title: ERC20 token code from the Ethereum website
body: <p>I took the code for the ERC20 token from the official ether website.
  link: <a href="https://www.ethereum.org/token" rel="nofollow noreferrer">https://www.ethereum.org/token</a> (the end of the page)
  My question is why do they have two contracts there, namely TokenERC20 and MyAdvancedToken and don't unite all functions under a single contract? Also, which contract should I deploy so all functions work properly? Thank you very much in advance. </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>pragma solidity ^0.4.16;
  
  contract owned {
      address public owner;
  
      function owned() public {
          owner = msg.sender;
      }
  
      modifier onlyOwner {
          require(msg.sender == owner);
          _;
      }
  
      function transferOwnership(address newOwner) onlyOwner public {
          owner = newOwner;
      }
  }
  
  interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) external; }
  
  contract TokenERC20 {
      // Public variables of the token
      string public name;
      string public symbol;
      uint8 public decimals = 18;
      // 18 decimals is the strongly suggested default, avoid changing it
      uint256 public totalSupply;
  
      // This creates an array with all balances
      mapping (address =&gt; uint256) public balanceOf;
      mapping (address =&gt; mapping (address =&gt; uint256)) public allowance;
  
      // This generates a public event on the blockchain that will notify clients
      event Transfer(address indexed from, address indexed to, uint256 value);
      
      // This generates a public event on the blockchain that will notify clients
      event Approval(address indexed _owner, address indexed _spender, uint256 _value);
  
      // This notifies clients about the amount burnt
      event Burn(address indexed from, uint256 value);
  
      /**
       * Constrctor function
       *
       * Initializes contract with initial supply tokens to the creator of the contract
       */
      function TokenERC20(
          uint256 initialSupply,
          string tokenName,
          string tokenSymbol
      ) public {
          totalSupply = initialSupply * 10 ** uint256(decimals);  // Update total supply with the decimal amount
          balanceOf[msg.sender] = totalSupply;                // Give the creator all initial tokens
          name = tokenName;                                   // Set the name for display purposes
          symbol = tokenSymbol;                               // Set the symbol for display purposes
      }
  
      /**
       * Internal transfer, only can be called by this contract
       */
      function _transfer(address _from, address _to, uint _value) internal {
          // Prevent transfer to 0x0 address. Use burn() instead
          require(_to != 0x0);
          // Check if the sender has enough
          require(balanceOf[_from] &gt;= _value);
          // Check for overflows
          require(balanceOf[_to] + _value &gt; balanceOf[_to]);
          // Save this for an assertion in the future
          uint previousBalances = balanceOf[_from] + balanceOf[_to];
          // Subtract from the sender
          balanceOf[_from] -= _value;
          // Add the same to the recipient
          balanceOf[_to] += _value;
          emit Transfer(_from, _to, _value);
          // Asserts are used to use static analysis to find bugs in your code. They should never fail
          assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
      }
  
      /**
       * Transfer tokens
       *
       * Send `_value` tokens to `_to` from your account
       *
       * @param _to The address of the recipient
       * @param _value the amount to send
       */
      function transfer(address _to, uint256 _value) public returns (bool success) {
          _transfer(msg.sender, _to, _value);
          return true;
      }
  
      /**
       * Transfer tokens from other address
       *
       * Send `_value` tokens to `_to` in behalf of `_from`
       *
       * @param _from The address of the sender
       * @param _to The address of the recipient
       * @param _value the amount to send
       */
      function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
          require(_value &lt;= allowance[_from][msg.sender]);     // Check allowance
          allowance[_from][msg.sender] -= _value;
          _transfer(_from, _to, _value);
          return true;
      }
  
      /**
       * Set allowance for other address
       *
       * Allows `_spender` to spend no more than `_value` tokens in your behalf
       *
       * @param _spender The address authorized to spend
       * @param _value the max amount they can spend
       */
      function approve(address _spender, uint256 _value) public
          returns (bool success) {
          allowance[msg.sender][_spender] = _value;
          emit Approval(msg.sender, _spender, _value);
          return true;
      }
  
      /**
       * Set allowance for other address and notify
       *
       * Allows `_spender` to spend no more than `_value` tokens in your behalf, and then ping the contract about it
       *
       * @param _spender The address authorized to spend
       * @param _value the max amount they can spend
       * @param _extraData some extra information to send to the approved contract
       */
      function approveAndCall(address _spender, uint256 _value, bytes _extraData)
          public
          returns (bool success) {
          tokenRecipient spender = tokenRecipient(_spender);
          if (approve(_spender, _value)) {
              spender.receiveApproval(msg.sender, _value, this, _extraData);
              return true;
          }
      }
  
      /**
       * Destroy tokens
       *
       * Remove `_value` tokens from the system irreversibly
       *
       * @param _value the amount of money to burn
       */
      function burn(uint256 _value) public returns (bool success) {
          require(balanceOf[msg.sender] &gt;= _value);   // Check if the sender has enough
          balanceOf[msg.sender] -= _value;            // Subtract from the sender
          totalSupply -= _value;                      // Updates totalSupply
          emit Burn(msg.sender, _value);
          return true;
      }
  
      /**
       * Destroy tokens from other account
       *
       * Remove `_value` tokens from the system irreversibly on behalf of `_from`.
       *
       * @param _from the address of the sender
       * @param _value the amount of money to burn
       */
      function burnFrom(address _from, uint256 _value) public returns (bool success) {
          require(balanceOf[_from] &gt;= _value);                // Check if the targeted balance is enough
          require(_value &lt;= allowance[_from][msg.sender]);    // Check allowance
          balanceOf[_from] -= _value;                         // Subtract from the targeted balance
          allowance[_from][msg.sender] -= _value;             // Subtract from the sender's allowance
          totalSupply -= _value;                              // Update totalSupply
          emit Burn(_from, _value);
          return true;
      }
  }
  
  /******************************************/
  /*       ADVANCED TOKEN STARTS HERE       */
  /******************************************/
  
  contract MyAdvancedToken is owned, TokenERC20 {
  
      uint256 public sellPrice;
      uint256 public buyPrice;
  
      mapping (address =&gt; bool) public frozenAccount;
  
      /* This generates a public event on the blockchain that will notify clients */
      event FrozenFunds(address target, bool frozen);
  
      /* Initializes contract with initial supply tokens to the creator of the contract */
      function MyAdvancedToken(
          uint256 initialSupply,
          string tokenName,
          string tokenSymbol
      ) TokenERC20(initialSupply, tokenName, tokenSymbol) public {}
  
      /* Internal transfer, only can be called by this contract */
      function _transfer(address _from, address _to, uint _value) internal {
          require (_to != 0x0);                               // Prevent transfer to 0x0 address. Use burn() instead
          require (balanceOf[_from] &gt;= _value);               // Check if the sender has enough
          require (balanceOf[_to] + _value &gt;= balanceOf[_to]); // Check for overflows
          require(!frozenAccount[_from]);                     // Check if sender is frozen
          require(!frozenAccount[_to]);                       // Check if recipient is frozen
          balanceOf[_from] -= _value;                         // Subtract from the sender
          balanceOf[_to] += _value;                           // Add the same to the recipient
          emit Transfer(_from, _to, _value);
      }
  
      /// @notice Create `mintedAmount` tokens and send it to `target`
      /// @param target Address to receive the tokens
      /// @param mintedAmount the amount of tokens it will receive
      function mintToken(address target, uint256 mintedAmount) onlyOwner public {
          balanceOf[target] += mintedAmount;
          totalSupply += mintedAmount;
          emit Transfer(0, this, mintedAmount);
          emit Transfer(this, target, mintedAmount);
      }
  
      /// @notice `freeze? Prevent | Allow` `target` from sending &amp; receiving tokens
      /// @param target Address to be frozen
      /// @param freeze either to freeze it or not
      function freezeAccount(address target, bool freeze) onlyOwner public {
          frozenAccount[target] = freeze;
          emit FrozenFunds(target, freeze);
      }
  
      /// @notice Allow users to buy tokens for `newBuyPrice` eth and sell tokens for `newSellPrice` eth
      /// @param newSellPrice Price the users can sell to the contract
      /// @param newBuyPrice Price users can buy from the contract
      function setPrices(uint256 newSellPrice, uint256 newBuyPrice) onlyOwner public {
          sellPrice = newSellPrice;
          buyPrice = newBuyPrice;
      }
  
      /// @notice Buy tokens from contract by sending ether
      function buy() payable public {
          uint amount = msg.value / buyPrice;               // calculates the amount
          _transfer(this, msg.sender, amount);              // makes the transfers
      }
  
      /// @notice Sell `amount` tokens to contract
      /// @param amount amount of tokens to be sold
      function sell(uint256 amount) public {
          address myAddress = this;
          require(myAddress.balance &gt;= amount * sellPrice);      // checks if the contract has enough ether to buy
          _transfer(msg.sender, this, amount);              // makes the transfers
          msg.sender.transfer(amount * sellPrice);          // sends ether to the seller. It's important to do this last to avoid recursion attacks
      }
  }</code></pre>
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