Sorting Arrays

COMP 1010 Home > Selection Sorting

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Selection Searching

Selection searches are often used in instances where the kth smallest item must be found. The easiest way to do this is to first sort the array into ascending (or descending) order, and then iterate through each item until the conditions have been satisfied.

The idea behind a selection sort is very simple: iterate through each element in the array, and for each element look at all remaining elements to see if there is something smaller that should be in this position. If there is, exchange the two values.

Example: sort this array

|30|10|20|40|60|

1) Outer loop: iterate through each element

i = 0 (30)

2) Inner loop: iterate through the remaining elements to find a smaller element that should be in this position

j = 1 (10)

3) Since the element at i is greater than the element at j, swap them - the array will now be:

|10|30|20|40|60|

4) Again with the inner loop, we will move to the next element and compare:

i = 0 (10), j = 2 (20)

Since all the other elements in the list are greater than the 0th (10), nothing else will happen for this outer loop.

Loop 2: In the outer loop, we will move to the next element (1) and loop through the remaining elements (starting at 2)

i = 1 (30), j = 2(20)

Since the element at i is greater than the element at j, swap them and continue, giving us this array (which is now sorted!)

|10|20|30|40|60|

Here is an example of a very simple selection sorting algorithm:

// Selection sort algorithm

public static void selectionSort(int[] haystack) {
    for (int i = 0; i < haystack.length - 1; i++) {
        for (int j = i + 1; j < haystack.length; j++) {
            if (haystack[i] > haystack[j]) {
                //... Exchange elements
                int temp = haystack[i];
                haystack[i] = haystack[j];
                haystack[j] = temp;
            }
        }
    }
}

After the list has been sorted, you can then iterate through the list simply using a for loop.

// selectionSearch - returns the index at which the value was found, or -1 if it was not found

public static int selectionSearch(int needle, int[] haystack) {
    for (int i = 0; i < haystack.length; i++) {
        if (needle == haystack[i]) {
            return i;  // if the value was found, return its index
        }
    }
    return -1;  // if the value has not been found return -1
}