diff --git a/arithmetic_analysis/gaussian_elimination.py b/arithmetic_analysis/gaussian_elimination.py new file mode 100644 index 000000000..51207686c --- /dev/null +++ b/arithmetic_analysis/gaussian_elimination.py @@ -0,0 +1,83 @@ +""" +Gaussian elimination method for solving a system of linear equations. +Gaussian elimination - https://en.wikipedia.org/wiki/Gaussian_elimination +""" + + +import numpy as np + + +def retroactive_resolution(coefficients: np.matrix, vector: np.array) -> np.array: + """ + This function performs a retroactive linear system resolution + for triangular matrix + + Examples: + 2x1 + 2x2 - 1x3 = 5 2x1 + 2x2 = -1 + 0x1 - 2x2 - 1x3 = -7 0x1 - 2x2 = -1 + 0x1 + 0x2 + 5x3 = 15 + >>> gaussian_elimination([[2, 2, -1], [0, -2, -1], [0, 0, 5]], [[5], [-7], [15]]) + array([[2.], + [2.], + [3.]]) + >>> gaussian_elimination([[2, 2], [0, -2]], [[-1], [-1]]) + array([[-1. ], + [ 0.5]]) + """ + + rows, columns = np.shape(coefficients) + + x = np.zeros((rows, 1), dtype=float) + for row in reversed(range(rows)): + sum = 0 + for col in range(row + 1, columns): + sum += coefficients[row, col] * x[col] + + x[row, 0] = (vector[row] - sum) / coefficients[row, row] + + return x + + +def gaussian_elimination(coefficients: np.matrix, vector: np.array) -> np.array: + """ + This function performs Gaussian elimination method + + Examples: + 1x1 - 4x2 - 2x3 = -2 1x1 + 2x2 = 5 + 5x1 + 2x2 - 2x3 = -3 5x1 + 2x2 = 5 + 1x1 - 1x2 + 0x3 = 4 + >>> gaussian_elimination([[1, -4, -2], [5, 2, -2], [1, -1, 0]], [[-2], [-3], [4]]) + array([[ 2.3 ], + [-1.7 ], + [ 5.55]]) + >>> gaussian_elimination([[1, 2], [5, 2]], [[5], [5]]) + array([[0. ], + [2.5]]) + """ + # coefficients must to be a square matrix so we need to check first + rows, columns = np.shape(coefficients) + if rows != columns: + return [] + + # augmented matrix + augmented_mat = np.concatenate((coefficients, vector), axis=1) + augmented_mat = augmented_mat.astype("float64") + + # scale the matrix leaving it triangular + for row in range(rows - 1): + pivot = augmented_mat[row, row] + for col in range(row + 1, columns): + factor = augmented_mat[col, row] / pivot + augmented_mat[col, :] -= factor * augmented_mat[row, :] + + x = retroactive_resolution( + augmented_mat[:, 0:columns], augmented_mat[:, columns : columns + 1] + ) + + return x + + +if __name__ == "__main__": + import doctest + + doctest.testmod()