Require, assert, and revert in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity function call?

Consider the following Solidity function:

function testRequire(uint x) public pure returns (string memory) {
    require(x > 10, "x must be greater than 10");
    return "Passed require";
}

What happens when testRequire(5) is called?

Blockchain / Solidity

function testRequire(uint x) public pure returns (string memory) {
    require(x > 10, "x must be greater than 10");
    return "Passed require";
}

A The function returns 'Passed require'.

B The transaction reverts with error message 'x must be greater than 10'.

C The function returns an empty string.

D The transaction succeeds but emits a warning.

💡 Hint

Remember, require checks a condition and reverts if false.

The require statement checks if x > 10. If false, it reverts the transaction with the given error message. Since 5 is not greater than 10, it reverts with 'x must be greater than 10'.

❓ Predict Output

intermediate

What error does this Solidity code raise?

Analyze this Solidity function:

function testAssert(uint x) public pure returns (string memory) {
    assert(x != 0);
    return "Passed assert";
}

What happens when testAssert(0) is called?

Blockchain / Solidity

function testAssert(uint x) public pure returns (string memory) {
    assert(x != 0);
    return "Passed assert";
}

A The transaction reverts with an 'assertion failed' error.

B The function returns 'Passed assert'.

C The function returns an empty string.

D The transaction reverts with a custom error message.

💡 Hint

assert is used to check for conditions that should never be false.

The assert statement checks if x != 0. If false, it triggers a panic error and reverts the transaction without a custom message. Since x is 0, it reverts with an 'assertion failed' error.

❓ Predict Output

advanced

What is the output of this Solidity function using revert?

Consider this Solidity function:

function testRevert(uint x) public pure returns (string memory) {
    if (x < 5) {
        revert("x must be at least 5");
    }
    return "Passed revert";
}

What happens when testRevert(3) is called?

Blockchain / Solidity

function testRevert(uint x) public pure returns (string memory) {
    if (x < 5) {
        revert("x must be at least 5");
    }
    return "Passed revert";
}

A The transaction reverts with error message 'x must be at least 5'.

B The function returns 'Passed revert'.

C The function returns an empty string.

D The transaction succeeds but emits a warning.

💡 Hint

revert immediately stops execution and reverts the transaction with a message.

The if condition is true because 3 is less than 5, so revert is called with the message 'x must be at least 5', reverting the transaction.

🔧 Debug

advanced

Why does this Solidity function always revert?

Look at this Solidity function:

function checkValue(uint x) public pure returns (string memory) {
    require(x > 0, "x must be positive");
    assert(x < 10);
    if (x >= 10) {
        revert("x is too large");
    }
    return "Value is valid";
}

Why does calling checkValue(10) always revert?

Blockchain / Solidity

function checkValue(uint x) public pure returns (string memory) {
    require(x > 0, "x must be positive");
    assert(x < 10);
    if (x >= 10) {
        revert("x is too large");
    }
    return "Value is valid";
}

A Because the require fails when x is 10.

B Because the revert statement is always executed regardless of x.

C Because the assert fails when x is 10, causing a revert before the revert statement.

D Because the function returns an invalid string causing revert.

💡 Hint

Check the order of assert and revert statements.

The assert(x < 10) fails when x is 10, causing an immediate revert with a panic error before the if and revert statements are reached.

🧠 Conceptual

expert

Which statement about require, assert, and revert is TRUE?

Choose the correct statement about require, assert, and revert in Solidity:

A <code>assert</code> should be used to check for conditions that should never be false and consumes all remaining gas on failure.

B <code>require</code> is used only for internal errors, while <code>assert</code> is for user input validation.

C <code>revert</code> cannot include an error message, unlike <code>require</code> and <code>assert</code>.

D <code>require</code> and <code>revert</code> both refund remaining gas when they fail, but <code>assert</code> does not.

💡 Hint

Think about gas consumption and error messages for each statement.

require and revert refund remaining gas and can include error messages. assert is for internal errors and uses all remaining gas on failure.