# Incrementing a counter

In this tutorial, you are going to write a dapp that provides a few basic functions to increment a counter and illustrates the persistence of a value.

For this tutorial, the dapp declares a COUNTER as a mutable variable to contain a natural number that represents the current value of the counter. This dapp supports the following functions:

• The increment function updates the current value, incrementing by 1 with no return value.

• The get function is a simple query that returns the current value of the counter.

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

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

Like the other sample dapps, this tutorial demonstrates a simple, but realistic, workflow in which you perform the following steps:

• Create a new project.

• Write a dapp that compiles into a WebAssembly module.

• Deploy the canister on the local canister execution environment.

• Invoke the canister methods to increment then read the value of a counter.

## Before you begin​

Before you start your project, verify the following:

• You have an internet connection and access to a shell terminal on your local macOS or Linux computer.

• You have downloaded and installed the Rust programming language and Cargo as described in the Rust installation instructions for your operating system.

curl --proto '=https' --tlsv1.2 https://sh.rustup.rs -sSf | sh

The Rust tool chain must be at version 1.46.0, or later.

• You have cmake installed. For example, use Homebrew with the following command:

brew install cmake

For instructions on how to install Homebrew, see the Homebrew Documentation.

• You have stopped any local canister execution environment processes running on your computer.

This tutorial takes approximately 20 minutes to complete.

## Create a new project​

To create a new project directory for this tutorial:

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

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

dfx new --type=rust rust_counter
3. Change to your project directory by running the following command:

cd rust_counter

## Modify the default project​

In the Hello, World! Rust CDK Quick Start, you went through the files in a default project with Rust type canister.

To complete this tutorial, you’ll need to complete the following steps:

### Replace the default dapp​

Now that you have the files in place for your Rust dapp, we can replace the template lib.rs dapp with the Rust dapp we want to deploy on the Internet Computer blockchain.

To replace the default dapp:

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

2. Open the template src/rust_counter_backend/src/lib.rs file in a text editor and delete the existing content.

The next step is to write a Rust dapp that declares the COUNTER variable and implements the increment, get, and set functions.

3. Copy and paste this code into the lib.rs file.

use ic_cdk::export::candid;use ic_cdk_macros::*;static mut COUNTER: Option<candid::Nat> = None;#[init]fn init() {    unsafe {        COUNTER = Some(candid::Nat::from(0));    }}#[update]fn increment() {    unsafe {        COUNTER.as_mut().unwrap().0 += 1u64;    }}#[query]fn get() -> candid::Nat {    unsafe { COUNTER.as_mut().unwrap().clone() }}#[update]fn set(input: candid::Nat) {    unsafe {        COUNTER.as_mut().unwrap().0 = input.0;    }}
4. Save your changes and close the lib.rs file to continue.

### Update interface description file​

Candid is an interface description language (IDL) for interacting with canisters running on the Internet Computer. Candid files provide a language-independent description of a canister’s interfaces including the names, parameters, and result formats and data types for each function a canister defines.

By adding Candid files to your project, you can ensure that data is properly converted from its definition in Rust to run safely on the Internet Computer blockchain.

To see details about the Candid interface description language syntax, see the Candid Guide or the Candid crate documentation.

To update the Candid file for this tutorial:

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

2. Open the src/rust_counter_backend/rust_counter_backend.did file in a text editor, then copy and paste the following service definition for the increment, get, and set functions:

service : {  "increment": () -> ();  "get": () -> (nat) query;  "set": (nat) -> ();}
3. Save your changes and close the rust_counter_backend.did file to continue.

## Start the local canister execution environment​

Before you can build the rust_counter project, you need to connect to the local canister execution environment running in your development environment or the decentralized Internet Computer blockchain mainnet.

To start the local canister execution environment:

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

2. Start the local canister execution environment on your computer in the background by running the following command:

dfx start --background

Depending on your platform and local security settings, you might see a warning displayed. If you are prompted to allow or deny incoming network connections, click Allow.

## Register, build, and deploy your project​

After you connect to the local canister execution environment running in your development environment, you can register, build, and deploy your project locally.

To register, build, and deploy:

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

2. Register, build, and deploy the canisters specified in the dfx.json file by running the following command:

dfx deploy

The dfx deploy command output displays information about each of the operations it performs similar to the following excerpt:

Creating a wallet canister on the local network.The wallet canister on the "local" network for user "default" is "rwlgt-iiaaa-aaaaa-aaaaa-cai"Deploying all canisters.Creating canisters...Creating canister rust_counter_backend...rust_counter_backend canister created with canister id: rrkah-fqaaa-aaaaa-aaaaq-caiCreating canister rust_counter_frontend...rust_counter_frontend canister created with canister id: ryjl3-tyaaa-aaaaa-aaaba-caiBuilding canisters...WARN: Cannot check for vulnerabilities in rust canisters because cargo-audit is not installed. Please run 'cargo install cargo-audit' so that vulnerabilities can be detected.Executing: cargo build --target wasm32-unknown-unknown --release -p rust_counter_backend --locked...    Finished release [optimized] target(s) in 53.36sBuilding frontend...Installing canisters...Creating UI canister on the local network.The UI canister on the "local" network is "r7inp-6aaaa-aaaaa-aaabq-cai"Installing code for canister rust_counter_backend, with canister ID rrkah-fqaaa-aaaaa-aaaaq-caiInstalling code for canister rust_counter_frontend, with canister ID ryjl3-tyaaa-aaaaa-aaaba-cai...Deployed canisters.

## Call functions and test the dapp​

After successfully deploying the canister, you can test the canister by invoking the functions it provides. For this tutorial:

• Call the get function to query the value of the counter.

• Call the increment function to increment the counter each time it is called.

• Call the set function to pass an argument to update the counter to an arbitrary value you specify.

To test the dapp:

1. Call the get function to read the current value of the COUNTER variable by running the following command:

dfx canister call rust_counter_backend get

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

(0 : nat)
2. Call the increment function to increment the value of the COUNTER variable by one:

dfx canister call rust_counter_backend increment

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

3. Rerun the command to call the get function to see the current value of the COUNTER variable:

dfx canister call rust_counter_backend get

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

(1 : nat)
4. Run additional commands to experiment with call the functions and using different values.

For example, try commands similar to the following to set and return the counter value:

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

Returns the current value of 987.

dfx canister call rust_counter_backend incrementdfx canister call rust_counter_backend get

Returns the incremented value of 988.

## Stop the local canister execution environment​

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

To stop the local canister execution environment:

1. In the terminal that displays network operations, press Control-C to interrupt the local canister execution environment process.

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

dfx stop