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11: Incrementing a natural number

Beginner
Motoko
Tutorial

Overview​

In this guide, you are going to write a program that creates a single actor and provides a few basic functions to increment a counter and illustrate persistence of a value.

For this example, the actor is named Counter. The program uses the currentValue variable to contain a natural number that represents the current value of the counter. This program supports the following function calls:

  • The increment function call updates the current value, incrementing it by 1 (no return value).

  • The get function call queries and returns the current value of the counter.

  • The set function call updates the current value to an arbitrary numeric value you specify as an argument.

This guide provides a simple example of how you can increment a counter by calling functions on a deployed canister. By calling the functions to increment and query the counter value multiple times, you can verify that the variable state; that is, the value of the variable between calls—persists.

Prerequisites​

Before getting started, assure you have set up your developer environment according to the instructions in the developer environment guide.

Create a new project​

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

Create a new project by running the following command:

Use dfx new <project_name> to create a new project:

dfx new my_counter

You will be prompted to select the language that your backend canister will use. Select 'Motoko':

? Select a backend language: ›
❯ Motoko
  Rust
  TypeScript (Azle)
  Python (Kybra)

Then, select a frontend framework for your frontend canister. Select 'No frontend canister':

  ? Select a frontend framework: ›
  SvelteKit
  React
  Vue
  Vanilla JS
  No JS template
❯ No frontend canister

Lastly, you can include extra features to be added to your project:

  ? Add extra features (space to select, enter to confirm) ›
⬚ Internet Identity
⬚ Bitcoin (Regtest)
⬚ Frontend tests

The command creates a new my_counter project for your project.

Then change into your project directory by running the following command:

cd my_counter

Modify the default configuration​

You have already seen that creating a new project adds a default dfx.json configuration file to your project directory. In this guide, you will modify the default settings to use a different name for the main program in your project.

To modify the dfx.json configuration file, open the dfx.json configuration file in a text editor and change the default main setting from main.mo to increment_counter.mo.

For example:

"main": "src/my_counter_backend/increment_counter.mo",

For this guide, changing the name of the source file from main.mo to increment_counter.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 with dependencies 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 be confusing.

You can leave the rest of the default settings as they are.

Save your change and close the dfx.json file to continue.

Change the name of the main program file in the source code directory src to match the name specified in the dfx.json configuration file by running the following command:

mv src/my_counter_backend/main.mo src/my_counter_backend/increment_counter.mo

Modify the default program​

So far, you have only changed the name of the main program for your project. The next step is to modify the code in the src/my_counter_backend/increment_counter.mo file to define an actor named Counter and implement the increment, get, and set functions.

To modify the default template source code, open the src/my_counter_backend/increment_counter.mo file in a text editor and delete the existing content.

Copy and paste this code into the increment_counter.mo file:

// Create a simple Counter actor.
actor Counter {
stable var currentValue : Nat = 0;

// Increment the counter with the increment function.
public func increment() : async () {
currentValue += 1;
};

// Read the counter value with a get function.
public query func get() : async Nat {
currentValue
};

// Write an arbitrary value with a set function.
public func set(n: Nat) : async () {
currentValue := n;
};
}

Let's take a closer look at this sample program:

  • You can see that the currentValue variable declaration in this example includes the stable keyword to indicate the state—the value that can be set, incremented, and retrieved—persists.

This keyword ensures that the value for the variable is unchanged when the program is upgraded.

  • The declaration for the currentValue variable also specifies that its type is a natural number (Nat).

  • The program includes two public update methods—the increment and set functions—and one a query method-the get function.

For more information about stable and flexible variables, see stable variables and upgrade methods in the Motoko programming language guide.

For more information about the differences between a query and an update, see query and update methods in canisters include both program and state.

Save your changes and close the file to continue.

Start the local canister execution environment​

Before you can build the my_counter project, you need to either connect to a local canister execution environment simulating the Internet Computer blockchain or to the Internet Computer blockchain mainnet.

Starting the local canister execution environment requires a dfx.json file, so you should be sure you are in your project’s root directory. For this guide, 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, open a new terminal window or tab on your local computer.

  • You should now have two terminals open.
  • You should have the project directory as your current working directory.

Start the local canister execution environment on your computer by running the following command:

dfx start

After you start the local canister execution environment, 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, build, and deploy the dapp​

After you connect to the local replica running in your development environment, you can register, build, and deploy your dapp locally.

Register, build, and deploy your dapp by running the following command in your project's directory:

dfx deploy

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

Invoke methods on the deployed canister​

After successfully deploying the canister, you can simulate an end-user invoking the methods provided by the canister. For this guide, you invoke the get method to query the value of a counter, the increment method that increments the counter each time it is called, and the set method to pass an argument to update the counter to an arbitrary value you specify.

Run the following command to invoke the get function, which reads the current value of the currentValue variable on the deployed canister:

dfx canister call my_counter_backend get

The command returns the current value of the currentValue variable as zero:

(0 : nat)

Run the following command to invoke the increment function to increment the value of the currentValue variable on the deployed canister by one:

dfx canister call my_counter_backend increment

This command increments the value of the variable, changing its state, but does not return the result.

Rerun the following command to get the current value of the currentValue variable on the deployed canister:

dfx canister call my_counter_backend get

The command returns the updated value of the currentValue variable as one:

(1 : nat)

Run additional commands to experiment with invoking other methods and using different values. For example, try commands similar to the following to set and return the counter value:

dfx canister call my_counter_backend set '(987)'
dfx canister call my_counter_backend get

This returns the updated value of the currentValue to be 987. Running the additional commands

dfx canister call my_counter_backend increment
dfx canister call my_counter_backend get

returns the incremented currentValue of 988.

Test your code using the Candid UI.​

To test your code, follow the instructions.

Next steps​

In the next guide, you'll learn about passing text arguments to a canister.