays can change size using push() and pop().

Use arrays to store lists of values in an ordered way.

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