Why logic controls execution in Blockchain / Solidity - See It in Action

Why Logic Controls Execution in Blockchain

📖 Scenario: Imagine you are creating a simple blockchain smart contract that controls how money is sent between users. The contract must check if the sender has enough money before allowing the transfer. This is like checking if you have enough cash in your wallet before buying something.

🎯 Goal: You will build a simple smart contract logic that controls the execution of a money transfer based on the sender's balance. This teaches why logic is important to control what happens in blockchain programs.

📋 What You'll Learn

Create a dictionary called balances with exact user balances

Create a variable called transfer_amount with the exact value 50

Write an if statement that checks if the sender has enough balance

Print "Transfer successful" if the transfer can happen, else print "Insufficient balance"

💡 Why This Matters

🌍 Real World

Smart contracts on blockchains use logic to control money transfers and other actions securely.

💼 Career

Understanding how logic controls execution is key for blockchain developers writing safe and correct smart contracts.

1

DATA SETUP: Create user balances

Create a dictionary called balances with these exact entries: 'Alice': 100, 'Bob': 30, 'Charlie': 75

Blockchain / Solidity

# Create the balances dictionary with exact values
# Your code here

Hint

Think of balances as a wallet for each user showing how much money they have.

2

CONFIGURATION: Set the transfer amount

Create a variable called transfer_amount and set it to 50

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 30, 'Charlie': 75}
# Set the transfer amount to 50
# Your code here

Hint

This is the amount Alice wants to send.

3

CORE LOGIC: Check if transfer can happen

Write an if statement that checks if balances['Alice'] is greater than or equal to transfer_amount

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 30, 'Charlie': 75}
transfer_amount = 50
# Write if statement to check if Alice has enough balance
# Your code here

Hint

This logic controls if the transfer can happen or not.

4

OUTPUT: Show the transfer result

Run the program to print the transfer result message

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 30, 'Charlie': 75}
transfer_amount = 50
if balances['Alice'] >= transfer_amount:
    print("Transfer successful")
else:
    print("Insufficient balance")
# Run the program to see the output
# Your code here

Hint

You should see the message that tells if the transfer can happen.

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