Selection Sort and In-Place Swap

Selection sort sorts an array by repeatedly finding the smallest remaining element and swapping it into the next sorted slot. After pass i, the first i + 1 elements are in their final positions.

The in-place swap uses a temporary variable to exchange two array positions without losing either value. It's a building block for sorting, shuffling, and many other algorithms.

On the AP exam, selection sort is one of two required sorting algorithms — and tracing it pass by pass is a common MCQ pattern.

Selection sort

public void selectionSort(int[] arr) {
    for (int i = 0; i < arr.length - 1; i++) {
        int minIndex = i;
        for (int j = i + 1; j < arr.length; j++) {
            if (arr[j] < arr[minIndex]) {
                minIndex = j;
            }
        }
        swap(arr, i, minIndex);
    }
}

The swap helper

private void swap(int[] arr, int a, int b) {
    int temp = arr[a];
    arr[a] = arr[b];
    arr[b] = temp;
}

The temporary variable holds one value while the other position is overwritten. Without it, one of the values is lost.

Outer loop stops at length - 1

When only one element remains unsorted, it's automatically in the right place. The last iteration is unnecessary.