C# - Fastest Way To Sort Array Of Primitives And Track Their Indices

Question

I need a float[] to be sorted. And I need to know where the old indices are in new array. That's why I can't use Array.Sort(); or whatever. So I would like to write a function that sorts the array for me and remembers from what index it took each value:

float[] input  = new float[] {1.5, 2, 0, 0.4, -1, 96, -56, 8, -45};
// sort
float[] output; // {-56, -45, -1, 0, 0.4, 1.5, 2, 8, 96};
int[] indices; // {6, 8, 4, 2, 3, 0, 1, 7, 5};

Size of arrays would be around 500. How should I approach this ? What sorting algorithm etc.

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After solved: It always surprises me how powerful C# is. I didn't even though of it being able to do that task on it's own. And since I already heard that Array.Sort() is very fast I'll take it.

Answer 1

float[] input = new float[] { 1.5F, 2, 0, 0.4F, -1, 96, -56, 8, -45 };
int[] indices = new int[input.Length];
for (int i = 0; i < indices.Length; i++) indices[i] = i;
Array.Sort(input, indices);
// input and indices are now at the desired exit state

Basically, the 2-argument version of Array.Sort applies the same operations to both arrays, running the actual sort comparisons on the first array. This is normally used the other way around - to rearrange something by the desired indices; but this works too.

Answer 2

You can use the overload of Array.Sort() which takes TWO arrays, and sorts the second one according to how it sorted the first one:

float[] input  = new [] { 1.5f, 2, 0, 0.4f, -1, 96, -56, 8, -45 };
int[] indices = Enumerable.Range(0, input.Length).ToArray();
Array.Sort(input, indices);

Answer 3

You can create a new array of indices, and then sort both of them using Array.Sort and treating input as keys:

float[] input = new float[] { 1.5F, 2, 0, 0.4F, -1, 96, -56, 8, -45 };
int[] indicies = Enumerable.Range(0, input.Length).ToArray();
Array.Sort(input, indicies);

Answer 4

if you use linq:

    float[] input = new float[] { 1.5F, 2, 0, 0.4F, -1, 96, -56, 8, -45 };

    var result = input.Select(x => new { Value = x, Index = input.ToList().IndexOf(x)}).OrderBy(x => x.Value).ToList();

    // sort
    float[] output = result.Select(x => x.Value).ToArray();
    int[] indices = result.Select(x => x.Index).ToArray();

in results you got objects with values and their indexes.

Answer 5

A List<KeyValuePair<int,float>> and a custom sorter would also work. the key for each pair holds the original index.

    private void Form1_Load(object sender, EventArgs e)
    {           
        List<KeyValuePair<int,float>> data = new List<KeyValuePair<int,float>>
        {
             new KeyValuePair<int,float>(0,1.5f),
             new KeyValuePair<int,float>(1,2),
             new KeyValuePair<int,float>(2,0),
             new KeyValuePair<int,float>(3,0.4f),
             new KeyValuePair<int,float>(4,-1),
             new KeyValuePair<int,float>(5,96),
             new KeyValuePair<int,float>(6,-56),
             new KeyValuePair<int,float>(7,8),
             new KeyValuePair<int,float>(8,-45)
        };
        data.Sort(SortByValue);
        foreach (KeyValuePair<int, float> kv in data)
        {
            listBox1.Items.Add(kv.Key.ToString() + " - " + kv.Value.ToString());
        }


    }
    private int SortByValue(KeyValuePair<int, float> a, KeyValuePair<int, float> b)
    {
        return a.Value.CompareTo(b.Value);
    }