Doubling a Matrix

Question

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,

Answer 1

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> startingMatrix = Arrays.asList(
        Arrays.asList(3, 4),
        Arrays.asList(2, 1)
    );

    List> doubleMatrix
        = startingMatrix.stream()
                        .map(innerList -> { //For each list
                          List 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);
  }
}

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.