Question

I’m trying to Populate an array of BitSets with information about an int array

    //ar is a SIZE*SIZE 1-D int array, SIZE is a constant

    //decare BitSet array
    BitSet bs[] = new BitSet[SIZE];

    //initialize BitSet array
    for (BitSet x:bs)
        x = new BitSet();

    //populate BitSet array
    for (int i = 0 ; i < ar.length ; i++)
        if (ar[i] > 0)
            bs[(i/SIZE)].set(SIZE-(i%SIZE));

The last line is what seems to be the problem but I can’t see anything wrong with the logic. Any ideas?

To make this a bit more clear, if I had an array like
0,3,4,5,0,1,0,0,1,2,7,0,2,3,0,3,2,0,0,0,2,1,5,8,0,0,0,0,0,1,0,0,0,6,0,0

I want to get a bitset array that could be represented as
011101,001110,110110,001111,000001,000100

Answer 1

You’re problem is a miss understanding of the enhanced for loop. Basically:

for (BitSet x:bs)
    x = new BitSet();

Is just a piece of syntatic sugar. The compiler expands it to the following:

for (int i = 0; i < bs.length; i++) {
    BitSet x = bs[i];
    x = new BitSet();
}

In this expansion it’s quite clear that you’re simply assigning the new bit set to x and throwing it away. You’re not actually modifying the array at all. You need to initialize the array as follows:

for (int i = 0; i < bs.length; i++) {
    bs[i] = new BitSet();
}