How to print first five elements of an matrix in Java

My goal is to implement the following method in parallel:
public static double[][] parallelAddMatrix(double[][] a, double[][] b), then test my program on randomly generated two lists of size 2000 x 2000. Finally I have to output the first 5 elements of matrix a and matrix b, and also the first five elements of the result matrix, which is what I'm having trouble with.

This is the part of my code where I create the first and second matrix.

public static void main(String[] args) {
int var1, var2;
final int matrices = 2000;
// creates first matrix
double[][] matrixA = new double[matrices][matrices];
for(var1 = 0; var1 < matrixA.length; var1++)
for (var2 = 0; var2 < matrixA[var1].length; var2++)
matrixA[var1][var2] = 1;
// creates second matrix
double[][] matrixB = new double[matrices][matrices];
for (var1 = 0; var1 < matrixB.length; var1++)
for (var2 = 0; var2 < matrixB[var1].length; var2++)
matrixB[var1][var2] = 1;

And then later created a function to create the result matrix...

public static double[][] parallelAddMatrix( double [][] a, double[][] b) {
//creates output matrix
double[][] resultMatrix = new double[a.length][a[0].length];
RecursiveAction task = new multiProcess(a, b, resultMatrix);
ForkJoinPool joinPool = new ForkJoinPool();
joinPool.invoke(task);
return resultMatrix;
}

How can I print out the first five elements for each of the three matrices?

I've tried stuff for the first and second matrix such as initializing var3, then under the "matrixA(orB)[var1][var2] = 1;", I put

for (var3 = 0; var3 < 5; var3++) {
System.out.println(var3);
}

and also tried

for (var3 = 0; var3 < 5; var3++) {
System.out.print(matrixA[var1][var2] + "");
}
System.out.println();

Please help on this, and please tell where it would be placed for each one (I might have trouble with brackets).

You'll need a nested for loop to iterate through the matrix, and a counter to see how many entries you've printed. Let's start with the easiest part: iterating over the matrix. I'll assume that the matrix is simply called matrix.

for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        System.out.println(matrix[i][j]);
    }
}

You probably already figured that out. Now we need a counter to count how many times we've printed out an entry from the matrix.

int num_printed = 0;
for (int i = 0; i < matrix.length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        System.out.println(matrix[i][j]);
        num_printed ++;
    }
}

Ok. So now we need to stop once we've reached the end. We can't just use one break statement, because, we have two for loops.

int num_printed = 0;

for (int i = 0; i < matrix.length; i++) { // iterate over the rows
    for (int j = 0; j < matrix[i].length; j++) { // iterate over the columns
        if (num_printed == 5) { // if we've already printed five items, stop
            break;
        } else { // otherwise, print the next item
            System.out.println(matrix[i][j]);
            num_printed ++; // increment the counter
        }
    }

    if (num_printed == 5) { // so that we don't go to the next row
        break;
    }
}

It's worth noting that you could create your own separate method, and only use a return statement:

public void print_five_elements() {
    int num_printed = 0;
    for (int i = 0; i < matrix.length; i++) { // iterate over the rows
        for (int j = 0; j < matrix[i].length; j++) { // iterate over the columns
            if (num_printed == 5) { // if we've already printed five items, stop
                return;
            } else { // otherwise, print the next item
                System.out.println(matrix[i][j]);
                num_printed ++; // increment the counter
            }
        }
    }
}

More Specialized Approach

This approach allows you to use matrices that have less than five columns. However, since your matrix is 2000x2000, you could go for a much simpler approach. Use zero as the first index, and then just iterate up to five. Just keep in mind that this won't work if you have less than five columns:

public void print_five_elements_for_wide_matrix() {
    for (int i = 0; i < 5; i++) {
        System.out.println(matrix[0][i]);
    }
}

</details>



# 答案2
**得分**: 1

由于这些矩阵的大小为 `2000 x 2000`，您不需要嵌套循环来显示每个矩阵的前 `5` 个元素。

```java
int i;

//显示 matrixA 的前 5 个元素
for(i=0; i<5; i++) {
    System.out.print(matrixA[0][i] + " ");
}
System.out.println();

//显示 matrixB 的前 5 个元素
for(i=0; i<5; i++) {
    System.out.print(matrixB[0][i] + " ");
}
System.out.println();

double[][] result = parallelAddMatrix(matrixA, matrixB);
//显示 result 的前 5 个元素
for(i=0; i<5; i++) {
    System.out.print(result[0][i] + " ");
}
System.out.println();

需要注意的是，上述循环打印了每个矩阵第一行（即索引为 0 的行）的前 5 个元素。然而，如果您想要打印前 5 行的第一个元素，只需交换索引，例如：

System.out.println(matrixA[i][0] + " ");

int i;
//Display first 5 elements of matrixA
for(i=0; i<5; i++) {
System.out.print(matrixA[0][i] + " ");
}
System.out.println();
//Display first 5 elements of matrixB
for(i=0; i<5; i++) {
System.out.print(matrixB[0][i] + " ");
}
System.out.println();
double[][] result = parallelAddMatrix(matrixA, matrixB);
//Display first 5 elements of result
for(i=0; i<5; i++) {
System.out.print(result[0][i] + " ");
}
System.out.println();

System.out.println(matrixA[i][0] + " ");

答案3

得分: 0

尝试这个：

将第一组括号视为行，第二组括号视为列。

for (int row = 0; row < 5; row++) {
   for (int col = 0; col < 5; col++) {
     System.out.print(matrixA[row][col] + " ");
   }
   System.out.println();
}

由于“多维”数组实际上是数组的数组，如果您想要打印整个矩阵，可以像这样做：

for (double[] row : matrixA) {
   System.out.println(Arrays.toString(row));
}

因此，每行的长度可以不同。因此，您可能需要获取长度以按照最初的打印方式打印它们。

for (int row = 0; row < matrixA.length; row++) {
   for (int col = 0; col < matrixA[row].length; col++) {
       System.out.print(matrixA[row][col] + " " );
   }
}

不同长度的行在“2D”数组中被称为“ragged-arrays”（不规则数组）。

for (int row = 0; row < 5; row++) {
for (int col = 0; col < 5; col++) {
System.out.print(matrixA[row][col] + " ");
}
System.out.println();
}

for (double[] row : matrixA) {
System.out.println(Arrays.toString(row));
}

for (int row = 0; row < matrixA.length; row++) {
for (int col = 0; col < matrixA[row].length; col++) {
System.out.print(matrixA[row][col] + " " );
}
}