refactor: InverseOfMatrix (#5446)

refactor: InverseOfMatrix

Author: alxkm

src/main/java/com/thealgorithms/misc/InverseOfMatrix.java

@@ -1,57 +1,29 @@
 package com.thealgorithms.misc;
 
-import java.util.Scanner;
-
-/*
- * Wikipedia link : https://en.wikipedia.org/wiki/Invertible_matrix
- *
- * Here we use gauss elimination method to find the inverse of a given matrix.
- * To understand gauss elimination method to find inverse of a matrix:
- * https://www.sangakoo.com/en/unit/inverse-matrix-method-of-gaussian-elimination
- *
- * We can also find the inverse of a matrix
+/**
+ * This class provides methods to compute the inverse of a square matrix
+ * using Gaussian elimination. For more details, refer to:
+ * https://en.wikipedia.org/wiki/Invertible_matrix
  */
 public final class InverseOfMatrix {
     private InverseOfMatrix() {
     }
 
-    public static void main(String[] argv) {
-        Scanner input = new Scanner(System.in);
-        System.out.println("Enter the matrix size (Square matrix only): ");
-        int n = input.nextInt();
-        double[][] a = new double[n][n];
-        System.out.println("Enter the elements of matrix: ");
-        for (int i = 0; i < n; i++) {
-            for (int j = 0; j < n; j++) {
-                a[i][j] = input.nextDouble();
-            }
-        }
-
-        double[][] d = invert(a);
-        System.out.println();
-        System.out.println("The inverse is: ");
-        for (int i = 0; i < n; ++i) {
-            for (int j = 0; j < n; ++j) {
-                System.out.print(d[i][j] + "  ");
-            }
-            System.out.println();
-        }
-        input.close();
-    }
-
     public static double[][] invert(double[][] a) {
         int n = a.length;
         double[][] x = new double[n][n];
         double[][] b = new double[n][n];
         int[] index = new int[n];
+
+        // Initialize the identity matrix
         for (int i = 0; i < n; ++i) {
             b[i][i] = 1;
         }
 
-        // Transform the matrix into an upper triangle
+        // Perform Gaussian elimination
         gaussian(a, index);
 
-        // Update the matrix b[i][j] with the ratios stored
+        // Update matrix b with the ratios stored during elimination
         for (int i = 0; i < n - 1; ++i) {
             for (int j = i + 1; j < n; ++j) {
                 for (int k = 0; k < n; ++k) {
@@ -60,7 +32,7 @@ public static double[][] invert(double[][] a) {
             }
         }
 
-        // Perform backward substitutions
+        // Perform backward substitution to find the inverse
         for (int i = 0; i < n; ++i) {
             x[n - 1][i] = b[index[n - 1]][i] / a[index[n - 1]][n - 1];
             for (int j = n - 2; j >= 0; --j) {
@@ -73,19 +45,20 @@ public static double[][] invert(double[][] a) {
         }
         return x;
     }
-
-    // Method to carry out the partial-pivoting Gaussian
-    // elimination.  Here index[] stores pivoting order.
-    public static void gaussian(double[][] a, int[] index) {
+    /**
+     * Method to carry out the partial-pivoting Gaussian
+     * elimination.  Here index[] stores pivoting order.
+     **/
+    private static void gaussian(double[][] a, int[] index) {
         int n = index.length;
         double[] c = new double[n];
 
-        // Initialize the index
+        // Initialize the index array
         for (int i = 0; i < n; ++i) {
             index[i] = i;
         }
 
-        // Find the rescaling factors, one from each row
+        // Find the rescaling factors for each row
         for (int i = 0; i < n; ++i) {
             double c1 = 0;
             for (int j = 0; j < n; ++j) {
@@ -97,22 +70,23 @@ public static void gaussian(double[][] a, int[] index) {
             c[i] = c1;
         }
 
-        // Search the pivoting element from each column
-        int k = 0;
+        // Perform pivoting
         for (int j = 0; j < n - 1; ++j) {
             double pi1 = 0;
+            int k = j;
             for (int i = j; i < n; ++i) {
-                double pi0 = Math.abs(a[index[i]][j]);
-                pi0 /= c[index[i]];
+                double pi0 = Math.abs(a[index[i]][j]) / c[index[i]];
                 if (pi0 > pi1) {
                     pi1 = pi0;
                     k = i;
                 }
             }
-            // Interchange rows according to the pivoting order
-            int itmp = index[j];
+
+            // Swap rows
+            int temp = index[j];
             index[j] = index[k];
-            index[k] = itmp;
+            index[k] = temp;
+
             for (int i = j + 1; i < n; ++i) {
                 double pj = a[index[i]][j] / a[index[j]][j];
 

src/test/java/com/thealgorithms/misc/InverseOfMatrixTest.java

@@ -0,0 +1,28 @@
+package com.thealgorithms.misc;
+
+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 InverseOfMatrixTest {
+
+    @ParameterizedTest
+    @MethodSource("provideTestCases")
+    void testInvert(double[][] matrix, double[][] expectedInverse) {
+        double[][] result = InverseOfMatrix.invert(matrix);
+        assertMatrixEquals(expectedInverse, result);
+    }
+
+    private static Stream<Arguments> provideTestCases() {
+        return Stream.of(Arguments.of(new double[][] {{1, 0, 0}, {0, 1, 0}, {0, 0, 1}}, new double[][] {{1, 0, 0}, {0, 1, 0}, {0, 0, 1}}), Arguments.of(new double[][] {{4, 7}, {2, 6}}, new double[][] {{0.6, -0.7}, {-0.2, 0.4}}));
+    }
+
+    private void assertMatrixEquals(double[][] expected, double[][] actual) {
+        for (int i = 0; i < expected.length; i++) {
+            assertArrayEquals(expected[i], actual[i], 1.0E-10, "Row " + i + " is not equal");
+        }
+    }
+}