If-else statements in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple if-else in Solidity

What is the output of this Solidity function when value = 10?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    function checkValue(uint value) public pure returns (string memory) {
        if (value > 10) {
            return "Greater";
        } else if (value == 10) {
            return "Equal";
        } else {
            return "Less";
        }
    }
}

A "Less"

B "Greater"

C "Equal"

D Compilation error

💡 Hint

Check the condition that matches exactly 10.

The function returns "Equal" because the value is exactly 10, matching the else if condition.

❓ Predict Output

intermediate

If-else behavior with boolean in smart contract

What will the function return when flag = false?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract FlagCheck {
    function checkFlag(bool flag) public pure returns (string memory) {
        if (flag) {
            return "True";
        } else {
            return "False";
        }
    }
}

A "False"

B Compilation error

C Runtime error

D "True"

💡 Hint

Think about what happens when the flag is false.

Since flag is false, the else branch runs and returns "False".

❓ Predict Output

advanced

Nested if-else output in Solidity

What does this function return when score = 75?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract ScoreCheck {
    function grade(uint score) public pure returns (string memory) {
        if (score >= 90) {
            return "A";
        } else {
            if (score >= 80) {
                return "B";
            } else {
                if (score >= 70) {
                    return "C";
                } else {
                    return "F";
                }
            }
        }
    }
}

A "A"

B "F"

C "B"

D "C"

💡 Hint

Check each condition from top to bottom.

75 is less than 80 but greater or equal to 70, so it returns "C".

🧠 Conceptual

advanced

Error type from missing else in Solidity if-else

What error will this Solidity code produce when compiled?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    function check(uint x) public pure returns (string memory) {
        if (x > 5)
            return "High";
        else
            return "Low";
    }
}

A SyntaxError: 'else' without 'if'

B No error, returns "Low" for x <= 5

C TypeError: incompatible types

D Runtime error: unreachable code

💡 Hint

Look at the placement of else after a return statement.

The else is correctly placed after the if statement, so no syntax error occurs.

❓ Predict Output

expert

Output of complex if-else with multiple conditions

What is the output of this Solidity function when value = 15?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Complex {
    function test(uint value) public pure returns (string memory) {
        if (value < 10) {
            return "Less than 10";
        } else if (value < 20) {
            if (value % 2 == 0) {
                return "Even and less than 20";
            } else {
                return "Odd and less than 20";
            }
        } else {
            return "20 or more";
        }
    }
}

A "Odd and less than 20"

B "Even and less than 20"

C "20 or more"

D "Less than 10"

💡 Hint

Check if 15 is less than 20 and if it is even or odd.

15 is less than 20 and odd, so it returns "Odd and less than 20".

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