Small Steps III — Thinking with Programs

Ordering from largest to smallest

Order values from largest to smallest.

What you will learn

Changing the comparison rule changes the order without changing the procedure. Listing larger values first is descending order.

Try it

void small_main()
{
    Array<int> numbers = {7, 2, 9, 4, 5};

    for (int i = 0; i < numbers.length(); i = i + 1)
    {
        int largest = i;

        for (int j = i + 1; j < numbers.length(); j = j + 1)
            if (numbers[j] > numbers[largest])
                largest = j;

        int temp = numbers[i];
        numbers[i] = numbers[largest];
        numbers[largest] = temp;
    }

    for (int i = 0; i < numbers.length(); i = i + 1)
        print(numbers[i]);
}

Now find the largest remaining value. The comparison changes from < to >, and the output becomes 9, 7, 5, 4, 2.

You do not need to rebuild the whole loop to change sorting direction. Change which value is selected first.

Exercise

Modify Selection Sort to arrange values from largest to smallest.

void small_main()
{
    Array<int> numbers = {10, 3, 8, 1, 6};

    // Sort from largest to smallest.

    for (int i = 0; i < numbers.length(); i = i + 1)
        print(numbers[i]);
}

Hint

Find largest instead of smallest, and change the comparison to >.

void small_main()
{
    Array<int> numbers = {10, 3, 8, 1, 6};

    for (int i = 0; i < numbers.length(); i = i + 1)
    {
        int largest = i;
        for (int j = i + 1; j < numbers.length(); j = j + 1)
            if (numbers[j] > numbers[largest])
                largest = j;

        int temp = numbers[i];
        numbers[i] = numbers[largest];
        numbers[largest] = temp;
    }

    for (int i = 0; i < numbers.length(); i = i + 1)
        print(numbers[i]);
}