Blockchain Development — Day 4

Mapping

In the last article, “Sending ether to a contract and withdrawing” we looked at a working example for using the address variable and msg object. In this article, we are going to extend our contract to store the balance for each addres sending money to the contract using the mapping variable.

Mappings Explained

Mappings act as hash tables which consist of key corresponding to value pairs. In our example, we will map an address to a balance.

mapping (address => uint)

This is the same code as the last contract with the new variable, balanceReceived that maps addresses to balances.

We modify the sendMoneyToContractfunction to increment the amount the address that sent money to the contract and

Lets now add a getBalanceForAddressfunction that gets the balance for a specified address

Finally, lets create a functionwithdrawMoneypassing the address we want to send the money too and the amount to send.

Line 34 : Checks that the msg sender has enough to send, otherwise it will send a message that there is not enough funds to transfer

Line 36: Updates the balance

Line 38: Sends the amount to the specified address

That’s covers extended smart contract to record each addresses balance sent to the smart contract using mapping.

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