Update README.md

unit08_blockchain/lab/README.md

@@ -525,6 +525,99 @@ Now answer the following:
 * Now send 0.1 of your token to someone else's wallet. If you want, you can send to your tutor's wallet. Bill's wallet is 0xbb15b38e4ef6af154b89a2e57e03cd5cbd752233
 * Did they receive the token?
 
+## Saving the state
+
+Smart contracts give us the opportunity to store data in a stateful way, and where we can add and delete data within the smart contact, and then view a current state. So let’s do a simple contact of adding cities to a string array. Overall it will cost us some gas to add and delete strings, but not to view the state. The contract we will use is:
+
+```Solidity
+pragma solidity ^0.4.18;
+
+contract ExampleApp {
+
+    string[]  myArray;
+
+    function add(string x) public {
+        myArray.push(x);
+       
+    }
+
+
+    function del(string x) public {
+        for (uint j = 0; j < myArray.length; j++) {
+        if (keccak256(abi.encodePacked(myArray[j])) == keccak256(abi.encodePacked(x))) {
+
+                delete myArray[j];
+        }
+        }
+   
+    }
+
+    function show() public view returns (uint256, string) {
+        string memory str;
+        for (uint j = 0; j < myArray.length; j++) {
+            str = string(abi.encodePacked(str, myArray[j]));
+
+        }
+
+        return(myArray.length,str);
+    }
+
+}
+```
+
+This has an add() method to add a new string to myArray, and a show() which will show the contents of myArray. The view element added to show() makes sure it is just a read function (and that we do not write to the function). We can then compile it in remix:
+
+![Alt text](https://asecuritysite.com/public/estate01.png)
+
+Now we can deploy:
+
+![Alt text](https://asecuritysite.com/public/estate02.png)
+
+This creates a new contact [here](https://ropsten.etherscan.io/address/0xc79961ad2fdf3fcb4a57ebeaab78a1e3f8bf373c):
+
+![Alt text](https://asecuritysite.com/public/estate03.png)
+
+If we look at the contract we get:
+
+![Alt text](https://asecuritysite.com/public/estate04.png)
+
+Next we can Verify the contract:
+
+![Alt text](https://asecuritysite.com/public/estate05.png)
+
+Note that the compiler we used is 0.4.24. We then add our code, and then the contact is validated:
+
+![Alt text](https://asecuritysite.com/public/estate06.png)
+
+It is now complete [here](https://ropsten.etherscan.io/address/0xc79961ad2fdf3fcb4a57ebeaab78a1e3f8bf373c):
+
+![Alt text](https://asecuritysite.com/public/estate07.png)
+
+This will cost us some gas as we change the state of the smart contact:
+
+![Alt text](https://asecuritysite.com/public/estate08.png)
+
+Once it has been mined, the smart contact will be updated:
+
+![Alt text](https://asecuritysite.com/public/estate09.png)
+
+We can then use the View() method to see the string:
+
+![Alt text](https://asecuritysite.com/public/estate10.png)
+
+Now we add “Glasgow”:
+
+![Alt text](https://asecuritysite.com/public/estate11.png)
+
+And once it has been mined, we can go back and show the new state:
+
+![Alt text](https://asecuritysite.com/public/estate12.png)
+
+Complete the following:
+
+* Setup your smart contact, and then add a few cities of the world, and prove that it works. 
+* Ask another person, or your tutor, to add a city to your smart contract, and prove that it works.
+
 # Additional Tutorial
 At the end of this lab, remember to stop your Blockchain (Control-C from the console that is running Geth), and shut down your VM. You may also want to use “rm -r mynapier” in order to delete your blockchain.
 ## Using Geth