At the Counter

Now the bank opens its counter. The new array today holds today's transactions, in the order in which they happened. Each one is a Transaction: a deposit ("deposit") or a withdrawal ("withdraw") of an amount of gold, by the owner of an account.

A deposit adds the gold to the account. A withdrawal takes it away, but only if the account has enough: otherwise, it fails with not enough gold, and the account stays as it was. A transaction by someone who has no account fails with no account. After the accounts in the morning, print a line for each transaction, and then the accounts and the bank's gold once more, as they are in the evening:

Kira: 120 gold
Bruno: 40 gold
Lena: 75 gold
The bank holds 235 gold.
Bruno deposits 30 gold. Balance: 70
Kira withdraws 50 gold. Balance: 70
Bruno can't withdraw 100 gold: not enough gold
Mira can't deposit 10 gold: no account
Kira: 70 gold
Bruno: 70 gold
Lena: 75 gold
The bank holds 215 gold.

A tip: something that can fail is a job for the type Result<T> from the Generics lesson. A method that withdraws gold can return a Result<number>: the new balance, or the reason why it failed.