Return values in Blockchain / Solidity - Mini Project: Build & Apply

Return Values in Blockchain Smart Contracts

📖 Scenario: You are creating a simple blockchain smart contract to store and retrieve a user's favorite number. This contract will help users save their number and get it back when needed.

🎯 Goal: Build a smart contract that stores a number and returns it when requested using a function with a return value.

📋 What You'll Learn

Create a state variable to store a number

Create a function to set the number

Create a function to get the stored number with a return value

Print the returned number in the final step

💡 Why This Matters

🌍 Real World

Smart contracts often store data and return values to users or other contracts. Understanding return values is key to building interactive blockchain applications.

💼 Career

Blockchain developers must write functions that return data correctly to enable decentralized apps to work smoothly and securely.

1

Create a state variable to store a number

Create a public state variable called favoriteNumber of type uint to store the user's favorite number.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumber {
    // Your code here
}

Hint

Use uint public favoriteNumber; inside the contract to create the variable.

2

Create a function to set the number

Add a public function called setNumber that takes a uint parameter named _number and sets favoriteNumber to this value.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumber {
    uint public favoriteNumber;

    // Your code here
}

Hint

Define function setNumber(uint _number) public and assign favoriteNumber = _number;.

3

Create a function to get the stored number with a return value

Add a public view function called getNumber that returns a uint and returns the value of favoriteNumber.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumber {
    uint public favoriteNumber;

    function setNumber(uint _number) public {
        favoriteNumber = _number;
    }

    // Your code here
}

Hint

Use function getNumber() public view returns (uint) and return favoriteNumber.

4

Print the returned number

Write a script to create the contract, call setNumber(42), then call getNumber() and print the returned value.

Blockchain / Solidity

// This step requires a script outside Solidity to interact with the contract
// Your code here

Hint

Use a script to call setNumber(42) then getNumber() and print the result.

Practice

1.

What does the return statement do in a blockchain smart contract function?

easy

A. It sends a value back to the caller of the function.

B. It stops the contract from deploying.

C. It creates a new transaction on the blockchain.

D. It changes the contract's address.

Solution

1. Step 1: Understand the purpose of return

   The return statement is used inside a function to send a value back to whoever called the function.

2. Step 2: Identify what return does in blockchain functions

   In smart contracts, returning a value allows the caller to receive data from the function execution.

3. Final Answer:

   It sends a value back to the caller of the function. -> Option A

4. Quick Check:

   Return sends value back [OK]

Hint: Return sends data back to caller, not change contract [OK]

Common Mistakes:

• Thinking return creates transactions
• Confusing return with contract deployment
• Believing return changes contract address

2.

Which of the following is the correct syntax to specify a function that returns an uint in Solidity?

function getValue() _____ { return 42; }

easy

A. returns uint

B. return uint

C. return (uint)

D. returns (uint)

Solution

1. Step 1: Recall Solidity function return syntax

   In Solidity, the correct way to specify a return type is returns (type) after the function parentheses.

2. Step 2: Match the syntax with options

   returns (uint) uses returns (uint), which is the correct syntax.

3. Final Answer:

   returns (uint) -> Option D

4. Quick Check:

   Solidity uses returns (type) [OK]

Hint: Remember: returns keyword with parentheses for type [OK]

Common Mistakes:

• Omitting parentheses around type
• Using return instead of returns
• Placing return type inside function body

3.

What will be the output of this Solidity function call?

function multiply(uint a, uint b) public pure returns (uint) { return a * b; }

Calling multiply(3, 4) returns:

medium

A. 7

B. 12

C. 34

D. Error

Solution

1. Step 1: Understand the function logic

   The function multiplies two unsigned integers a and b and returns the product.

2. Step 2: Calculate the product of 3 and 4

   3 multiplied by 4 equals 12, so the function returns 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs and return result [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Expecting syntax error

4.

Identify the error in this Solidity function that is supposed to return a bool:

function isEven(uint num) public pure returns (bool) { if (num % 2 == 0) { true; } else { false; } }

medium

A. Missing return statements before true and false.

B. The function should return uint instead of bool.

C. The modulus operator % is not allowed in Solidity.

D. The function should be marked as view instead of pure.

Solution

1. Step 1: Check the return statements

   The function uses true; and false; without return, so it does not actually return any value.

2. Step 2: Understand Solidity return requirements

   Every return value must be preceded by the return keyword to send the value back.

3. Final Answer:

   Missing return statements before true and false. -> Option A

4. Quick Check:

   Return keyword needed before values [OK]

Hint: Always use return before the value to send back [OK]

Common Mistakes:

• Omitting return keyword
• Confusing pure and view modifiers
• Thinking modulus operator is invalid

5.

You want to write a Solidity function that returns the sum of all elements in a uint[] array. Which of the following implementations correctly returns the sum?

function sumArray(uint[] memory arr) public pure returns (uint) { uint total = 0; for (uint i = 0; i < arr.length; i++) { total += arr[i]; } _____ }

hard

A. return arr.length;

B. return i;

C. return total;

D. return arr;

Solution

1. Step 1: Understand the function logic

   The function sums all elements in the array by adding each element to total.

2. Step 2: Identify the correct return value

   After the loop, the sum is stored in total, so returning total gives the correct sum.

3. Final Answer:

   return total; -> Option C

4. Quick Check:

   Return the sum variable total [OK]

Hint: Return the accumulator variable holding the sum [OK]

Common Mistakes:

• Returning array length instead of sum
• Returning loop index variable
• Returning the whole array instead of a value