"Hello World" with counter

Friend of mine pointed out that the "Hello World" contract described in my previous post does not keep state. Reading and writing data to the blockchain is important and can be challenging. In this post I will add a counter to count number of times the sayHello() function was called.   

Step 1. Create  a new contract

Let's create a file HelloWithCounter.sol and put it into the "contracts" directory.

pragma solidity ^0.4.4;

contract HelloWithCounter {
  uint private helloCounter;

  function Hello() public {
    // constructor
    helloCounter = 0;
  }

  function sayHello() public returns (string) {
     helloCounter = helloCounter+1;
     return strConcat('I say Hello #', uintToString(helloCounter));
  }

  function getHelloCounter() constant public returns (string) {
     return uintToString(helloCounter);
  }

  function uintToString(uint v) internal pure returns (string str) {
      uint maxlength = 100;
      bytes memory reversed = new bytes(maxlength);
      uint i = 0;
      while (v != 0) {
        uint remainder = v % 10;
          v = v / 10;
          reversed[i++] = byte(48 + remainder);
      }
      bytes memory s = new bytes(i);
      for (uint j = 0; j < i; j++) {
          s[j] = reversed[i - 1 - j];
      }
      str = string(s);
  }

  function strConcat(string _a, string _b) internal pure returns (string) {
        bytes memory _ba = bytes(_a);
        bytes memory _bb = bytes(_b);
 
        string memory ab = new string(_ba.length + _bb.length);
        bytes memory bab = bytes(ab);
        uint k = 0;
        for (uint i = 0; i < _ba.length; i++) {
          bab[k++] = _ba[i];
        }
        for (i = 0; i < _bb.length; i++) {
          bab[k++] = _bb[i];
        }
        return string(bab);
    }
}

Step 2. Update 2_deploy_contracts.js (in migrations directory) to include new contract. 

Here is how the file should look after the update:

var Hello1 = artifacts.require("./Hello.sol");
var Hello2 = artifacts.require("./HelloWithCounter.sol");


module.exports = function(deployer) {
  deployer.deploy(Hello1);
  deployer.deploy(Hello2);
};

Step 3. Start the console

$ truffle console↵

Step 4. Deploy new contract 

truffle(development)> truffle migrate --reset↵
Using network 'development'.

Running migration: 1_initial_migration.js
  Replacing Migrations...
  ... 0x2dd16d6427594963fce6c0b876bb97eccb686665d28e8a0fb474e8aaef63fd8c
  Migrations: 0xac37a9866815625c4a4c6aa0fc60116e495b1df5
Saving successful migration to network...
  ... 0x5cb753bc95a68ae74a6153b5253686e2286cbe28fa823656f0efcdfa86b010fb
Saving artifacts...
Running migration: 2_deploy_contracts.js
  Replacing Hello...
  ... 0x32884e4eafe5754c0144daa90a74812b53baea3e132f8a160ee99672e91503cc
  Hello: 0x320b839e89a460384ec01919614c7399f80c5213
  Replacing HelloWithCounter...
  ... 0xda8e7f43f62fcd94ffe478b1f135b31640ef8872c97278f10eab7c47b71405f8
  HelloWithCounter: 0xec2a49fb14e9e5b52d4236f9e27e3c45dfeb44b1
Saving successful migration to network...
  ... 0xda21ca9b4bae886293e7f7f3b06a3beaa6b33e67d9f97d88b5a9e5d35ba6ad86
Saving artifacts...

Step 5. Check the contract 

First let's see the counter before any calls to the sayHello()

truffle(development)> var hc = HelloWithCounter.at(HelloWithCounter.address)↵
undefined
truffle(development)> hc.getHelloCounter()↵
''

No surprises here. Counter is empty.


Now let's  say "Hello" for couple times:

truffle(development)> hc.sayHello()↵
{ tx: '0xd2579761083d00e3d2130cc59bba08ea1e6befa3323f1836c60e99f95da03cfe',
  receipt:
   { transactionHash: '0xd2579761083d00e3d2130cc59bba08ea1e6befa3323f1836c60e99f95da03cfe',
     transactionIndex: 0,
     blockHash: '0x8bb3e28112468c8ed46ea875e157112cc1c79a258062ffeb6756da2706f9c42a',
     blockNumber: 70,
     gasUsed: 47195,
     cumulativeGasUsed: 47195,
     contractAddress: null,
     logs: [],
     status: 1 },
  logs: [] }
truffle(development)> hc.sayHello()
{ tx: '0x57f25d5faf5efaf14d6c77698edb3ed7f6ee2a2cb7f7fa29d87970571ba9e19f',
  receipt:
   { transactionHash: '0x57f25d5faf5efaf14d6c77698edb3ed7f6ee2a2cb7f7fa29d87970571ba9e19f',
     transactionIndex: 0,
     blockHash: '0x16e9d46ca85c902663812cb60e7a0885c0647b2e9196e32b9e698707c24b1f3c',
     blockNumber: 71,
     gasUsed: 32195,
     cumulativeGasUsed: 32195,
     contractAddress: null,
     logs: [],
     status: 1 },
  logs: [] }

Note that some gas is needed to call this function. That's the price for writing data to the Etherium blockhain and (a little bit) for the time of running this function within virtual machines on the network,

Let's check the counter again:

truffle(development)> hc.getHelloCounter()↵
'2'