使用插入排序对随机数数组进行排序

Question

Trouble Shooting My insertionSort

Here is my code...

        ArrayList<Integer> num2 = new ArrayList<Integer>();
        for (int b = 0; b < 300; b++)
        {
            num2.add((int)(Math.random() * 1000));
        }

        if (choice == 2)
        {
            System.out.println("Insertion Sort chosen\nUnsorted Array List: " + num2);
            insertionSort(num2);
            System.out.println("Insertion sorted Array List: " + num2);
        }//choice 2 end

public static void insertionSort(ArrayList<Integer> num2)
{
    for(int f = 1; f < num2.size(); f++)
    {
        int key = num2.get(f);
        for (int g = f-1; g >= 0; g--)
        {
            if (key < num2.get(g))
            {
                num2.set(g+1, num2.get(g));
                if (g == 0)
                {
                    num2.set(0, key);
                }
            }
            else
            {
                num2.set(g+1, key);
            }
        }
    }
}

Output:
Insertion Sort chosen
Unsorted Array List: [42, 944, 500, 442, 277, 178, 881, 267, 705, 365, 481, 882, 221, 835, 329, 312, 97, 1,...
Insertion sorted Array List: [1, 479, 479, 479, 479, 718, 785, 785, 988, 988, 988, 988, 988, 988, 988, 988, 988,...


if anyone can help me figure out why a bunch of the larger numbers just start repeating themselves because it is frustrating.

Answer 1

When you cannot find where the problems is, you can make use of the breakpoint to help you. Set one or several breakpoints where you think the codes here are critical. For example, when you debug your insertionSort function, you can set your breakpoint at Line 5 to see what changes the inner loop have made on your array. And you will find that the inner loop have done something outside your expectation. Then you can continue to set the breakpoint inside the inner loop to see where the problem is.

Actually, your problem is that when you find the right position to put your key , you should break the inner loop. Otherwise, since the key is larger than all the numbers before the proper position, all the numbers before except the first one (since your code is num2.set(g+1, key); , and when g = 0, it changes the second element) will be replaced by the key .

So, you only need to add a break .

public static void insertionSort(ArrayList<Integer> num2)
{
    for(int f = 1; f < num2.size(); f++)
    {
        int key = num2.get(f);
        for (int g = f-1; g >= 0; g--)
        {
            if (key < num2.get(g))
            {
                num2.set(g+1, num2.get(g));
                if (g == 0)
                {
                    num2.set(0, key);
                }
            }
            else
            {
                num2.set(g+1, key);
                break;
            }
        }
    }
}