Use integers in 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 SDK package as described in Download and install.

• You have stopped any canister execution environment 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:

1. Open a terminal shell on your local computer, if you don’t already have one open.

2. Change to the folder you are using for your Internet Computer projects, if you are using one.

3. Create a new project by running the following command:

   dfx new calc

4. 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:

1. Open the dfx.json configuration file in a text editor.

2. 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, changing the name of the source file from main.mo to calc_main.mo simply illustrates how the setting in the dfx.json configuration file determines the source file to be compiled.

   In a more complex dapp, you might have multiple source files instead of a single main program file. More complex applications might also have specific dependencies between multiple source files that you need to manage using settings in the dfx.json configuration file. In a scenario like that—with multiple canisters and programs defined in your dfx.json file—having multiple files all named main.mo might make navigating your workspace more difficult. The name you choose for each program isn’t significant, but it is important that the name you set in the dfx.json file matches the name of your program in the file system.

3. 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:

1. Check that you are still in your project directory, if needed.

2. 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

3. Open the src/calc/calc_main.mo file in a text editor and delete the existing content.

4. Copy and paste this code into the calc_main.mo file.

   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.

5. Save your changes and close the file to continue.

Start the local canister execution environment

Before you can build the calc project, you need to connect to the canister execution environment running locally in your development environment, or you need to connect toa subnet that you can access.

Starting the network locally requires a dfx.json file, so you should be sure you are in your project’s root directory. For this tutorial, you should have two separate terminal shells, so that you can start and see network operations in one terminal and manage your project in another.

To start the local canister execution environment:

1. Open a new terminal window or tab on your local computer.

2. Navigate to the root directory for your project, if necessary.

   • You should now have two terminals open.

   • You should have the project directory as your current working directory.

3. Start the local canister execution environment on your machine by running the following command:

   dfx start

   After you start the local network, the terminal displays messages about network operations.

4. Leave the terminal that displays network operations open and switch your focus to your original terminal where you created your new project.

Register, build, and deploy the dapp

After you connect to the local canister execution environment, you can register, build, and deploy your dapp locally.

To deploy the dapp locally:

1. Check that you are still in the root directory for your project, if needed.

2. Register, build, and deploy your dapp by running the following command:

   dfx deploy

   The dfx deploy command output displays information about the operations it performs.

Verify calculator functions on the canister

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

To test the program you have deployed:

1. 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)

2. 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)

3. 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)

4. 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)

5. 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)

Stop the local canister execution environment

After you finish experimenting with the dapp, you can stop the local canister execution environment so that it does not continue running in the background.

To stop the local canister execution environment:

1. In the terminal that displays the operations, press Control-C to interrupt the process.

2. Stop the local canister execution environment by running the following command:

   dfx stop