矩阵相乘时接收错误

Question

在我的计算机科学 A AP 中，我们必须使用两个类进行矩阵乘法。

我一直遇到问题，使用该函数将矩阵相乘并得到乘积。 这是第一个类，声明并初始化矩阵：

public class Main {
    public static void main(String[] args) {
        int r1 = 3, c1 = 4;
        int r2 = 4, c2 = 2;
        int[][] a = {{1, 2, -2, 0}, {-3, 4, 7, 2}, {6, 0, 3, 1}};
        int[][] b = {{-1, 3}, {0, 9}, {1, -11}, {4, -5}};

        // So there are the matrices
        int[][] mult = matrixMult(a, b, r1, c1, c2);
        displayProduct(product);
    }

    public static void displayProduct(int[][] product) {
        System.out.println("");
        for (int[] row : product) {
            for (int column : row) {
                System.out.println(column + "   ");
            }
            System.out.println();
        }
    }
}

如果矩阵成一个矩阵，这就是我打印乘积的方式。

然后这是应该与矩阵相乘的第二个类：

public class MatrixMult {
    public static int[][] matrixMult(int[][] a, int[][] b) {
        int[][] product = new int[r1][c2];
        for (int i = 0; i < r1; i++) {
            for (int j = 0; j < c2; j++) {
                for (int k = 0; k < c1; k++) {
                    product[i][j] += a[i][k] * b[k][j];
                }
            }
        }
        return product;
    }
}

我尝试运行代码时收到的错误是：

Main.java:14: error: cannot find symbol
int[][] mult = matrixMult(a, b, r1, c1, c2);
               ^
symbol:   method matrixMult(int[][],int[][],int,int,int)
location: class Main
Main.java:16: error: cannot find symbol
displayProduct(product);
               ^
symbol:   variable product
location: class Main
MatrixMult.java:9: error: cannot find symbol
int[][] product = new int[r1][c2];
                          ^
symbol:   variable r1
location: class MatrixMult
MatrixMult.java:9: error: cannot find symbol
int[][] product = new int[r1][c2];
                              ^
symbol:   variable c2
location: class MatrixMult
MatrixMult.java:10: error: cannot find symbol
for (int i = 0; i < r1; i++)  {
                    ^
symbol:   variable r1
location: class MatrixMult
MatrixMult.java:12: error: cannot find symbol
  for (int j = 0; j < c2; j++)  {
                      ^
symbol:   variable c2
location: class MatrixMult
MatrixMult.java:14: error: cannot find symbol
    for (int k = 0; k < c1; k++)  {
                        ^
symbol:   variable c1
location: class MatrixMult

Answer 1

代码审查。您的代码可能如下所示：

public class MatrixMult {
    public static int[][] matrixMult(
            // correct number, type and order of variables
            int[][] a, int[][] b, int r1, int c1, int c2)  {
        int[][] product = new int[r1][c2];
        for (int i = 0; i < r1; i++)  {
            for (int j = 0; j < c2; j++)  {
                for (int k = 0; k < c1; k++)  {
                    product[i][j] += a[i][k] * b[k][j];
                }
            }
        }
        return product;
    }
}

public class Main {
    public static void main(String[] args) {
        int r1 = 3, c1 = 4;
        int r2 = 4, c2 = 2;

        int[][] a = {{1, 2, -2, 0}, {-3, 4, 7, 2}, {6, 0, 3, 1}};
        int[][] b = {{-1, 3}, {0, 9}, {1, -11}, {4, -5}};

        int[][] mult = matrixMult(a, b, r1, c1, c2);

        displayProduct(mult);
    }

    public static void displayProduct(int[][] product) {
        for (int[] row : product) {
            for (int column : row) {
                // print as a single line without
                // a line separator at the end
                System.out.print(column + "   ");
            }
            // print a line separator
            System.out.println();
        }
    }
}

输出：

-3   43   
18   -60   
1   -20   

---

^{另见：Matrix multiplication using streams}

Answer 2

变量声明在错误的类和方法中。以下声明：

int r1 = 3, c1 = 4;
int r2 = 4, c2 = 2;
int[][] a = {{1, 2, -2, 0}, {-3, 4, 7, 2}, {6, 0, 3, 1}};
int[][] b = {{-1, 3}, {0, 9}, {1, -11}, {4, -5}};

这些是在Main.main（主类，主方法）中声明的，而您试图在MatrixMult.matrixMult 中调用它们。相反，在matrixMult（方法）中声明它们或在 MatrixMult（类）中声明它们但将它们设为静态。

另一个问题是你写了matrixMult(a, b, r1, c1, c2);，当matrixMult只接受两个参数时，将a, b, r1, c1, c2作为参数传递：public static int[][] matrixMult(int[][] a, int[][] b)。

堆栈跟踪告诉您：它找不到symbol: method matrixMult(int[][],int[][],int,int,int。所以只需将public static int[][] matrixMult(int[][] a, int[][] b) 重写为public static int[][] matrixMult(int[][] a, int[][] b, int r1, c1, c2)。

请记住，matrixMult 并不“知道”这些变量是什么，因为它们是在不同的方法中声明的。只有当它们作为参数传入时它才知道它们。