If-else statements in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the transaction amount is greater than 1000.

Blockchain / Solidity

if (transaction.amount [1] 1000) {
    approveTransaction();
} else {
    rejectTransaction();
}

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

A>

B!=

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will check the wrong condition.

Using '==' checks equality, not greater than.

The code checks if the transaction amount is greater than 1000 using the > operator.

2fill in blank

medium

Complete the code to set the status to 'approved' if the balance is sufficient.

Blockchain / Solidity

if (user.balance [1] transaction.amount) {
    status = 'approved';
} else {
    status = 'denied';
}

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

A>=

B==

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will approve when balance is less, which is wrong.

Using '==' only approves if balance is exactly equal.

The code checks if the user's balance is greater than or equal to the transaction amount to approve it.

3fill in blank

hard

Fix the error in the if-else condition to check if the sender is the owner.

Blockchain / Solidity

if (sender [1] owner) {
    execute();
} else {
    revert();
}

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

A=

B==

C===

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' causes assignment, not comparison.

Using '===' is invalid syntax in blockchain smart contract languages.

In blockchain smart contracts, '==' is used to check equality between sender and owner.

4fill in blank

hard

Fill both blanks to approve if the amount is less than 500 and the sender is verified.

Blockchain / Solidity

if (transaction.amount [1] 500 && sender.verified [2] true) {
    approve();
} else {
    deny();
}

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

A<

B==

C!=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' reverses the amount check.

Using '!=' for verification checks the wrong condition.

The code approves if the amount is less than 500 and the sender is verified (true).

5fill in blank

hard

Fill all three blanks to reject if the balance is less than amount or sender is not verified.

Blockchain / Solidity

if (user.balance [1] transaction.amount || sender.verified [2] [3]) {
    reject();
} else {
    accept();
}

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

A<

B==

Cfalse

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' reverses the verification logic.

Using 'true' instead of 'false' checks the wrong condition.

The code rejects if balance is less than amount or sender is not verified (verified == false).

Practice

1.

What does an if-else statement do in a blockchain smart contract?

easy

A. It chooses between two actions based on a condition.

B. It repeats a set of instructions multiple times.

C. It stores data permanently on the blockchain.

D. It creates a new blockchain network.

Solution

1. Step 1: Understand the role of if-else

   An if-else statement checks a condition and decides which code to run next.

2. Step 2: Apply to blockchain context

   In smart contracts, it helps decide actions like approving transactions or rejecting them.

3. Final Answer:

   It chooses between two actions based on a condition. -> Option A

4. Quick Check:

   If-else = choose action [OK]

Hint: If-else picks one path out of two based on condition [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else stores data
• Believing if-else creates blockchains

2.

Which of the following is the correct syntax for an if-else statement in Solidity?

uint balance = 100;

easy

A. if (balance > 50) then { /* code */ } else { /* code */ }

B. if balance > 50 then { /* code */ } else { /* code */ }

C. if balance > 50: { /* code */ } else: { /* code */ }

D. if (balance > 50) { /* code */ } else { /* code */ }

Solution

1. Step 1: Recall Solidity if-else syntax

   Solidity uses parentheses around conditions and braces for code blocks without 'then'.

2. Step 2: Compare options

   Only if (balance > 50) { /* code */ } else { /* code */ } matches Solidity syntax: if (condition) { } else { }.

3. Final Answer:

   if (balance > 50) { /* code */ } else { /* code */ } -> Option D

4. Quick Check:

   Solidity if-else = parentheses + braces [OK]

Hint: Solidity if-else uses parentheses and braces, no 'then' [OK]

Common Mistakes:

• Using 'then' keyword
• Missing parentheses around condition
• Using colons instead of braces

3.

What will be the output of this Solidity function when amount = 150?

function checkAmount(uint amount) public pure returns (string memory) {
  if (amount > 100) {
    return "Large amount";
  } else {
    return "Small amount";
  }
}

medium

A. Compilation error

B. "Small amount"

C. "Large amount"

D. No output

Solution

1. Step 1: Check the condition with amount = 150

   Since 150 > 100, the if condition is true.

2. Step 2: Determine which return runs

   The function returns "Large amount" because the if branch runs.

3. Final Answer:

   "Large amount" -> Option C

4. Quick Check:

   150 > 100 -> "Large amount" [OK]

Hint: Check condition value to pick correct return [OK]

Common Mistakes:

• Confusing greater than with less than
• Assuming else runs when if is true
• Expecting syntax errors

4.

Find the error in this Solidity snippet:

function checkBalance(uint balance) public pure returns (string memory) {
  if balance > 0 {
    return "Positive";
  } else {
    return "Zero or Negative";
  }
}

medium

A. Missing parentheses around the if condition

B. Missing semicolon after return statements

C. Incorrect return type

D. Missing else keyword

Solution

1. Step 1: Check if statement syntax

   In Solidity, the condition must be inside parentheses: if (condition).

2. Step 2: Identify the error

   The code has if balance > 0 without parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the if condition -> Option A

4. Quick Check:

   if condition needs parentheses [OK]

Hint: Always put condition inside parentheses in if [OK]

Common Mistakes:

• Omitting parentheses in if condition
• Thinking semicolons are missing after return
• Confusing return type errors

5.

You want to write a Solidity function that returns "Access granted" only if userRole is exactly "admin", otherwise returns "Access denied". Which code snippet correctly uses if-else to do this?

hard

A. if (userRole = "admin") { return "Access granted"; } else { return "Access denied"; }

B. if (userRole == "admin") { return "Access granted"; } else { return "Access denied"; }

C. if userRole == "admin" { return "Access granted"; } else { return "Access denied"; }

D. if (userRole.equals("admin")) { return "Access granted"; } else { return "Access denied"; }

Solution

1. Step 1: Understand string comparison in Solidity

   Solidity uses == to compare strings for equality, not = or methods like equals().

2. Step 2: Check syntax correctness

   if (userRole == "admin") { return "Access granted"; } else { return "Access denied"; } uses == with parentheses and braces correctly.

3. Final Answer:

   if (userRole == "admin") { return "Access granted"; } else { return "Access denied"; } -> Option B

4. Quick Check:

   Use == for string comparison in if [OK]

Hint: Use == to compare strings in Solidity if conditions [OK]

Common Mistakes:

• Using single = instead of ==
• Omitting parentheses in if
• Using Java-style equals() method