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Tutorial: Counter

In this tutorial, you learn how to implement a smart contract that stores and manipulates an integer value on the Stacks 2.0 Blockchain. By the end of this tutorial, you will ...

Have experienced test-driven development with Clarity
Understand more Clarity language design principles
Have a working Clarity counter smart contract

Overview

TOC {:toc}

Pre-requisites

Before you get started, you should complete the Hello World tutorial.

Step 1: Download counter tutorial project

In this step, you initialize a starter project with additional counter tutorial files:

Using your terminal, run the following command:
```
npm init clarity-starter
```
You have to select a template and a name for your local folder. For the counter template used in this tutorial, ensure to type counter and hit ENTER:
```
? Template - one of [hello-world, counter]: counter
? Project name: (clarity-counter-project)
```
Finally, the project dependencies are installed and your project is ready for development. Because you already completed the Hello World tutorial, the project structure is familiar to you. The main difference is that we have additional tests for a new counter smart contract.

Step 2: Running tests

Smart contracts are often developed in a test-driven approach to ensure code quality but also to speed up the development cycle by removing the need to push every change to the Blockchain before executing it. We will do the same in this project. Now, let's run the tests and review the results:

Still in the project root directory, run the following command:

npm test

You should see the following response:

counter contract test suite
    ✓ should have a valid syntax
    deploying an instance of the contract
    1) should start at zero
    2) should increment
    3) should decrement

hello world contract test suite
    ✓ should have a valid syntax
    deploying an instance of the contract
    ✓ should return 'hello world'
    ✓ should echo number


4 passing (455ms)
3 failing

It looks like we see some failed test! That is on purpose - we will implement the new smart contract in the next steps! After every increment of the contract, we will rerun the tests to ensure we're on the right track.

Step 3: Developing a smart contract

Let's get familiar with the tests to understand what the new smart contract should look like

Take a quick look at the test file associated with the counter smart contract:
```
cat test/counter.ts
```
You should be familiar with the test set up from the Hello World tutorial. Notice how the instance of the smart contract is created on line 8:
```
counterClient = new Client("SP3GWX3NE58KXHESRYE4DYQ1S31PQJTCRXB3PE9SB.counter", "counter", provider);
```
That tells us that the new smart contract is named counter and that we need to start by creating a new file for the smart contract: contracts/counter.clar. Note that the contracts folder is assumed as the base folder and that every Clarity file has the suffix .clar.
Let's create the new file:
```
touch contracts/counter.clar
```
With the editor of your choice, open the file and add the following lines of code:
```
(define-data-var counter int 0)

(define-public (get-counter)
    (ok (var-get counter))
)
```
The first line initializes a new integer variable counter with the value set to 0 using the define-data-var statement. It is important to note that all definition statements in Clarity need to be at the top of the file.

To provide access to the variable from outside of the current smart contract, we need to declare a public get method for this variable. The last lines of the code add a public get-counter method. The var-get statement looks for a variable in the contract's data map and returns it.

With that, you are ready to rerun the tests!
Run the tests and review the results:
```
npm test
```
You should now only see 2 failing tests! should start at zero is passing, and you successfully build your first part of the contract. Congrats!

However, we don't stop here. Let's implement increment and decrement methods.
Add the following lines to the bottom of the counter.clar file and take a few seconds to review them:
```
(define-public (increment)
    (begin
        (var-set counter (+ (var-get counter) 1))
        (ok (var-get counter))
    )
)
```
First, the begin statement evaluates the multi-line expressions and returns the value of the last expression. In this case, it is used to set a new value and return the new value.

Next, a var-set is used to set a new value for the counter variable. The new value is constructed using the + (add) statement. This statement takes a number of integers and returns the result. Along with add, Clarity provides statements to subtract, multiply, and divide integers. Find more details in the Clarity language reference.

Finally, take a few minutes and implement a new public method decrement to subtract 1 from the counter variable. You should have all knowledge needed to succeed at this!

Done? Great! Run the tests and make sure all of them are passing. You are looking for 7 successful tests:

counter contract test suite
    ✓ should have a valid syntax
    deploying an instance of the contract
    ✓ should start at zero
    ✓ should increment (95ms)
    ✓ should decrement (92ms)

hello world contract test suite
    ✓ should have a valid syntax
    deploying an instance of the contract
    ✓ should return 'hello world'
    ✓ should echo number


7 passing (518ms)

Congratulations! You just implemented your first Clarity smart contract.

Here is how the final smart contract file should look like. Note that you can find the decrement method in here - in case you want to compare with your own implementation:

(define-data-var counter int 0)
(define-public (increment)
    (begin
        (var-set counter (+ (var-get counter) 1))
        (ok (var-get counter))
    )
)
(define-public (decrement)
    (begin
        (var-set counter (- (var-get counter) 1))
        (ok (var-get counter))
    )
)
(define-public (get-counter)
    (ok (var-get counter))
)

Where to go next

{:.no_toc}

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