Return values in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the stored value from the function.

Blockchain / Solidity

function getValue() public view returns (uint) {
    return [1];
}

Drag options to blanks, or click blank then click option'

AgetValue

Buint

Cvalue

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the function name instead of the variable.

Returning the type instead of the variable.

Using the keyword 'return' twice.

The function returns the variable value which holds the stored number.

2fill in blank

medium

Complete the code to return the sum of two numbers.

Blockchain / Solidity

function add(uint a, uint b) public pure returns (uint) {
    return a [1] b;
}

Drag options to blanks, or click blank then click option'

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition.

Using multiplication or division operators by mistake.

The function returns the sum of a and b using the plus operator +.

3fill in blank

hard

Fix the error in the return statement to correctly return the balance.

Blockchain / Solidity

function getBalance() public view returns (uint) {
    uint balance = 100;
    return [1];
}

Drag options to blanks, or click blank then click option'

Abalance

Bbal

Cbalance()

DgetBalance

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a variable name that does not exist.

Adding parentheses to a variable name.

Returning the function name instead of the variable.

The variable balance holds the value to return. Using balance() or other names causes errors.

4fill in blank

hard

Fill both blanks to return the product of two numbers plus 1.

Blockchain / Solidity

function multiply(uint x, uint y) public pure returns (uint) {
    return x [1] y [2] 1;
}

Drag options to blanks, or click blank then click option'

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication first.

Using subtraction or division operators.

The function multiplies x and y using * and then adds 1 with +.

5fill in blank

hard

Fill both blanks to return a formatted string with the user's name and age.

Blockchain / Solidity

function getUserInfo(string memory name, uint age) public pure returns (string memory) {
    return string(abi.encodePacked([1], ": ", [2], " years old"));
}

Drag options to blanks, or click blank then click option'

Aname

Bage

Cstring(age)

DtoString(age)

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to concatenate a number without converting it to string.

Using the wrong function to convert number to string.

Missing the variable name for the first blank.

The function returns a string combining the name and the age converted to string using toString(age).

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