4. Write Smart Contract

1. Background

2. Define the variable

3. Define functions

4. Let's do something more 4.1. Add a variable 4.2. Update functions

1) Background

We will make a super simple contract called "Count".

a. There would be just one storage variable called count. b. Users can increase count variable by 1 or decrease it by 1. So there would be two functions, plus function which increases count variable by 1, and minus function which decreases count variable by 1. That's all!

2) Define the variable

Before setting a variable, we should specify the solidity version. Let's use 0.5.6 stable version.

 solidity 0.5.6; // Specify solidity's version

Then we will name our contract "Count".

pragma solidity 0.5.6;

contract Count { // set contract names to "Count"

}

We need to declare the variable count as uint(unsigned integer) type, and initialize it to be 0.

pragma solidity 0.5.6;

contract Count {
  uint public count = 0; // Declare count variable as uint type and initialize its value to 0.
}

3) Define functions

We need two functions, plus and minus. Each function's role is: plus - increase the count by 1. (count = count + 1) minus - decrease the count by 1. (count = count - 1)

pragma solidity 0.5.6;

contract Count {
  uint public count = 0;

  function plus() public { // Make a public function called 'plus'
    count = count + 1; // 'plus' function increases count variable by 1.
  }

  function minus() public { // Make a public function called 'plus'
    count = count - 1; // 'minus' function decreases count variable by 1.
  }
}

NOTE To allow the functions to be called outside the contract, functions should be declared as public.

function plus() public { … }

4) Let's do something more

We want to add one more feature. How about remembering the last participant's wallet address?

4-1) Add a variable

So we will have a variable, lastParticipant as address type: address public lastParticipant;

pragma solidity 0.5.6;

contract Count {
  uint public count = 0;
  address public lastParticipant;

  function plus() public { // Make a public function called 'plus'
    count = count + 1; // 'plus' function increases count variable by 1.
  }

  function minus() public { // Make a public function called 'plus'
    count = count - 1; // 'minus' function decreases count variable by 1.
  }
}

4-2) Update functions

To track the last participant's address, we store the address to lastParticipant like the below:

pragma solidity 0.5.6;

contract Count {
  uint public count = 0;
  address public lastParticipant;

  function plus() public {
    count = count + 1;
    lastParticipant = msg.sender; // store msg.sender to lastParticipant
  }

  function minus() public {
    count = count - 1;
    lastParticipant = msg.sender; // store msg.sender to lastParticipant
  }
}

NOTE 1) public If you declare a variable or a function as public, you can access them outside the blockchain, i.e., you can access this variable or function from your frontend application. You will see how to interact with the contract public methods and variables from the frontend application in the Count componenent chapter.

2) msg.sender msg.sender is the address that initiated the current transaction. To get the address of the transaction sender we can use msg.sender variable.

lastParticipant = msg.sender;

This line will make the lastParticipant to have the msg.sender.