assert.md

Chapter 9: Assert

In the previous chapter, we added WildPokemons interface in the trainer contract. In the next chapters, we will start writing the function to battle with the wild pokemons using the battle function of the WildPokemon interface.

In the current contract, when we call the createTrainer function, a new trainer is created with a single pokemon.

Now a trainer should be able to use their pokemon to battle with wild pokemons. If the trainer's pokemon wins then the trainer captures the wild pokemon. If the trainer's pokemon loses, the trainer gets nothing.

assert

The assert statement makes an assertion about a given condition. If the condition evaluates to False then the transaction is reverted.

You can also add an optional error message.

@external
def applyForDriversLicense(String[32]: name, uint256: age):
    assert age < 18 , "not eligible for driver's license"

Here if age provided to the applyForDriversLicense function is less than 18 then the transaction gets reverted with the message "not eligible for driver's license".

NOTE: If a transaction is reverted then any changes made to the contract state by the transaction will be canceled. In other words, the state of the contract will be restored to the state before the transaction occurred.

Put it to the test

A trainer can battle, capture, and own multiple pokemons. So, a trainer has to choose which pokemon they want to use to battle with the wild pokemon.

All the pokemons of a trainer are stored in trainerToPokemon mapping:

trainerToPokemon: HashMap[address, HashMap[uint256, Pokemon]]

To select a specific Pokemon of a trainer we need 2 things:

• address of the trainer calling the contract.

• A uint256 index that will choose a specific pokemon of the trainer.

You may recall from Chapter 2 that you can get the address of the contract caller using msg.sender.

1. Create an external function named battleWildPokemon which takes a single parameter: pokemonIndex of uint256 type.

A trainer who calls the battleWildPokemon can pass pokemonIndex to select which pokemon to use to battle with the wild pokemon.

Now, suppose a trainer has a single pokemon. Let us examine the following lines of code from createTrainer:

self.trainerToPokemon[msg.sender][self.trainerPokemonCount[msg.sender]] = newPokemon
self.trainerPokemonCount[msg.sender] += 1

When a new trainer is created, the value of self.trainerPokemonCount[msg.sender] for the trainer is 0.

So, the first pokemon is added to the index 0 of the trainerToPokemon mapping.

As we increment the trainerPokemonCount, subsequent pokemons are added to higher indexes.

What happens if a trainer with single pokemon calls battleWildPokemon function with pokemonIndex greater than 0?

As there is no pokemon associated with pokemonIndex greater than 0 for this trainer, the transaction would result in a runtime error!

2. To mitigate such a situation, inside the battleWildPokemon function, add an assert statement which checks if pokemonIndex is less than self.trainerPokemonCount[msg.sender]. If the condition is False, return an error message: "Invalid Index Provided".

** Template **

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** Solution **

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** Previous Chapter Solution **

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