二进制搜索多维数组中的第一个元素

Question

My goal is to perform a binary search for only the first element in a 2D array. I have been searching all day to find if it is possible using BinarySearch() in .NET but I can't find a thing.

To make this clearer. Imagine I had a 1D array, unsorted. If I sort the array, I lose the original index. I would like to create a second element of my array to hold the original index (this I can do) then sort by first element, then binary search over the first elements.

If anyone could push me in the right direction I'd be very grateful. Thanks

Answer 1

Well, if I understand you correctly, you need something like this:

// initialize the array and the indexes array
var a2D = new int[2][];
a2D[0] = new[] { 3, 14, 15, 92, 65, 35 }; // <-- your array (fake data here)
a2D[1] = Enumerable.Range(0, a2D[0].Length).ToArray(); // create the indexes row

// sort the first row and the second one containing the indexes
Array.Sort(a2D[0], a2D[1]);

// now a2D array contains:
//  row 0: 3, 14, 15, 35, 65, 92
//  row 1: 0,  1,  2,  5,  4,  3

// and you can perform binary search on the first row:
int columnIndexOf35 = Array.BinarySearch(a2D[0], 35);
// columnIndexOf35 = 3
// 
// a2D[0][columnIndexOf35] = 35 <- value
// a2D[1][columnIndexOf35] = 5  <- original index

Answer 2

As per MSDN , Array.BinarySearch method operates only with one-dimensional arrays, so it is impossible to use it directly in your case. Some of the options you have are:

1. Extract first column into a separate array and call Array.BinarySearch on it.
2. Define custom class Pair that implements interface IComparable and construct your array with the instances of this class.
3. Implement binary search on two dimensional array by yourself.

Answer 3

It looks like you want to have object that holds data and "original index" and than sort/search array of objects by data.

(This answer shows Andrei's option 2)

class IndexedData:IComparable
{
  public MyType Data;
  public int OriginalIndex;

  public int CompareTo(object obj) {
    // add correct checks for null,.. here
    // and return correct comparison result. 
    // I.e. if MyType is IComparable - just delegate.
    return Data.CompareTo(obj);
}

Check IComparable on MSDN for implementation/usage details.

Answer 4

Depending on what you're planning to do with the arrays afterwards, another solution might be to use LINQ.

var unsortedStartingArray = new[] {3, 6, 2, 1, 20, 20};
var q = unsortedStartingArray
        .Select((item, index) => new {item, index})
        .ToLookup(x => x.item, x => x.index);

var notFound = q[30]; // An empty array. Nothing found
var indexOf1 = q[1].First(); // returns 3
var multipleIndexsOf20 = q[20]; // Returns an array with 4, 5

The index into the lookup would then be the value you're searching for. Performance wise I would guesstimate this to be faster aswell about 5 times slower from my crude testing.