Issue with multidimensional arrays/ arraylist of arrays

Question

I'm trying to write a program to print all possible combinations of 28 variables that can be either 1 or -1 using a somewhat roundabout method, but the program isn't working. Specifically, I'm using an arraylist to store the solutions, but the stored arrays seem to be changing without my telling them to. I also tried a multidimensional array with the same result. Any help would be appreciated.

Here's my code:

    int[] vals = new int [28]; 
    Arrays.fill(vals, 1);
    ArrayList <int[]> solutions = new  ArrayList<int[]> (756);
    long c=0; //counter
    int ns=0; //number of solutions found

    while (ns<756){
        c++;
        for(int i=0;i<28;i++){
            if (c%(i+1)==0){
                vals[i]*=-1;
            }
        }
        boolean unique = true;
        for(int i=0;i<ns;i++){
            if(Arrays.equals(vals, solutions.get(i)) ){
                unique = false;
            }
        }
        if(unique==true){
            solutions.add(vals);
            ns++;
        }
    }
    for(int i=0;i<756;i++){
        System.out.println( "Solution "+ i);
        for(int j=0;j<28;j++){
            System.out.println("1: " + solutions.get(i)[j]);
        }
        System.out.println();
    }

You're storing the same array over and over again, so of course any change you make through one "view" would be visible from the others. — awksp
– awksp, Commented Jun 23, 2014 at 22:30
An array in Java is an object, and therefore vals is an object reference. Every time you say solutions.add(vals), you are adding a reference to that same array to the ArrayList; you are not making a copy of the array. Look at the Arrays.copyOf methods. — ajb
– ajb, Commented Jun 23, 2014 at 22:33
Side note: don't use if(unique==true); instead use if(unique). — Carl Manaster
– Carl Manaster, Commented Jun 24, 2014 at 0:58

ajb · Accepted Answer · 2014-06-23 23:54:11Z

3

An array in Java is an object, and therefore vals is an object reference. Every time you say solutions.add(vals), you are adding a reference to that same array to the ArrayList; you are not making a copy of the array.

The Arrays class has some static copyOf methods to copy arrays for you. This should work, but I haven't tested it:

solutions.add(Arrays.copyOf(vals, vals.length));

answered Jun 23, 2014 at 23:54

ajb

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David Ehrmann · Accepted Answer · 2014-06-24 00:42:46Z

0

I'm trying to write a program to print all possible combinations of 28 variables that can be either 1 or -1.

final int vars = 28;

for (int i = 0; i < (1 << vars); i++) {
    int temp = i;
    for (int j = 0; j < vars; j++) {
        System.out.print((temp % 2 == 0 ? "-1" : "1"));
        System.out.print(' ');
        temp = temp / 2;
    }
    System.out.println();
}

answered Jun 24, 2014 at 0:42

David Ehrmann

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