What is wrong with this Selection Sort Code?

Question

I was asked to make a selection sort algorithm, but it's not working and I don't know why. Here's the code:

    int count = 0;
    int count2;
    int min;
    int size = scan.nextInt();
    int temp = 0;
    int[] numbers = new int[size];

    while (count < size) {
        numbers[count] = scan.nextInt();
        count ++;
    }
    count = 0;
    while (count < size) {
        count2 = size;
        min = numbers[count];
        while (count < count2) {
            count2 --;
            if (numbers[count2] < numbers[min]) {
                min = count2;
            }
        }
        temp = numbers[temp];
        numbers[temp] = numbers[count];
        numbers[count] = temp;
        count ++;
    }

    count = 0;
    while (count < size) {
        System.out.println(numbers[count]);
        count ++;
    }   
}

Input: 10 1 0 2 9 3 8 4 7 5 6

Output: 1 2 9 8 3 3 8 4 7 4

Do you want a valuable tip for free? Use a debugger! Follow it and you'll better.. much better.. understand your code and your error. — Maroun
– Maroun, Commented Sep 17, 2013 at 13:56
It seems like for-loops would be better choices than most of those while-loops. — Bernhard Barker
– Bernhard Barker, Commented Sep 17, 2013 at 13:58

gifpif · Accepted Answer · 2013-09-17 14:10:09Z

1

In while loop you try to use numbers[0] element as the index of numbers[] numbers[min]

min = numbers[count]; \\min is value of first element of numbers[]
    while (count < count2) {
        count2 --;
        if (numbers[count2] < numbers[min]) {  \\ you try to use value of numbers[0] element as index of numbers[] aray.
            min = count2;
        }

replace if (numbers[count2] < numbers[min]) with if (numbers[count2] < min)

answered Sep 17, 2013 at 14:10

gifpif

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Comments

Cruncher · Accepted Answer · 2013-09-17 14:02:31Z

0

temp = numbers[temp];
numbers[temp] = numbers[count];
numbers[count] = temp;
count ++;

Take a second look at this code.

If you ran this where the first input was 11, it would error with index out of range.

In particular, what do you plan to accomplish with temp = numbers[temp]? You're assigning temp to essentially an arbitrary number, because numbers[0] could be anything.

answered Sep 17, 2013 at 14:02

Cruncher

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Comments

Bernhard Barker · Accepted Answer · 2013-09-17 14:05:19Z

0

The fact that you're getting duplicate values in your output (that aren't in your input) means that your swap doesn't work, thus:

temp = numbers[temp];
numbers[temp] = numbers[count];
numbers[count] = temp;

is wrong.

It should be min, not temp:

temp = numbers[min];
numbers[min] = numbers[count];
numbers[count] = temp;

answered Sep 17, 2013 at 14:05

Bernhard Barker

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Comments

Marc · Accepted Answer · 2013-09-17 14:29:11Z

0

Here is an algorithm that works and make use of a great functionality of for in java. To go through an array or a list you can do for (int current : array) This is far more convinient than using while or for like you do.

    int min;
    int mem = 0;
    int size = 11;
    int [] numbers = {10, 1, 0, 2, 9, 3, 8, 4, 7, 5, 6};

    for (int i=0; i<size-1; i++){
        min =i;
        for(int j=i+1; j<size; j++){
            if (numbers[j]<numbers[min]){
                min = j;
            }
        }

        // swap
        mem= numbers[i];
        numbers[i] = numbers[min];
        numbers[min] = mem;

    }

    for (int toPrint : numbers){
        System.out.println(toPrint);
    }

}

edited Sep 17, 2013 at 14:29

answered Sep 17, 2013 at 14:09

Marc

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