Java ArrayList String Selection Sort

user_2951723 0
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I'm struggling mightly on doing selection sort on an ArrayList of Strings to alphabetize them. I have no idea what I'm doing wrong. But its just not working properly for me. Heres my code.

    ArrayList<String> list = new ArrayList<String>();
    list.add("a");
    list.add("d");
    list.add("f");
    list.add("c");
    System.out.println(list);
    int i;
    int j;
    int minValue;
    int minIndex;

    for (i=0; i<list.size(); i++) {
        System.out.println(list.get(i));
        char iLetter = (list.get(i).charAt(0));
        int iValue = (int) iLetter;
        minValue = iValue;
        minIndex = i;
        for(j=i; j<list.size(); j++) {
            char jLetter = list.get(j).charAt(0);
            int jValue = (int) jLetter;
            if (jValue < minValue) {
                minValue = jValue;
                minIndex = j;
            }
        }
        if(minValue < iValue) {
            int temp = iValue;
            char idx = list.get(minIndex).charAt(0);
            int idxValue = (int) idx;
            iValue = idxValue;
            idxValue = temp;

        }
    }
    System.out.println(list);
}

It still prints it out as ["a", "d", "f", "c"]

selection-sort arraylist java
2 Answers

You are not updating your list anywhere in your loop, so it remains unsorted.

In order to actually swap elements of the list, replace:

if(minValue < iValue) {
    int temp = iValue;
    char idx = list.get(minIndex).charAt(0);
    int idxValue = (int) idx;
    iValue = idxValue;
    idxValue = temp;
}

with:

if(minValue < iValue) {
    Collections.swap (list, i, minIndex);
}

Collections.swap performs the following modification:

list.set(i, list.set(minIndex, list.get(i)));

Now the output will be

[a, c, d, f]
user_12215710

As mentioned, you need to do the actual swapping in the list, not just the temporary variables (doh!).

public static void main(String[] args) {

    List<String> list = new ArrayList<>();
    list.add("a");
    list.add("d");
    list.add("f");
    list.add("c");

    System.out.println(list);

    for (int i = 0; i < list.size(); i++) {
        String smallest = list.get(i);
        int smallestIndex = i;
        for (int j = i; j < list.size(); j++) {
            String value = list.get(j);
            if (value.compareTo(smallest) < 0) {
                smallest = value;
                smallestIndex = j;
            }
        }

        if (smallestIndex != i) {
            String head = list.get(i);
            list.set(i, smallest);
            list.set(smallestIndex, head);
        }
    }

    System.out.println(list);
}

Additionally, your code is just a single method, AKA spaghetti code. To make it more object-oriented you could make the following changes.

import java.util.*;

public class SelectionSort<T extends Comparable> {

    private List<T> values;

    public SelectionSort(List<T> values) {
        this.values = values;
    }

    private void sort() {
        for (int headIndex = 0; headIndex < values.size(); headIndex++) {
            sortFrom(headIndex);
        }
    }

    private void sortFrom(int headIndex) {
        int smallestIndex = findSmallestFrom(headIndex);
        if (smallestIndex != headIndex) {
            swap(headIndex, smallestIndex);
        }
    }

    private int findSmallestFrom(int i) {
        int smallestIndex = i;
        T smallest = values.get(i);
        for (int j = i; j < values.size(); j++) {
            T value = values.get(j);
            if (value.compareTo(smallest) < 0) {
                smallest = value;
                smallestIndex = j;
            }
        }
        return smallestIndex;
    }

    private void swap(int i, int j) {
        T head = values.get(i);
        values.set(i, values.get(j));
        values.set(j, head);
    }

    public static void main(String[] args) {

        List<String> values = createTestData();
        System.out.println(values);

        SelectionSort selectionSort = new SelectionSort<>(values);
        selectionSort.sort();

        System.out.println(values);
    }

    private static List<String> createTestData() {
        List<String> values = new ArrayList<>();
        values.add("a");
        values.add("d");
        values.add("f");
        values.add("c");
        return values;
    }
}

Some of the changes I made:

  • Separate method for creation of test data
  • Separate method for printing the before and after state of the list and calling the sort
  • Create an instance instead of only static code
  • separate iterations and logic into meaningful methods
  • Rename the 'list' variable to 'values'. The fact that it's a list is already clear. The convention is to name a collection by the meaning of the data it contains
  • Introduced a generic type variable on the class (<T extends Comparable>). This allows any type of data to be sorted, as long as it implements the Comparable interface
user_1813360
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