Arrays (fixed and dynamic) in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a fixed-size array of 5 integers.

Blockchain / Solidity

int[[1]] numbers;

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

A0

B10

C5

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero or negative numbers for array size.

Forgetting to specify the size.

The fixed-size array must specify the size inside square brackets. Here, 5 means the array holds 5 integers.

2fill in blank

medium

Complete the code to declare a dynamic array of addresses.

Blockchain / Solidity

address[] public [1];

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

Ausers

Bbalances

Cowners

Daccounts

Attempts:

3 left

💡 Hint

Common Mistakes

Using fixed-size brackets instead of empty brackets.

Using singular names for arrays.

Dynamic arrays are declared with empty square brackets []. The name 'users' is a common choice for an array of addresses.

3fill in blank

hard

Fix the error in the code to add an element to a dynamic array.

Blockchain / Solidity

users.[1](msg.sender);

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

Apush

Bappend

Cadd

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add' or 'append' which are not valid methods in Solidity.

Trying to assign directly without using a method.

In Solidity, the correct method to add an element to a dynamic array is 'push'.

4fill in blank

hard

Fill both blanks to declare a fixed-size array and assign a value to its first element.

Blockchain / Solidity

uint256[[1]] scores;
scores[[2]] = 100;

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

A3

B0

C1

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 instead of 0 for the first element.

Using zero or negative number for array size.

The array size is 3, and the first element index is 0. So, scores[0] = 100 assigns 100 to the first element.

5fill in blank

hard

Fill all three blanks to create a dynamic array, add an element, and get its length.

Blockchain / Solidity

address[] public [1];

function addUser(address user) public {
    [1].[2](user);
}

function getUserCount() public view returns (uint) {
    return [1].[3];
}

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

Ausers

Bpush

Clength

Daccounts

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the array in different places.

Using 'append' or 'add' instead of 'push'.

Trying to call length as a function instead of a property.

The dynamic array is named 'users'. We add elements with 'push' and get the number of elements with 'length'.

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