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.

Practice

1. Why is logic important in blockchain programming?

easy

A. It creates new cryptocurrencies automatically

B. It stores data permanently on the blockchain

C. It controls which parts of the code run based on conditions

D. It speeds up the blockchain network

Solution

1. Step 1: Understand the role of logic in code

   Logic decides which code runs by checking conditions, like a traffic light controlling cars.

2. Step 2: Apply this to blockchain context

   In blockchain, logic ensures only allowed actions happen, protecting assets and data.

3. Final Answer:

   It controls which parts of the code run based on conditions -> Option C

4. Quick Check:

   Logic controls execution = D [OK]

Hint: Logic decides code flow by conditions, like a gatekeeper [OK]

Common Mistakes:

• Confusing logic with data storage
• Thinking logic speeds network
• Assuming logic creates cryptocurrencies

2. Which of the following is the correct syntax for an if statement in Solidity (blockchain language)?

easy

A. if (balance > 0) { sendFunds(); }

B. if balance > 0 then sendFunds()

C. if balance > 0 { sendFunds(); }

D. if (balance > 0) sendFunds;

Solution

1. Step 1: Recall Solidity if statement syntax

   Solidity uses C-style syntax: condition in parentheses, code block in braces.

2. Step 2: Check each option

   if (balance > 0) { sendFunds(); } matches correct syntax with parentheses and braces; others miss parentheses, braces, or semicolon.

3. Final Answer:

   if (balance > 0) { sendFunds(); } -> Option A

4. Quick Check:

   Solidity if syntax = C [OK]

Hint: If statements need parentheses and braces in Solidity [OK]

Common Mistakes:

• Omitting parentheses around condition
• Missing braces for code block
• Leaving out semicolons

3. What will be the output of this Solidity function when amount = 50?

function checkAmount(uint amount) public pure returns (string memory) {
  if (amount > 100) {
    return "High";
  } else if (amount > 20) {
    return "Medium";
  } else {
    return "Low";
  }
}

medium

A. "High"

B. Compilation error

C. "Low"

D. "Medium"

Solution

1. Step 1: Evaluate the if conditions with amount = 50

   50 > 100 is false, so skip first if; 50 > 20 is true, so return "Medium".

2. Step 2: Confirm no errors in code

   Code syntax is correct, so no compilation error.

3. Final Answer:

   "Medium" -> Option D

4. Quick Check:

   50 > 20 = true, output = "Medium" [OK]

Hint: Check conditions top to bottom; first true runs [OK]

Common Mistakes:

• Choosing "High" because 50 is large
• Ignoring else if condition order
• Thinking code has syntax errors

4. Identify the error in this Solidity code snippet:

function transfer(uint amount) public {
  if amount > balance {
    revert("Insufficient balance");
  }
  balance -= amount;
}

medium

A. Missing parentheses around the if condition

B. Using revert instead of throw

C. balance should be a local variable

D. Missing semicolon after revert statement

Solution

1. Step 1: Check if statement syntax

   In Solidity, conditions must be inside parentheses: if (condition) {...}

2. Step 2: Verify other parts

   Using revert is correct for errors; semicolon is present; balance can be state variable.

3. Final Answer:

   Missing parentheses around the if condition -> Option A

4. Quick Check:

   If condition needs parentheses = A [OK]

Hint: If conditions always need parentheses in Solidity [OK]

Common Mistakes:

• Thinking revert is wrong
• Assuming balance must be local
• Missing that parentheses are required

5. You want to write a Solidity function that only allows transfers if the sender is the owner and the amount is less than the balance. Which logic correctly controls execution?

hard

A. if (msg.sender == owner || amount < balance) { transfer(amount); } else { revert("Not allowed"); }

B. if (msg.sender == owner && amount < balance) { transfer(amount); } else { revert("Not allowed"); }

C. if (msg.sender != owner && amount > balance) { transfer(amount); } else { revert("Not allowed"); }

D. if (msg.sender != owner || amount > balance) { transfer(amount); } else { revert("Not allowed"); }

Solution

1. Step 1: Understand the required conditions

   Both conditions must be true: sender is owner AND amount less than balance.

2. Step 2: Check each option's logic

   if (msg.sender == owner && amount < balance) { transfer(amount); } else { revert("Not allowed"); } uses && (AND) correctly; B uses || (OR) which allows wrong cases; C and D use negations incorrectly.

3. Final Answer:

   if (msg.sender == owner && amount < balance) { transfer(amount); } else { revert("Not allowed"); } -> Option B

4. Quick Check:

   Use AND for both conditions true = A [OK]

Hint: Use AND (&&) to require all conditions true [OK]

Common Mistakes:

• Using OR (||) instead of AND (&&)
• Negating conditions incorrectly
• Allowing transfers when conditions fail