Remix Warning : Assert

Question

I am trying to compile a contract on remix and have run into the following warning :

Use assert(x) if you never ever want x to be false, not in any circumstance (apart from a bug in your code). Use require(x) if x can be false, due to e.g. invalid input or a failing external component.

The parts of my code that use assert are :

assert(checkPlayerExists(msg.sender) == false);
assert(number >= 1 && number <= 10);
assert(msg.value >= minimumBet);

Would love someone to explain this as it seems a bit cryptic.

Answer 1

This is a misuse of assert because, depending on input, the expressions may evaluate to false. Use require() instead.

assert() is meant to check for logical errors in the contract itself. Asserts state facts that must always be true under any circumstances, so any possibility of false would be a logic issue.

Consider:

uint balanceAlice = 10;
uint balanceBob = 5;
uint conserveFunds = balanceAlice + balanceBob;

// do stuff
uint conservedFunds = balanceAlice + balanceBob;

// whatever we did, we should always end up with the same funds accounted for.
// ANY departure from this principal means the contract contains a logic error. 
assert(conserveFunds == conservedFunds)

Hope it helps.