## Wednesday, September 17, 2014

### Selection Sort Implementation [Java]

Selection sort is a sorting algorithm, specifically an in-place comparison sort. It has O(n2) time complexity, making it inefficient on large lists, and generally performs worse than the similar insertion sort. Selection sort is noted for its simplicity, and it has performance advantages over more complicated algorithms in certain situations, particularly where auxiliary memory is limited.

The algorithm divides the input list into two parts: the sublist of items already sorted, which is built up from left to right at the front (left) of the list, and the sublist of items remaining to be sorted that occupy the rest of the list. Initially, the sorted sublist is empty and the unsorted sublist is the entire input list. The algorithm proceeds by finding the smallest (or largest, depending on sorting order) element in the unsorted sublist, exchanging it with the leftmost unsorted element (putting it in sorted order), and moving the sublist boundaries one element to the right.
Here is an example of this sort algorithm sorting five elements:
```64 25 12 22 11 // this is the initial, starting state of the array

11 25 12 22 64 // sorted sublist = {11}

11 12 25 22 64 // sorted sublist = {11, 12}

11 12 22 25 64 // sorted sublist = {11, 12, 22}

11 12 22 25 64 // sorted sublist = {11, 12, 22, 25}

11 12 22 25 64 // sorted sublist = {11, 12, 22, 25, 64}
```

Code:
```import java.util.Arrays;

public class SelectionSort {

public static int getMinIndex(int[] array, int from){
int min = Integer.MAX_VALUE, minIndex = -1;
for(int i=from;i<array.length;++i){
if(array[i]<min){
min = array[i];
minIndex = i;
}
}
return minIndex;
}
public static void selectionSort(int[] array){
for(int i=0;i<array.length;++i){
int minIndex = getMinIndex(array, i);
int temp = array[i];
array[i] = array[minIndex];
array[minIndex] = temp;
}
}
public static void main(String[] args){
int arr[] = {5, 4, 3, 2, 1};
selectionSort(arr);
System.out.println(Arrays.toString(arr));
}
}```

Output:
[1, 2, 3, 4, 5]
as expected!