Question

I’m trying to turn an array like this:

0, 1, 2, 2, 1, 0, 1, 0, 0, 1, 2

into this:

0 0 0 0 1 1 1 1 2 2 2

Here is my code:

public static int[] sortDNF(int[] tape) {
    int smaller = 0; // everything with index < smaller is 0
    int bigger = tape.length - 1; // everything with index > bigger is 2
    int current = 0; // where we are looking now
    int tmp;

    while (current <= bigger) {
        if (tape[current] == 0) {
            tmp = tape[smaller];
            tape[smaller] = tape[current];
            tape[current] = tmp;
            smaller++;
        }
        if (tape[current] == 2) {
            tmp = tape[bigger];
            tape[bigger] = tape[current];
            tape[current] = tmp;
            bigger--;
        }
        current++;
    }

    return tape;
}

This is what it produces:

 0  0  0  1  1  1  1  0  2  2  2 

What is my problem?

Answer 1

A guess:

You should not be increasing current every time through the loop since that is supposed to represent the partition between the 1’s and the unknowns. For example, when you hit the first 2 you end up swapping it with another 2 and then moving on. The fact that the 2 later gets swapped for a 0 is accidental. The elements between smaller and current should always be 1’s and that is broken.

current++ should only be done in the tape[current] != 2 case. It’s ok to do it when tape[current] = 0 because you haven’t changed the [smaller -> current] = 1-only condition. And it’s ok to move it when tape[current] = 1 because that satisfies the 1-only condition.

…but I haven’t tried it.