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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a user has enough balance before sending tokens.

Blockchain / Solidity

if user_balance [1] amount_to_send:
    send_tokens()

Drag options to blanks, or click blank then click option'

A!=

B>=

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will allow sending tokens even if balance is less, causing errors.

Using '==' is too strict; user can have more than the amount.

The condition must check if the user balance is greater than or equal to the amount to send to allow the transaction.

2fill in blank

medium

Complete the code to execute a smart contract function only if the sender is the owner.

Blockchain / Solidity

if sender_address [1] contract_owner:
    execute_function()

Drag options to blanks, or click blank then click option'

A!=

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' would block the owner from executing the function.

Using '>' or '<' does not make sense for addresses.

The function should run only if the sender address matches the contract owner address.

3fill in blank

hard

Fix the error in the condition to prevent unauthorized access.

Blockchain / Solidity

if not (user_role [1] 'admin'):
    deny_access()

Drag options to blanks, or click blank then click option'

A==

B!=

Cin

Dnot in

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' inside with 'not' causes double negation and wrong logic.

Using 'in' or 'not in' is incorrect for string equality here.

The code denies access if the user role is not 'admin'. The condition uses 'not' outside, so inside it must check equality.

4fill in blank

hard

Fill both blanks to create a condition that executes only if the transaction amount is positive and sender is verified.

Blockchain / Solidity

if transaction_amount [1] 0 and sender_verified [2] True:
    process_transaction()

Drag options to blanks, or click blank then click option'

A>

B<

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' for amount allows zero or negative amounts.

Using '!=' for sender_verified allows unverified senders.

The transaction amount must be greater than zero and sender_verified must be exactly True to process the transaction.

5fill in blank

hard

Complete the code to create a dictionary comprehension that maps user IDs to their balances only if balance is positive and user is active.

Blockchain / Solidity

user_balances = {user_id: balance for user_id, balance, active in users if balance [1] 0 and active}

Drag options to blanks, or click blank then click option'

A.upper()

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Adding '.upper()' changes user_id to uppercase unnecessarily.

Using '==' instead of '>' allows zero balances.

The dictionary keys are user_id as is (no method call), values are balance as is, and the condition checks if balance is greater than zero.

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