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Counter

View this sample's code on GitHub

Overview​

This example demonstrates a counter application. It uses an orthogonally persistent counter variable to store an arbitrary precision natural number that represents the current value of the counter.

By using the Motoko keyword stable when declaring the counter variable, the value of this variable will automatically be preserved whenever your canister code is upgraded. Without the stable keyword, a variable is deemed flexible, and its value is reinitialized on every canister upgrade, i.e. whenever new code is deployed to the canister.

The application provides an interface that exposes the following methods:

  • set: sets the value of the counter.
  • inc: increments the value of the counter.
  • get: gets the value of the counter.

Prerequisites​

This example requires an installation of:

  • Install the IC SDK.
  • Clone the example dapp project: git clone https://github.com/dfinity/examples

Begin by opening a terminal window.

Step 1: Navigate into the folder containing the project's files and start a local instance of the Internet Computer with the command:​

cd examples/motoko/counter
dfx start --background

Step 2: Deploy the canister:​

dfx deploy

Step 3: Set the value of the counter:​

dfx canister call counter set '(7)'

Step 4: Increment the value of the counter:​

dfx canister call counter inc

Step 5: Get the value of the counter:​

dfx canister call counter get

The following output should be returned:

(8 : nat)

Resources​

To learn more about these features of Motoko, see:

Security considerations and best practices​

If you base your application on this example, we recommend you familiarize yourself with and adhere to the security best practices for developing on the Internet Computer. This example may not implement all the best practices.