加倍矩阵 [英] Doubling a Matrix

问题描述

我创建了一个二维矩阵，并用随机数填充它。然后我把它打印出来了。我需要帮助创建第二个矩阵，其大小是第一个矩阵的两倍，并且填充了第一个矩阵（现在是2x2）。例如：

I've created a two dimensional matrix and populated it with random numbers. I've then printed it out. I need help creating a second matrix that is twice the size of the first, and is populated with the numbers from the first one (which are now 2x2). For example:

起始矩阵：

3   4  
2   1 

Doubled Matrix：

Doubled Matrix:

3 3 4 4

3 3 4 4

2 2 1 1 

2 2 1 1

推荐答案

在Java 8中，您可以使用地图轻松处理收藏家。以下是一个完整的示例：

In Java 8 you can handle this pretty easily using maps and collectors. Here is a full example:

public class DoubleMatrix {
  public static void main(String[] args) {
    List<List<Integer>> startingMatrix = Arrays.asList(
        Arrays.asList(3, 4),
        Arrays.asList(2, 1)
    );

    List<List<Integer>> doubleMatrix
        = startingMatrix.stream()
                        .map(innerList -> { //For each list
                          List<Integer> doubled = innerList.stream()
                                                           .map(element -> Arrays.asList(element, element)) //Return a list doubling each element
                                                           .flatMap(l -> l.stream())  //Flatten out/join all doubled lists
                                                           .collect(Collectors.toList());
                          return Arrays.asList(doubled, doubled); //Double the list
                        })
                        .flatMap(l -> l.stream())
                        .collect(Collectors.toList()); //Collect into a final list

    System.out.println(doubleMatrix);
  }
}

这样可以避免事先知道列表的大小，并且也容忍矩阵的宽度和高度之间存在差异 - 只需将两个方向上的每个元素加倍。

This avoids needing to know the size of the list beforehand, and is also tolerant of there being a difference between the width and height of your matrix - simply doubling every element in both directions.

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