Trying to swap values in an array in Java

Question

I use the selection method to solve this problem, but I meet some problems. After seeing answer for it, I found two bugs in my program and then fix it. I am confused about why these two bugs occur, can anyone give me a hand to explain them?

Here is my code:

import java.util.Scanner;
public class JavaTest{
    public static void main(String[] args){
    Scanner input = new Scanner(System.in);
    int[] myList = new int[5];
    for (int i = 0; i < 5; i++){
        myList[i] = input.nextInt();
    }

    for (int i = 0; i < 4; i++){
        int j = 0;

        // Why do I need to put this outside inner for loop?
        int smallest = myList[i];

        // Why do I need to give the value of i to index after i increase by 1?
        int index = i;

        for (j = i + 1; j < 5; j++){
            if (smallest > myList[j]){
                smallest = myList[j];
                index = j;
            }
        }

        if (index != i){
            myList[index] = myList[i];
            myList[i] = smallest;
        }

    }

    for (int i = 0; i < 5; i++){
        System.out.print(myList[i] + " ");
    }

    input.close();
}
}

@immibis he wrote the bugs in the comments inside his code.... — Old Fox
– Old Fox, Commented Mar 23, 2015 at 23:45
@immibis from his questions it's clearly that "int smallest = myList[i];" was inside the inner loop and "int index = i;" wasn't changed after each iteration — Old Fox
– Old Fox, Commented Mar 23, 2015 at 23:52

Old Fox · Accepted Answer · 2015-03-24 00:26:49Z

1

Selection algorithm is an algorithm for finding the kth smallest number in a list or array.

Each iteration your algorithm find the smallest number i to myList.length:

First iteration is between 0 to 4

Second iteration is between 1 to 4 and etc...

    // Why do I need to put this outside inner for loop?
    int smallest = myList[i];

After the first iteration the value of "smallest" will be the real smallest number in the list instead of the smallest in indexes 1 to 4, and then the inner loop won't do anything.

    // Why do I need to give the value of i to index after i increase by 1?
    int index = i;

you initialize the index to the current position of your (suspected)smallest number. if the current value of "smallest" is the smallest number in the iteration then the second "for loop" won't do anything.

and then the condition:

(index != i)

will be true even though it shouldn't

edited Mar 24, 2015 at 0:26

answered Mar 23, 2015 at 23:43

Old Fox

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