Not like bitcoin, at all!

Ethereum isn't like bitcoin where you can combine inputs and all funds at an address go into a transaction, with multiple outputs.

With Ethereum, all addresses are their own wallets and have their own funds, and when you spend, you only send what you need.

Creating addresses for users

If you want to allow users to deposit to their own address, then you can simply create a new address for them.

You don't need to give the users control of their private keys, however there is no combined pool of funds. Ethereum won't magically work out how to combine addresses as inputs to make the required balance for you.

Another, better way...

When you move to Ethereum, you need to start thinking in a different way.

• Ethereum is about tokens and contracts.
• Ethereum is about putting your server logic into the blockchain.

With Ethereum, people transfer funds to your contract, and your contract records the payment, usually (but not necessarily) by giving them a virtual token.

Then people who hold tokens (or other interest or control) can call your contract and ask to perform actions, or direct it to transfer control or tokens to other addresses.

They don't send you funds and get nothing. They send you funds and get a promise and control of something equivalent in value, or a tangible or physical item.

Onsite integration into your website

What's more, you can even integrate the blockchain into your website. This technology is called Web3 (as opposed to Web 2.0).

It means that customers can view their balances of your tokens on your website, and authorise actions against those funds using their wallet(+private key) from their browser.

An example of a physical purchase:

• User has loaded up a bunch of things in their shopping cart.
• Your site can prompt them to (optionally?) make a payment from their Ethereum wallet.
• Website creates the transaction, including:
  • Your contract (the "TO") address.
  • The request method (eg: makePayment).
  • The invoice number, or transaction id etc.
  • The amount of funds required.
• Customer authorises the transaction and fund transfer.
• The smart contract gets called and receives the funds.
• Contract emits an event logging the payment action.
• Website is subscribed to listen for the payment event from your contract.
• Website processes the payment and ships the customer their goods.

You see, most of the logic was handled by the blockchain.

All your site really needed to do was create a transaction that would make a payment to the contract, and then listen for an event saying that payment was made.

Ethereum is all about putting more control in the hand of the customer and taking away the requirement for managing balances, credit card details and other chicanery from your backend servers.

1. Use mapping (address => uint256) accountBalance; where an address is obviously the users' address and uint256 is the amount that the users has as balance. When a user sends ether to your contract update the amount by uint256 oldBalance = accountBalance[msg.sender]; uint256 newBalance = msg.value + oldBalance; accountBalance[msg.sender] = newBalance; Note that I have not thought about re-entrancy or over/underflow of the uint values.

2. Using an array is not optimal but you could indeed use an array for this. For instance, you could create an array of structs which have the address and balance of the user. But again, this isn't optimal.