Adding ArrayLists to another ArrayList in java

Question

I want to find the minimum and the maximum from a matrix from each column. My function

public static void findMinMax(double[][] matrix) {
 ArrayList<Double> column = new ArrayList<Double>();
 ArrayList<Double> minmax = new ArrayList<Double>();
 ArrayList<ArrayList<Double>> minMaxValues = new ArrayList<ArrayList<Double>>();
 for (int j=0;j<matrix[1].length;j++) {
 column.clear();
 minmax.clear();
 for (int i=0;i<matrix.length;i++) {
 column.add(matrix[i][j]);
 }
 double max = Collections.max(column);
 double min = Collections.min(column);
 minmax.add(min);
 minmax.add(max);
 try {
 System.out.println("adding to final minmax " + j);
 minMaxValues.add(minmax);
 System.out.println(minMaxValues);
 } catch (Exception ex) {
 System.out.println(ex.getMessage());
 }
 }
 System.out.println(minMaxValues);
 }

The problem is that at the last iteration it adds to minMaxValues only the last minmax ArrayList. Here is an example of the output

adding to final minmax 0
[[-3.5, 4.5]]
adding to final minmax 1
[[-3.5, 4.5], [-3.5, 4.5]]
adding to final minmax 2
[[-3.5, 4.5], [-3.5, 4.5], [-3.5, 4.5]]
adding to final minmax 3
[[-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2]]
adding to final minmax 4
[[-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2]]
adding to final minmax 5
[[-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2]]
adding to final minmax 6
[[-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2], [-3.5, 4.2]]
adding to final minmax 7
[[-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2]]
adding to final minmax 8
[[-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2]]
adding to final minmax 9
[[-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2], [-3.9, 4.2]]
adding to final minmax 10
[[-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5], [-3.9, 4.5]]

As you can see it works fine until minmax 10 where I don't understand what happens. Can anyone help me?

• Robby Cornelissen: So what does matrix[1].length evaluate to? 11?
• dres: Yes, matrix has 10 rows and 11 columns
• Sumit Singh: What is going wrong? your problem is not clear
• dres: @RobbyCornelissen I don't understand what you mean...Why it works fine until the last column and the adds only the last pair of minmax?
• Robby Cornelissen: See my answer. You're just storing 11 references to the same minmax list. The contents of the list itself are overwritten in every iteration.

Answer 1

You need to reinitialize ArrayList<Double> minmax for every iteration. Otherwise your result list will just contain 11 references to the same list, containing the last values that you put in.

You need to move

ArrayList<Double> minmax = new ArrayList<Double>();

inside your loop.

• dres: Thanks, that was the problem. I'm still trying to understand why :)
• Robby Cornelissen: Try to wrap your head around the fact that your minMaxValues just contains 11 references to the same minmax list. That list is in the state that you leave after the last iteration, hence containing the values of the last iteration.