Here is my code:
package algorithms;
import Jama.Matrix;
import java.io.File;
import java.util.Arrays;
public class ThetaGetter {
//First column is one, second is price and third is BHK
private static double[][] variables = {
{1,1130,2},
{1,1100,2},
{1,2055,3},
{1,1047,2},
{1,1927,3},
{1,2667,3},
{1,1146,2},
{1,2020,3},
{1,1190,2},
{1,2165,3},
{1,1250,2},
{1,1185,2},
{1,2825,4},
{1,1200,2},
{1,1580,3},
{1,3200,3},
{1,715,1},
{1,1270,2},
{1,2403,3},
{1,1465,3},
{1,1345,2}
};
private static double[][] prices = {
{69.65},
{60},
{115},
{55},
{140},
{225},
{76.78},
{120},
{73.11},
{140},
{56},
{79.39},
{161},
{73.69},
{80},
{145},
{34.87},
{77.72},
{165},
{98},
{82}
};
private static Matrix X = new Matrix(variables);
private static Matrix y = new Matrix(prices);
public static void main(String[] args) {
File file = new File("theta.dat");
if(file.exists()){
System.out.println("Theta has already been calculated!");
return;
}
//inverse(Tra(X)*X)*tra(X)*y
Matrix transposeX = X.transpose();
Matrix inverse = X.times(transposeX).inverse();
System.out.println(y.getArray().length);
System.out.println(X.getArray().length);
Matrix test = inverse.times(transposeX);
Matrix theta = test.times(y);
System.out.println(Arrays.deepToString(theta.getArray()));
}
}
This algorithm basically tries to take housing prices and then get a few constants which are then used to guess prices of houses. However I am getting an exception on the line 'Matrix theta = test.times(y);' The error message is pretty much what's in the question. Is there some sort issue with the dimensions? Both of them have 21 items, so I don't know what's going on.
The mistake you are making is in the following line of code:
Matrix inverse = X.times(transposeX).inverse();
The formula you commented above is:
//inverse(Tra(X)*X)*tra(X)*y
but what you are actually calculating in code is: (X*Tra(X) instead of Tra(X)*X)
//inverse(X*Tra(X))*tra(X)*y
If the dimension of X is (m,n) where
and the dimension of Y is (m,1), using the multiplications you used above you will have the following:
inverse(X * Tra(X)) *Tra(X)*Y = inverse * Tra(X) * Y = result * y
inverse((m,n)(n,m))(n,m)*(m,1)= (m,m) * (n,m) => which results in the error because the inner dimensions for a matrix multiplication must be equal
What would fix your code would be replacing the following line:
Matrix inverse = X.times(transposeX).inverse();
with
Matrix inverse = transposeX.times(X).inverse();