Why logic controls execution in Blockchain / Solidity - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this smart contract function?

Consider this simplified smart contract function that controls token transfer based on a condition.

function transfer(uint amount) public returns (string memory) {
    if (amount > 100) {
        return "Transfer denied: amount too large";
    } else {
        return "Transfer successful";
    }
}

What will be the output when transfer(150) is called?

Blockchain / Solidity

function transfer(uint amount) public returns (string memory) {
    if (amount > 100) {
        return "Transfer denied: amount too large";
    } else {
        return "Transfer successful";
    }
}

A Runtime error

B "Transfer successful"

C Compilation error

D "Transfer denied: amount too large"

💡 Hint

Think about what the if condition checks and what happens when the amount is 150.

The function checks if the amount is greater than 100. Since 150 > 100, it returns "Transfer denied: amount too large".

❓ Predict Output

intermediate

What does this Solidity code output?

Look at this Solidity function that uses logic to decide the fee.

function calculateFee(uint amount) public pure returns (uint) {
    if (amount <= 50) {
        return 1;
    } else if (amount <= 100) {
        return 2;
    } else {
        return 5;
    }
}

What is the output of calculateFee(75)?

Blockchain / Solidity

function calculateFee(uint amount) public pure returns (uint) {
    if (amount <= 50) {
        return 1;
    } else if (amount <= 100) {
        return 2;
    } else {
        return 5;
    }
}

A 2

B 5

C 1

D 0

💡 Hint

Check which condition 75 satisfies.

75 is greater than 50 but less than or equal to 100, so the function returns 2.

🔧 Debug

advanced

Why does this smart contract function always revert?

Here is a Solidity function meant to approve spending only if the amount is positive.

function approve(uint amount) public returns (bool) {
    require(amount < 0, "Amount must be positive");
    return true;
}

Why does this function always revert?

Blockchain / Solidity

function approve(uint amount) public returns (bool) {
    require(amount < 0, "Amount must be positive");
    return true;
}

A Because amount can never be less than 0 since it's unsigned, so require always fails

B Because the function should use if instead of require

C Because the function is missing a return statement

D Because require needs a semicolon after the message string

💡 Hint

Think about the type of amount and what the condition checks.

In Solidity, uint is unsigned and cannot be negative. The condition amount < 0 is always false, so require always reverts.

📝 Syntax

advanced

Which option causes a syntax error in this Solidity function?

Look at this function that uses logic to check ownership.

function checkOwner(address user) public view returns (bool) {
    if user == owner {
        return true;
    } else {
        return false;
    }
}

Which option shows the code with a syntax error?

Blockchain / Solidity

function checkOwner(address user) public view returns (bool) {
    if user == owner {
        return true;
    } else {
        return false;
    }
}

A if (user == owner) { return true; } else { return false; }

B return user == owner;

C if user == owner { return true; } else { return false; }

D if (user == owner) return true; else return false;

💡 Hint

Remember how conditions are written in Solidity.

In Solidity, the condition in an if statement must be inside parentheses. Option C misses parentheses, causing a syntax error.

🚀 Application

expert

How many times will the event be emitted in this Solidity loop?

Consider this Solidity function that emits an event based on a logic condition inside a loop.

event LogNumber(uint number);

function emitEvenNumbers() public {
    for (uint i = 0; i < 5; i++) {
        if (i % 2 == 0) {
            emit LogNumber(i);
        }
    }
}

How many times will LogNumber be emitted when emitEvenNumbers() is called?

Blockchain / Solidity

event LogNumber(uint number);

function emitEvenNumbers() public {
    for (uint i = 0; i < 5; i++) {
        if (i % 2 == 0) {
            emit LogNumber(i);
        }
    }
}

A 4

B 3

C 2

D 5

💡 Hint

Count how many numbers from 0 to 4 are even.

The loop runs with i = 0,1,2,3,4. Even numbers are 0, 2, and 4, so the event emits 3 times.