Library contracts in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a Solidity library function call

What is the output of this Solidity code when calling TestLibrary.getDouble(5)?

Blockchain / Solidity

library TestLibrary {
    function getDouble(uint x) internal pure returns (uint) {
        return x * 2;
    }
}

contract Test {
    function callDouble(uint val) public pure returns (uint) {
        return TestLibrary.getDouble(val);
    }
}

A 0

B 5

C Compilation error

D 10

💡 Hint

Remember that the library function multiplies the input by 2.

The library function getDouble returns the input multiplied by 2. So input 5 returns 10.

🧠 Conceptual

intermediate

Why use library contracts in Solidity?

Which of the following is the main reason to use library contracts in Solidity?

A To manage private keys securely

B To store large amounts of data persistently

C To reuse code without deploying multiple copies

D To create user interfaces

💡 Hint

Think about how libraries help reduce contract size and gas costs.

Libraries allow code reuse by deploying once and linking, saving gas and avoiding code duplication.

🔧 Debug

advanced

Identify the error in this library usage

What error will this Solidity code produce when compiled?

Blockchain / Solidity

library MathLib {
    function add(uint a, uint b) public pure returns (uint) {
        return a + b;
    }
}

contract Calc {
    function sum(uint x, uint y) public pure returns (uint) {
        return MathLib.add(x, y);
    }
}

A TypeError: Library function cannot be public

B No error, compiles successfully

C Warning: Function should be external

D Runtime error: Division by zero

💡 Hint

Check the visibility of library functions in Solidity.

Library functions must be internal or external, not public. Public visibility causes a compilation error.

📝 Syntax

advanced

Correct syntax for using a library with 'using for'

Which option correctly applies the library ArrayLib to type uint[] using using for syntax?

Blockchain / Solidity

library ArrayLib {
    function sum(uint[] storage arr) internal view returns (uint) {
        uint s = 0;
        for (uint i = 0; i < arr.length; i++) {
            s += arr[i];
        }
        return s;
    }
}

contract Test {
    uint[] data;
    // Which line correctly applies the library?
}

A using ArrayLib on uint[];

B using ArrayLib for uint[];

C import ArrayLib for uint[];

D apply ArrayLib to uint[];

💡 Hint

The correct keyword is 'using' followed by the library name and 'for' the type.

The correct syntax is using ArrayLib for uint[]; to attach library functions to the type.

🚀 Application

expert

How many deployed contracts exist after deploying this code?

Given this Solidity code, how many contracts are deployed on the blockchain after deploying MainContract?

Blockchain / Solidity

library HelperLib {
    function multiply(uint a, uint b) internal pure returns (uint) {
        return a * b;
    }
}

contract MainContract {
    function calc(uint x, uint y) public pure returns (uint) {
        return HelperLib.multiply(x, y);
    }
}

A 1

B 2

C 0

D 3

💡 Hint

Consider if the library code is deployed separately or embedded.

Since the library function is internal, its code is embedded in the contract, so only one contract is deployed.