Return values in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity function call?

Consider the following Solidity function that returns a value. What will be the output when getValue() is called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    function getValue() public pure returns (uint) {
        uint a = 5;
        uint b = 10;
        return a + b;
    }
}

A 10

B 5

C 15

D Compilation error

💡 Hint

Look at the return statement carefully. It adds two numbers.

The function adds a and b which are 5 and 10 respectively, so it returns 15.

❓ Predict Output

intermediate

What does this smart contract function return?

Given the following Solidity function, what is the output when getGreeting() is called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Greeter {
    function getGreeting() public pure returns (string memory) {
        return "Hello, Blockchain!";
    }
}

A "Hello, Blockchain!"

B "Hello, World!"

C Compilation error due to missing memory keyword

D Empty string

💡 Hint

Check the string returned exactly as written.

The function returns the string literal "Hello, Blockchain!" exactly as coded.

❓ Predict Output

advanced

What is the output of this Solidity function with multiple returns?

What will be the output of calling getData() in the following contract?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Data {
    function getData() public pure returns (uint, bool) {
        uint x = 42;
        bool y = true;
        return (x, y);
    }
}

A Compilation error due to missing tuple return

B (42, true)

C 42

D (true, 42)

💡 Hint

Look at the order of the returned values in the function signature.

The function returns a tuple with uint first and bool second, so the output is (42, true).

❓ Predict Output

advanced

What error does this Solidity function produce?

What error will occur when calling brokenFunction() in this contract?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Broken {
    function brokenFunction() public pure returns (uint) {
        uint a = 10;
        // Missing return statement
    }
}

A No error, returns 10

B Runtime error: Division by zero

C Returns 0 by default

D Compilation error: Missing return statement

💡 Hint

Functions with a return type must have a return statement.

The function declares it returns a uint but does not have a return statement, causing a compilation error.

🧠 Conceptual

expert

Which option correctly describes the behavior of return values in Solidity functions?

Choose the statement that best describes how return values work in Solidity functions.

A Functions must always explicitly return a value if a return type is declared; otherwise, a compilation error occurs.

B Functions can omit return statements even if a return type is declared; the default zero value is returned automatically.

C Functions can return multiple values only if they are declared as external functions.

D Functions without a return type can still return values without errors.

💡 Hint

Think about Solidity's strict typing and return requirements.

In Solidity, if a function declares a return type, it must explicitly return a value of that type; otherwise, the compiler throws an error.

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