Exploring calculator functions

In this tutorial, you are going to write a simple calculator program that creates a single actor with several public entry-point functions to perform basic arithmetic operations.

For this tutorial, the actor is named Calc. The program uses the cell variable to contain an integer number that represents the current result of a calculator operation.

This program supports the following function calls:

The add function call accepts input and performs addition.
The sub function call accepts input and performs subtraction.
The mul function call accepts input and performs multiplication.
The div function call accepts input and performs division.
The clearall function clears the cell value stored as the result of previous operations, resetting the cell value to zero.

The div function also includes code to prevent the program from attempting to divide by zero.

Before you begin

Before starting the tutorial, verify the following:

You have downloaded and installed the DFINITY Canister SDK package as described in Download and install.
You have stopped any Internet Computer network processes running on the local computer.

This tutorial takes approximately 20 minutes to complete.

Create a new project

To create a new project for this tutorial:

Open a terminal shell on your local computer, if you don’t already have one open.
Change to the folder you are using for your Internet Computer sample projects.
Create a new project by running the following command:
```
dfx new calc
```
Change to your project directory by running the following command:
```
cd calc
```

Modify the default configuration

For this tutorial, let’s modify the default dfx.json configuration file to use a more specific name for its main program.

To modify the default dfx.json configuration file:

Open the dfx.json configuration file in a text editor.
Change the main key setting from the default main.mo program name to calc_main.mo.

For example:
```
"main": "src/calc/calc_main.mo",
```
For this tutorial, you are changing the name of the source file from main.mo to calc_main.mo to illustrate how this setting in the dfx.json file determines the source file to be compiled.
Save your changes and close the file to continue.

Modify the default program

For this tutorial, you need to replace the default program with a program that performs basic arithmetic operations.

To replace the default program:

Check that you are still in your project directory, if needed.
Copy the template main.mo file to create a new file named calc_main.mo by running the following command:
```
cp src/calc/main.mo src/calc/calc_main.mo
```
Open the src/calc/calc_main.mo file in a text editor and delete the existing content.

Copy and paste the following sample code into the calc_main.mo file:

// This single-cell calculator defines one calculator instruction per
// public entry point (add, sub, mul, div).

// Create a simple Calc actor.
actor Calc {
  flexible var cell : Int = 0;

  // Define functions to add, subtract, multiply, and divide
  public func add(n:Int) : async Int { cell += n; cell };
  public func sub(n:Int) : async Int { cell -= n; cell };
  public func mul(n:Int) : async Int { cell *= n; cell };
  public func div(n:Int) : async ?Int {
    if ( n == 0 ) {
      // null encodes div-by-zero error
      return null
    } else {
      cell /= n; ?cell
    }
  };

    // Clear the calculator and reset to zero
    public func clearall() : async Int {
    if (cell : Int != 0)
       cell -= cell;
    return cell};
 };

You might notice that this sample code uses integer (Int) data types, enabling you to use positive or negative numbers. If you wanted to restrict the functions in this calculator code to only use positive numbers, you could change the data type to only allow natural (Nat) data.

Save your changes and close the file to continue.

Start the local network

Before you can build the calc project, you need to connect to the Internet Computer network either running locally in your development environment or running remotely on a sub-network that you can access.

To start the network locally:

Open a new terminal window or tab on your local computer and navigate to your project directory.

For example, you can do either of the following if running Terminal on macOS:
- Click Shell, then select New Tab to open a new terminal in your current working directory.
- Click Shell and select New Window, then run cd ~/ic-projects/calc in the new terminal if your calc project is in the ic-projects working folder.
You should now have two terminals open with your project directory as your current working directory.
Start the Internet Computer network on your local computer by running the following command:
```
dfx start
```
After you start the local network, the terminal displays messages about network operations.
Leave the terminal that displays network operations open and switch your focus to your original terminal where you created your new project.

Register canister identifiers

After you connect to the Internet Computer network running locally in your development environment, you can register with the network to generate unique, network-specific canister identifiers for your project.

To register canister identifiers for the local network:

Check that you are still in your project directory, if needed.
Register unique canister identifiers for the canisters in the project by running the following command:
```
dfx canister create --all
```
The command displays the network-specific canister identifiers for the canisters defined in the dfx.json configuration file.
```
"calc" canister created with canister id: "75hes-oqbaa-aaaaa-aaaaa-aaaaa-aaaaa-aaaaa-q"
"calc_assets" canister created with canister id: "cxeji-wacaa-aaaaa-aaaaa-aaaaa-aaaaa-aaaaa-q"
```

Build and deploy the program

You now have a program that you can compile into an executable WebAssembly module that you can deploy on your local Internet Computer network.

To build and deploy the program:

Check that you are still in your project directory by running the pwd command, if necessary.
Build the WebAssembly executable by running the following command:
```
dfx build --skip-frontend
```
For this tutorial, you can build your project using the --skip-frontend option because the project is a simple terminal-based applications that doesn’t include any front-end assets.
Deploy your calc project on the local network by running the following command:
```
dfx canister install calc
```

Verify calculator functions on the canister

You now have a program deployed as a canister on your local Internet Computer network. You can test the program by using dfx canister call commands.

To test the program you have deployed:

Use the dfx canister call command to call the calc canister add function and pass it the input argument 10 by running the following command:
```
dfx canister call calc add '(10)'
```
When you pass an argument enclosed by the single quotation marks and parentheses,the interface description language (IDL) parses the argument type, so you don’t need to specify the argument type manually.

Verify that the command returns the value expected for the add function. For example, the program displays output similar to the following:
```
(10)
```
Call the mul function and pass it the input argument 3 by running the following command:
```
dfx canister call calc mul '(3)'
```
Verify that the command returns the value expected for the mul function. For example, the program displays output similar to the following:
```
(30)
```
Call the sub function and pass it the input argument 5 of type number by running the following command:
```
dfx canister call calc sub '(5)'
```
Verify that the command returns the value expected for the sub function. For example, the program displays output similar to the following:
```
(25)
```
Call the div function and pass it the input argument 5 by running the following command:
```
dfx canister call calc div '(5)'
```
Verify that the command returns the value expected for the div function. For example, the program displays output similar to the following:
```
(opt 5)
```
You might notice that the div function returns an optional result. The program makes the result optional to enable the div function to return null in the case of a division-by-zero error.

Because the cell variable in this program is an integer, you can also call its functions and specify negative input values. For example, you might run the following command:
```
dfx canister call calc mul '(-4)'
```
which returns:
```
(-20)
```
Call the clearall function and verify it resets the cell value to zero:
```
dfx canister call calc clearall
```
For example, the program displays output similar to the following:
```
(0)
```

Test functions in a browser

The canister interface description language—often referred to as Candid or more generally as the IDL—provides a common language for specifying the signature of a canister service. The language provides a unified way of interacting with canisters in various languages and tools, such as the dfx command-line interface, JavaScript, and Motoko.

Based on the type signature of the actor, Candid provides a web interface that allows you to call canister functions for testing and debugging.

To use the Candid web interface to test canister functions:

Deploy your project using the dfx canister install command and copy the canister identifier associated with the main actor for your application.
Open a browser and navigate to the address and port number specified in the dfx.json configuration file.

By default, the local network binds to the 127.0.0.1:8000 address and port number.
Add the candid endpoint to access the Candid web interface followed by the required canisterId parameter and canister identifier.

For example, the full URL should look similar to the following but with the canister_identifier that was returned by the dfx canister install command:
```
http://127.0.0.1:8000/candid?canisterId=<YOUR-CANISTER-IDENTIFIER>
```
Review the list of function calls and types defined in the program.
Type a value of the appropriate type for a function or click Lucky to generate a value, then click Call or Query to see the result.

Note that depending on the data type, the Candid interface might display additional configuration settings for testing functions. For example, if a function takes an array, you might need to specify the number of items in the array before entering values.

Stop the local network

After you finish experimenting with your program, you can stop the local Internet Computer network so that it doesn’t continue running in the background.

To stop the local network:

In the terminal that displays network operations, press Control-C to interrupt the local network process.
Stop the Internet Computer network by running the following command:
```
dfx stop
```