diff --git a/src/main/java/com/thealgorithms/matrix/SolveSystem.java b/src/main/java/com/thealgorithms/matrix/SolveSystem.java new file mode 100644 index 000000000000..9e683bc4dc5c --- /dev/null +++ b/src/main/java/com/thealgorithms/matrix/SolveSystem.java @@ -0,0 +1,71 @@ +package com.thealgorithms.matrix; + +/** + * This class implements an algorithm for solving a system of equations of the form Ax=b using gaussian elimination and back substitution. + * + * @link Gaussian Elimination Wiki + * @see InverseOfMatrix finds the full of inverse of a matrice, but is not required to solve a system. + */ +public final class SolveSystem { + private SolveSystem() { + } + + /** + * Problem: Given a matrix A and vector b, solve the linear system Ax = b for the vector x.\ + *

+ * This OVERWRITES the input matrix to save on memory + * + * @param matrix - a square matrix of doubles + * @param constants - an array of constant + * @return solutions + */ + public static double[] solveSystem(double[][] matrix, double[] constants) { + final double tol = 0.00000001; // tolerance for round off + for (int k = 0; k < matrix.length - 1; k++) { + // find the largest value in column (to avoid zero pivots) + double maxVal = Math.abs(matrix[k][k]); + int maxIdx = k; + for (int j = k + 1; j < matrix.length; j++) { + if (Math.abs(matrix[j][k]) > maxVal) { + maxVal = matrix[j][k]; + maxIdx = j; + } + } + if (Math.abs(maxVal) < tol) { + // hope the matrix works out + continue; + } + // swap rows + double[] temp = matrix[k]; + matrix[k] = matrix[maxIdx]; + matrix[maxIdx] = temp; + double tempConst = constants[k]; + constants[k] = constants[maxIdx]; + constants[maxIdx] = tempConst; + for (int i = k + 1; i < matrix.length; i++) { + // compute multipliers and save them in the column + matrix[i][k] /= matrix[k][k]; + for (int j = k + 1; j < matrix.length; j++) { + matrix[i][j] -= matrix[i][k] * matrix[k][j]; + } + constants[i] -= matrix[i][k] * constants[k]; + } + } + // back substitution + double[] x = new double[constants.length]; + System.arraycopy(constants, 0, x, 0, constants.length); + for (int i = matrix.length - 1; i >= 0; i--) { + double sum = 0; + for (int j = i + 1; j < matrix.length; j++) { + sum += matrix[i][j] * x[j]; + } + x[i] = constants[i] - sum; + if (Math.abs(matrix[i][i]) > tol) { + x[i] /= matrix[i][i]; + } else { + throw new IllegalArgumentException("Matrix was found to be singular"); + } + } + return x; + } +} diff --git a/src/test/java/com/thealgorithms/matrix/SolveSystemTest.java b/src/test/java/com/thealgorithms/matrix/SolveSystemTest.java new file mode 100644 index 000000000000..c8d289bd8339 --- /dev/null +++ b/src/test/java/com/thealgorithms/matrix/SolveSystemTest.java @@ -0,0 +1,21 @@ +package com.thealgorithms.matrix; + +import static org.junit.jupiter.api.Assertions.assertArrayEquals; + +import java.util.stream.Stream; +import org.junit.jupiter.params.ParameterizedTest; +import org.junit.jupiter.params.provider.Arguments; +import org.junit.jupiter.params.provider.MethodSource; + +class SolveSystemTest { + + @ParameterizedTest + @MethodSource({"matrixGenerator"}) + void solveSystem(double[][] matrix, double[] constants, double[] solution) { + double[] expected = SolveSystem.solveSystem(matrix, constants); + assertArrayEquals(expected, solution, 1.0E-10, "Solution does not match expected"); + } + private static Stream matrixGenerator() { + return Stream.of(Arguments.of(new double[][] {{-5, 8, -4}, {0, 6, 3}, {0, 0, -4}}, new double[] {38, -9, 20}, new double[] {-2, 1, -5}), Arguments.of(new double[][] {{-2, -1, -1}, {3, 4, 1}, {3, 6, 5}}, new double[] {-11, 19, 43}, new double[] {2, 2, 5})); + } +}