# 修改默认程序

到目前为止，只修改了项目主程序的名称。接下来需要修改src/my\_counter/increment\_counter.mo文件来定义一个名为Counter的actor并实现increment ，get 和set函数。

源码修改步骤

1. 检查是否当前目录为项目根目录
2. 编辑器中打开src/my\_counter/increment\_counter.mo 源码文件，并删除已有内容
3. 复制粘贴下面的示例代码到increment\_counter.mo文件

   ```
   // 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;
     };
   }
   ```

   示例代码解析

   * 实例中定义的currentValue变量包含stable关键字代表状态-这个值可以被修改增加并获取-也就是可维护

   这个关键字保证了变量值在程序升级后不可改变

   * currentValue定义也指定其类型为自然数（Nat）
   * 程序包含2个公共update方法-increment和set函数-以及一个query方法，在这里是get函数

    需要更多关于stable和flexible变量的信息，请 在[*Motoko Programming Language Guide*](http://language-guide/:motoko.html/)查看[Stable variables and upgrade methods](http://language-guide/:upgrades.html/)
4. 保存并关闭文件。


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