asked on

Find the top N values of a 2D array

Dear experts:
Basically I would like to implement the following method:

public List<SearchResults> FindTopValues(int[,] data, int howManyTops)
{
    List<SearchResults> results = new List<SearchResults>();
    //Your code goes here.
    return results;
}

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where

public class SearchResults
    {
        public int Row { get; set; }
        public int Col { get; set; }
        public int Value { get; set; }
    }

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For example given the data array
[ 0 3 2 1
4 2 1 5
1 6 2 1 ]

If I want to find the top 3 values, the method call will be:
FindTopValues(data, 3)
and the results will be: (row,col, value)
2,1,6
1,3,5
1,0,4

Further array details:
- Number of rows >= 5* Number of rows
- Values in array between 0 and 10000.

Thanks,
MAS
Code is in C# , but VB.NET/C++ answers are OK.

kaufmed

Using a LINQ approach you could do:

public List<SearchResults> FindTopValues(int[,] data, int howManyTops)
{
    List<SearchResults> results = new List<SearchResults>();
    int r = 0;
    int c = 0;
    int rowSize = data.GetUpperBound(1) + 1;    // 1 is 2nd dimension (i.e. column count); + 1 since GetUpperBound returns highest index, not # of items

    var rowColQuery = from int item in data
                      let row = (r / rowSize)
                      let column = (r++ % rowSize)    // Increment "r" here since it's the last place r is used, per iteration
                      select new SearchResults()
                      {
                          Row = row,
                          Col = column,
                          Value = item,
                      };

    var maxQuery = rowColQuery.OrderByDescending(sr => sr.Value)
                              .Take(howManyTops);

    results = maxQuery.ToList();

    return results;
}

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Fernando Soto

Hi mas_oz2003;

Here is another solution.

public List<SearchResults> FindTopValues(int[,] data, int howManyTops)
{
    List<SearchResults> results = new List<SearchResults>();
    
    for(int i = 0; i < data.GetUpperBound(1); i++ ){
        for(int j = 0; j <= data.GetUpperBound(1); j++ ){
            results.Add(new SearchResults() { Col = j, Row = i, Value = data[i,j]});
        }
    }    
    
    return results.OrderByDescending(r => r.Value).Take(howManyTops).ToList();
}

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kaufmed

@FenandoSoto

Shouldn't your first GetUpperBound call be looking at the first dimension (i.e. zero)?

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kaufmed

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SOLUTION

Fernando Soto

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