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.