Arrays (fixed and dynamic) in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of fixed-size array initialization

What is the output of this Solidity code snippet when the function getArray() is called?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    uint[3] fixedArray = [1, 2, 3];
    function getArray() public view returns (uint[3] memory) {
        return fixedArray;
    }
}

A Compilation error

B [0, 0, 0]

C [1, 2, 3]

D [1, 2]

💡 Hint

Fixed-size arrays in Solidity hold the exact number of elements declared.

The fixed-size array fixedArray is initialized with three elements: 1, 2, and 3. The function returns this array as is.

❓ Predict Output

intermediate

Dynamic array length after push operations

What is the length of the dynamic array dynamicArray after executing the addElements() function?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    uint[] public dynamicArray;
    function addElements() public {
        dynamicArray.push(10);
        dynamicArray.push(20);
        dynamicArray.push(30);
    }
    function getLength() public view returns (uint) {
        return dynamicArray.length;
    }
}

A Compilation error

B 0

C 2

D 3

💡 Hint

Each push adds one element to the dynamic array.

The addElements() function pushes three elements, so the length becomes 3.

🔧 Debug

advanced

Identify the error in fixed-size array assignment

What error will this Solidity code produce when compiled?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    uint[2] fixedArray;
    function setArray() public {
        fixedArray = [1, 2, 3];
    }
}

A TypeError: Type uint256[3] memory is not implicitly convertible to expected type uint256[2] storage pointer

B Runtime error: Array index out of bounds

C No error, compiles and runs fine

D SyntaxError: Missing semicolon

💡 Hint

Fixed-size arrays must match the exact length when assigned.

The code tries to assign an array of length 3 to a fixed-size array of length 2, which causes a type mismatch error.

❓ Predict Output

advanced

Output after deleting an element from dynamic array

What is the content of dynamicArray after calling removeFirst()?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    uint[] public dynamicArray;
    constructor() {
        dynamicArray.push(5);
        dynamicArray.push(10);
        dynamicArray.push(15);
    }
    function removeFirst() public {
        delete dynamicArray[0];
    }
    function getArray() public view returns (uint[] memory) {
        return dynamicArray;
    }
}

A [5, 10, 15]

B [0, 10, 15]

C [10, 15]

D Runtime error: Cannot delete element

💡 Hint

Deleting an element sets it to default value but does not change array length.

The delete keyword sets the first element to zero but keeps the array length unchanged.

🧠 Conceptual

expert

Gas cost behavior of fixed vs dynamic arrays

Which statement correctly describes the gas cost difference between fixed-size and dynamic arrays in Solidity when storing data on-chain?

A Fixed-size arrays always cost less gas to store because their size is known at compile time.

B Both fixed-size and dynamic arrays cost the same gas regardless of size.

C Dynamic arrays always cost less gas because they can resize and use less storage.

D Dynamic arrays cost more gas when resizing, but fixed-size arrays cost more gas when accessing elements.

💡 Hint

Knowing size at compile time helps optimize storage layout.

Fixed-size arrays have a known size, allowing Solidity to optimize storage and reduce gas costs compared to dynamic arrays which require extra gas for resizing and length management.