永不终止的递归

Question

我的程序有问题，它应该解决数独并在完成时打印，但问题是，程序看起来好像永远不会结束，打印解决后的数据后会出现错误。

这是程序正在制作的循环：如果我在开始时删除System.out.print(f+"\n");它也不打印任何错误，它也不打印字段。代码：

        public class Sudoku {


  public static void main(String[] args) throws SolvedException {
    Field field = new Field();
    field.fromFile("test1.txt");
    SudokuSolver solver = new SudokuSolver();

    solve(field, 0, 0, solver);

    System.out.println(field);

  }




public static void solve(Field f, int i, int j, SudokuSolver solver) {

    System.out.print(f+"\n");

    if ( j >= Field.SIZE) {

        //we are done (return true now!)
        solver.done=true;
        return;

    } 

    if (f.isEmpty(i, j)) {

        for (int val = 1; val <=9; val++) {

            if (f.tryValue(val, i, j)){

                if (j>=Field.SIZE-1){

                    solve (f, i+1, 0, solver);

                    if ( solver.done ) {

                        // This halts the loop here:
                        return;
                    }

                    f.clear(i, j);

                } else {

                    solve(f,i,j+1, solver);

                    if ( solver.done ) {

                        // This halts the loop here:
                        return;
                    }

                    f.clear(i, j);

                }

            }
        }

    } else if (j>=Field.SIZE-1) {

        solve(f,i+1,0, solver);

    } else {

        solve(f,i,j+1, solver);

    }
}
}

SudokuSolver

public class SudokuSolver{

    /* Set true when the solve is done */
    public boolean done;

}

错误：

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 9
    at Field.isEmpty(Field.java:101)
    at Sudoku.solve(Sudoku.java:30)
    at Sudoku.solve(Sudoku.java:38)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:67)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:38)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:38)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:38)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:67)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:67)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:38)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:38)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:71)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.solve(Sudoku.java:50)
    at Sudoku.main(Sudoku.java:9)

任何想法我怎么能停止循环并打印正确的字段？解决方法中的System.out.println(field)而不是这个？

Answer 1

永不终止的递归

（在这种情况下，它会在错误时终止。）

这部分代码需要实际做一些事情来表明它已经完成了：

if ( j >= Field.SIZE) {

    //we are done

}

否则该循环只是继续进行，不知道它应该已经停止（缩短）：

for (int val = 1; val <=9; val++) {

     ...

     solve (f, i+1, 0); // Maybe this call is 'done', but this loop will keep going

     ...
}

因此，基于throws SolvedException，您的代码应该在那里抛出异常：

if ( j >= Field.SIZE) {

    //we are done
    throw new SolvedException();

}

但这是一个坏主意。用于控制代码流的例外不 - 它们用于completely unexpected situations。

表示已完成

相反，我们需要有一些方法来了解代码何时达到“完成”的目的。条件。在经典递归中，这是通过返回一些东西来执行的。我们只想知道它是否已经完成，bool处理就好了：

// Type changed to bool, removed throws:

public static bool solve(Field f, int i, int j) {

    System.out.print(f+"\n");

    if ( j >= Field.SIZE) {

        //we are done (return true now!)
        return true;

    } 

    if (f.isEmpty(i, j)) {

        for (int val = 1; val <=9; val++) {

            if (f.tryValue(val, i, j)){

                if (j>=Field.SIZE-1){

                    if( solve (f, i+1, 0) ){
                        // This halts the loop here:
                        return true;
                    }

                    f.clear(i, j);

                } else {

                    if( solve(f,i,j+1) ){
                        // This halts the loop here:
                        return true;
                    }

                    f.clear(i, j);

                }

            }
        }

    } else if (j>=Field.SIZE-1) {

        // (Side note: This one is tail recursion)
        return solve(f,i+1,0);

    }

    // (Side note: This one is tail recursion)
    return solve(f,i,j+1);

}

在被叫方中，你也有：

try {
  solve(field, 0, 0);
} 
catch (SolvedException e) { }

您还可以换掉：

if( solve(field, 0, 0) ){

    // It was solved!

}

返回void

您提到您仍然想要返回void。好的，所以，我们需要跟踪那些已经完成的事情。在其他地方陈述 - 例如在我们称之为SudokuSolver的某个对象中：

public class SudokuSolver{

    /* Set true when the solve is done */
    public bool done;

}

使用它会使代码看起来更像这样：

// Type changed to void, added our solver arg:

public static void solve(Field f, int i, int j, SudokuSolver solver) {

    System.out.print(f+"\n");

    if ( j >= Field.SIZE) {

        //we are done (return true now!)
        solver.done=true;
        return;

    } 

    if (f.isEmpty(i, j)) {

        for (int val = 1; val <=9; val++) {

            if (f.tryValue(val, i, j)){

                if (j>=Field.SIZE-1){

                    solve (f, i+1, 0, solver);

                    if ( solver.done ) {

                        // This halts the loop here:
                        return;
                    }

                    f.clear(i, j);

                } else {

                    solve(f,i,j+1, solver);

                    if ( solver.done ) {

                        // This halts the loop here:
                        return;
                    }

                    f.clear(i, j);

                }

            }
        }

    } else if (j>=Field.SIZE-1) {

        solve(f,i+1,0, solver);

    } else {

        solve(f,i,j+1, solver);

    }

}

现在呼叫网站看起来像这样：

SudokuSolver solver=new SudokuSolver();

solve(field, 0, 0, solver);

if ( solver.done ) {
    // It was solved!
}

Answer 2

某处试图访问不存在的数组上层元素。尝试调试并找到。

可能val是寻找的关键。

我假设 for（int val = 1; val＆lt; = 9 ; val ++）应该 for（int val = 1; val＆lt; 9 ; val ++）

我的循环永远不会结束

2 个答案:

永不终止的递归

表示已完成

返回void