Reference types behavior in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity code snippet?

Consider the following Solidity contract snippet. What will be the value of arr[0] after calling modifyArray()?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint[] public arr = [1, 2, 3];

    function modifyArray() public {
        uint[] memory temp = arr;
        temp[0] = 99;
    }
}

A 1

B Compilation error

C 0

D 99

💡 Hint

Think about whether temp is a reference or a copy.

In Solidity, memory creates a copy of the array. Modifying temp does not affect the original arr. So, arr[0] remains 1.

❓ Predict Output

intermediate

What happens when modifying a storage reference in Solidity?

Given this Solidity code, what will be the value of arr[0] after modifyArray() is called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint[] public arr = [1, 2, 3];

    function modifyArray() public {
        uint[] storage temp = arr;
        temp[0] = 99;
    }
}

A Compilation error

B 0

C 1

D 99

💡 Hint

Consider what storage means for temp.

temp is a storage reference to arr. Modifying temp changes arr directly. So, arr[0] becomes 99.

🔧 Debug

advanced

Why does this Solidity function revert when modifying a struct?

Examine the code below. Why does calling updatePerson() cause a revert?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract People {
    struct Person {
        string name;
        uint age;
    }

    Person public person = Person("Alice", 30);

    function updatePerson() public {
        Person memory p = person;
        p.age = 31;
    }
}

A Because <code>person</code> is not updated, so revert occurs

B Because the function tries to modify a copy, not the original, no revert occurs

C Because <code>memory</code> structs cannot be modified

D Because <code>string</code> in <code>memory</code> cannot be assigned

💡 Hint

Think about whether modifying p affects person.

The function modifies a memory copy of person. This does not affect the stored person. No revert happens; the change is simply lost.

📝 Syntax

advanced

Which option correctly declares a storage reference to a mapping in Solidity?

Given the mapping mapping(address => uint) balances;, which declaration correctly creates a storage reference to it inside a function?

A mapping(address => uint) memory ref = balances;

B mapping(address => uint) ref = balances;

C mapping(address => uint) storage ref = balances;

D mapping(address => uint) calldata ref = balances;

💡 Hint

Remember that mappings can only be stored in storage or calldata, not memory.

Mappings cannot be stored in memory. Only storage or calldata are allowed. To create a reference, storage is required.

🚀 Application

expert

How many items are in the array after this Solidity function runs?

Consider this Solidity contract. After calling addItem(), how many elements does items contain?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Inventory {
    string[] public items;

    function addItem() public {
        string[] storage ref = items;
        ref.push("apple");
        string[] memory copy = ref;
        // copy.push("banana"); // This line causes a compilation error
    }
}

A Compilation error

B 2

C 0

D 1

💡 Hint

Think about which array is modified and which is a copy.

ref is a storage reference to items, so pushing "apple" adds to items. copy is a memory copy; pushing "banana" to it does not affect items. However, pushing to a memory array is not allowed and causes a compilation error.