Why logic controls execution in Blockchain / Solidity

Introduction

Logic controls execution to decide what actions happen and when. It helps programs make choices and follow steps in the right order.

When you want a smart contract to perform different actions based on user input.

When you need to check if a transaction meets certain rules before processing it.

When you want to control the flow of a blockchain program to avoid errors.

When you want to automate decisions like releasing funds only if conditions are met.

Syntax

Blockchain / Solidity

if condition {
    // do something
} else {
    // do something else
}

Logic uses conditions like 'if' to choose between actions.

This helps the program run different code depending on what happens.

Examples

This checks if there is enough balance before sending money.

Blockchain / Solidity

if balance >= amount {
    send(amount)
} else {
    reject()
}

This lets only the owner change settings.

Blockchain / Solidity

if isOwner {
    changeSettings()
} else {
    denyAccess()
}

Sample Program

This smart contract lets a user withdraw money only if they have enough balance. The logic controls whether the withdrawal happens or not.

Blockchain / Solidity

contract SimpleWallet {
    uint balance = 100;
    
    function withdraw(uint amount) public returns (string memory) {
        if (balance >= amount) {
            balance -= amount;
            return "Withdrawal successful";
        } else {
            return "Insufficient balance";
        }
    }
}

Important Notes

Logic helps avoid mistakes by checking conditions before actions.

Without logic, programs would do everything the same way, which can cause errors.

Summary

Logic controls what code runs and when.

It helps programs make decisions based on conditions.

In blockchain, logic ensures safe and correct execution of contracts.

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