Re: 2D lookup table

From:
"Daniel Pitts" <googlegroupie@coloraura.com>
Newsgroups:
comp.lang.java.programmer
Date:
31 Jan 2007 10:56:15 -0800
Message-ID:
<1170269775.116629.252280@v45g2000cwv.googlegroups.com>
On Jan 31, 9:16 am, "Mize-ze" <zahy.bn...@gmail.com> wrote:

If I want to store and fetch information from a matrix like structure
where the indexes of the matrix are not integers but other data
structures, how such thing can be done without a translation table?

Example:

If I have a list of doubles and I want to store a value per each pairs
of values
What is the best way to do this?

Thanks

Well, it kind of depends on the situation.
You can't reliably use doubles as a key to anything. A pair of doubles
is, *ahem*, doublely unreliable.
But, if you have a String as an x coordinate, and a String as the y
coordinate, then you have a few options.

If there is a finite set of x coordinates and a finite set of y
coordinates, and the matrix is "mostly" full, then you would be best
to use a translation table.
Map<String, Integer> xCoordinates;
Map<String, Integer> yCoordinates;

Object[][] matrix = new Object[yCoordinates.size()]
[xCoordinates.size()];

Alternatively, if your matrix is "mostly" empty, you may wish to use
nested maps:

Map<String, Map<String, Object>> matrix;

As another alternative, you may want to use a different "structure"
altogether:

final class Coordinate<X, Y> {
   final X x;
   final Y y;
   public Coordinate(X x, Y y) {
      this.x = x;
      this.y = y;
   }

   public boolean equals(Object o) {
     if (o == null || !(o instanceof Coordinate)) {
        return false;
     }
     Coordinate c = (Coordinate)o;
     return c.x.equals(x) && c.y.equals(y);
   }

   public int hashCode() {
      return x.hashCode() * 39 + y.hashCode();
   }
}

Map<Coordinate, Object> matrix;

Hope this helps,
Daniel.

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