Selection Sort in AP CSA: Trace After Each Pass

Selection sort is one of two sorting algorithms tested on the 2026 AP CSA exam. You need to trace the array state after each pass, count comparisons, and recognize the algorithm from code. This page breaks down exactly how it works with the trace tables the exam expects.

🔧 How Selection Sort Works

Each pass through the array does two things:

1. Find: Scan the unsorted portion to find the index of the minimum value
2. Swap: Swap that minimum value with the first element of the unsorted portion

After pass k, the first k elements are in their final sorted positions and will never move again.

📋 Complete Trace Example

Sort the array {6, 3, 2, 5, 4, 1} using selection sort:

Notice pass 3: the minimum of the unsorted portion IS already in the correct position, so it “swaps with itself.” The swap still happens — the algorithm does not skip it.

💻 The Code (AP Exam Version)

public static void sort(int[] data)
{
    for (int j = 0; j < data.length - 1; j++)
    {
        int minIdx = j;
        for (int k = j + 1; k < data.length; k++)
        {
            if (data[k] < data[minIdx])  /* Compare values */
            {
                minIdx = k;
            }
        }
        int temp = data[minIdx];   /* Assign to temp */
        data[minIdx] = data[j];
        data[j] = temp;
    }
}

❌ Common Pitfalls

✏ Practice Questions

for (int j = 0; j < data.length - 1; j++)
{
    int maxIdx = j;
    for (int k = j + 1; k < data.length; k++)
    {
        if (data[k] > data[maxIdx])
            maxIdx = k;
    }
    int temp = data[maxIdx];
    data[maxIdx] = data[j];
    data[j] = temp;
}

❓ Frequently Asked Questions