# 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 <a href="#id-1-background" id="id-1-background"></a>

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 <a href="#id-2-define-the-variable" id="id-2-define-the-variable"></a>

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 <a href="#id-3-define-functions" id="id-3-define-functions"></a>

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 <a href="#id-4-let-s-do-something-more" id="id-4-let-s-do-something-more"></a>

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

### 4-1) Add a variable <a href="#id-4-1-add-a-variable" id="id-4-1-add-a-variable"></a>

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 <a href="#id-4-2-update-functions" id="id-4-2-update-functions"></a>

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](https://archive-docs.klaytn.foundation/content/dapp/tutorials/count-dapp/5.-frontend-code-overview/5-3.-count-component) 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`.