Return values in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a return value in a blockchain smart contract function?

A return value is the data that a smart contract function sends back after it finishes running. It tells you the result of the function's work.

beginner

Why are return values important in blockchain programming?

Return values let other parts of the program or users know what happened inside the contract, like if a transaction succeeded or what data was found.

intermediate

Can a blockchain smart contract function return multiple values?

Yes, some blockchain languages like Solidity allow functions to return multiple values using tuples or structs.

beginner

What happens if a smart contract function does not have a return statement?

If a function does not return anything, it means it only changes the contract's state or performs actions without sending back data.

intermediate

How do return values affect gas costs in blockchain transactions?

Returning data can increase gas costs because the blockchain stores or sends the returned data, so simpler return values can save gas.

What does a return value from a smart contract function represent?

A The result or output of the function

B The amount of cryptocurrency sent

C The transaction fee paid

D The contract's address

A return value is the output data that a function sends back after execution.

Can a smart contract function return more than one value?

A Yes, using tuples or structs

B Only if the contract is deployed on Ethereum

C No, only one value can be returned

D Only if the function is marked payable

Functions can return multiple values using tuples or structs in languages like Solidity.

What happens if a function has no return statement?

A It returns the contract's balance

B It returns nothing and only performs actions

C It returns zero by default

D It causes an error

Functions without return statements perform actions but do not send back data.

How do return values affect gas costs?

A Gas costs depend only on contract size

B Returning data always reduces gas costs

C Gas costs are not affected by return values

D Returning data can increase gas costs

Returning data can increase gas costs because the blockchain processes and stores the returned data.

Which of these is a reason to use return values in smart contracts?

A To send cryptocurrency to users

B To change the contract's address

C To provide information about the function's result

D To increase transaction fees

Return values provide information about what the function did or found.

Explain what a return value is in a blockchain smart contract and why it matters.

Think about how a function tells you what it did.

Describe how returning multiple values from a smart contract function works and when you might use it.

Consider a function that needs to send back more than one piece of information.

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