Require, assert, and revert in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Require, assert, and revert

Start Transaction

↓

Check require(condition)

No→Revert with message

Yes↓

Execute code

↓

Check assert(condition)

No→Revert without message

Yes↓

Continue execution

↓

End Transaction

The transaction starts, checks require conditions first, reverts with a message if false, then executes code, checks assert conditions, reverts without message if false, else continues and ends.

Execution Sample

Blockchain / Solidity

function test(uint x) public {
  require(x > 0, "x must be positive");
  assert(x != 42);
  // code continues
}

This function checks if x is positive using require, then asserts x is not 42, reverting if any check fails.

Execution Table

Step	Check	Condition	Result	Action
1	require	x > 0	True	Continue execution
2	assert	x != 42	True	Continue execution
3	End	-	-	Function completes successfully

💡 Execution stops if require or assert condition is false, reverting the transaction.

Variable Tracker

Variable	Start	After require	After assert	Final
x	input value	unchanged	unchanged	unchanged

Key Moments - 3 Insights

Why does require include an error message but assert does not?

Require is used for input validation and user errors, so it provides a message (see step 1 in execution_table). Assert is for internal errors and reverts without a message (see step 2).

What happens if require condition is false?

The transaction immediately reverts with the provided message, stopping all execution (see step 1 in execution_table where condition is false).

Can assert fail due to user input?

No, assert is meant to check for internal errors and invariants, not user input. It should never fail if the code is correct (see step 2).

Visual Quiz - 3 Questions

Look at the execution table, what happens if x = 0 at step 1?

A Assert condition is false, transaction reverts without message

B Require condition is false, transaction reverts with message

C Function completes successfully

D Require condition is true, continue execution

💡 Hint

Check step 1 in execution_table where require checks x > 0

At which step does the transaction revert without an error message?

A Step 1 require failure

B Step 3 end

C Step 2 assert failure

D No revert happens

💡 Hint

See step 2 in execution_table where assert condition is checked

If x = 42, what does the variable_tracker show after assert?

A x unchanged, but transaction reverts at assert

B x changes to 0

C x changes to 42

D x is removed

💡 Hint

Variable x does not change during checks, see variable_tracker

Concept Snapshot

Require checks user inputs and reverts with message if false.
Assert checks internal errors and reverts without message.
Revert stops execution and undoes changes.
Use require for validation, assert for invariants.
Failed require shows error; failed assert does not.
Execution stops immediately on failure.

Full Transcript

This concept explains how require, assert, and revert work in blockchain smart contracts. The transaction starts and first checks the require condition. If require fails, it reverts with a message and stops. If require passes, the code runs and then assert checks internal conditions. If assert fails, it reverts without a message. If both pass, the function completes successfully. Variables like input x remain unchanged during these checks. Require is for user input validation with error messages, assert is for internal errors without messages. Revert undoes all changes and stops execution immediately.

Practice

1. Which statement best describes the purpose of require in Solidity smart contracts?

easy

A. It checks user inputs or conditions and stops execution with an error message if the condition is false.

B. It is used to check for internal errors that should never happen in the code.

C. It immediately stops execution without any error message.

D. It logs information but does not stop the contract execution.

Solution

1. Step 1: Understand the role of require

   require is used to validate inputs or conditions before continuing execution.

2. Step 2: Compare with other statements

   assert checks internal errors, and revert stops execution with a message, but require is specifically for input validation.

3. Final Answer:

   It checks user inputs or conditions and stops execution with an error message if the condition is false. -> Option A

4. Quick Check:

   require = input check with error [OK]

Hint: Require is for input checks and stops if false [OK]

Common Mistakes:

• Confusing require with assert for internal errors
• Thinking require does not stop execution
• Believing require logs info without stopping

2. Which of the following is the correct syntax to use assert in Solidity?

easy

A. assert(condition);

B. assert(condition, "Error message");

C. assert {condition};

D. assert condition;

Solution

1. Step 1: Recall assert syntax

   In Solidity, assert takes only a condition without an error message.

2. Step 2: Check each option

   assert(condition, "Error message"); wrongly includes an error message, C and D have invalid syntax.

3. Final Answer:

   assert(condition); -> Option A

4. Quick Check:

   assert syntax = assert(condition); [OK]

Hint: Assert takes only a condition, no message [OK]

Common Mistakes:

• Adding error messages to assert
• Using braces or missing parentheses
• Writing assert as a statement without parentheses

3. What will happen when the following Solidity code runs?

function test(uint x) public {
    require(x > 10, "x must be greater than 10");
    assert(x != 15);
    if (x == 20) {
        revert("x cannot be 20");
    }
}

Input: x = 15

medium

A. Execution stops at require with error 'x must be greater than 10'.

B. Execution stops at revert with error 'x cannot be 20'.

C. Execution stops at assert because x equals 15.

D. Function completes successfully without errors.

Solution

1. Step 1: Check require condition

   Input x=15 is greater than 10, so require passes.

2. Step 2: Check assert condition

   assert(x != 15) fails because x is 15, so execution stops here.

3. Final Answer:

   Execution stops at assert because x equals 15. -> Option C

4. Quick Check:

   assert fails if condition false [OK]

Hint: Assert fails if condition is false, stopping execution [OK]

Common Mistakes:

• Thinking require fails for x=15
• Assuming revert triggers for x=15
• Believing function runs fully without errors

4. Identify the error in this Solidity function:

function check(uint value) public {
    require(value > 0, "Value must be positive");
    assert(value < 100, "Value too large");
}

medium

A. The require statement is missing a message string.

B. There is no error; the code is correct.

C. The function should use revert instead of require.

D. The assert statement incorrectly includes an error message.

Solution

1. Step 1: Review require usage

   Require correctly uses a condition and an error message.

2. Step 2: Review assert usage

   Assert does not accept an error message; including one causes a syntax error.

3. Final Answer:

   The assert statement incorrectly includes an error message. -> Option D

4. Quick Check:

   Assert has no message parameter [OK]

Hint: Assert never takes error messages, only conditions [OK]

Common Mistakes:

• Adding messages to assert
• Confusing require and revert usage
• Assuming no error in assert syntax

5. You want to write a Solidity function that checks if a user has enough balance to withdraw an amount. If the balance is insufficient, it should stop with a clear error message. If an unexpected internal error occurs, it should stop immediately without a message. Which combination of require, assert, and revert is best to use?

hard

A. Use assert for balance check and require for internal errors.

B. Use require to check balance and assert for internal errors; no need for revert.

C. Use revert to check balance with message and assert for internal errors.

D. Use only revert for both balance check and internal errors.

Solution

1. Step 1: Identify correct use of require

   Require is ideal for checking user inputs like balance with error messages.

2. Step 2: Identify correct use of assert

   Assert is for internal errors that should never happen, stopping immediately without messages.

3. Step 3: Consider revert usage

   Revert can be used for complex checks but is not necessary here if require and assert cover cases.

4. Final Answer:

   Use require to check balance and assert for internal errors; no need for revert. -> Option B

5. Quick Check:

   Require for inputs, assert for internal errors [OK]

Hint: Require for inputs, assert for internal errors, revert optional [OK]

Common Mistakes:

• Using assert for input validation
• Using revert unnecessarily for simple checks
• Mixing assert and require roles