A-M baby

Task/Matrix-arithmetic/C/matrix-arithmetic-1.c

@@ -0,0 +1,45 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+double det_in(double **in, int n, int perm)
+{
+	if (n == 1) return in[0][0];
+
+	double sum = 0, *m[--n];
+	for (int i = 0; i < n; i++)
+		m[i] = in[i + 1] + 1;
+
+	for (int i = 0, sgn = 1; i <= n; i++) {
+		sum += sgn * (in[i][0] * det_in(m, n, perm));
+		if (i == n) break;
+
+		m[i] = in[i] + 1;
+		if (!perm) sgn = -sgn;
+	}
+	return sum;
+}
+
+/* wrapper function */
+double det(double *in, int n, int perm)
+{
+	double *m[n];
+	for (int i = 0; i < n; i++)
+		m[i] = in + (n * i);
+
+	return det_in(m, n, perm);
+}
+
+int main(void)
+{
+	double x[] = {	0, 1, 2, 3, 4,
+			5, 6, 7, 8, 9,
+			10, 11, 12, 13, 14,
+			15, 16, 17, 18, 19,
+			20, 21, 22, 23, 24 };
+
+	printf("det:  %14.12g\n", det(x, 5, 0));
+	printf("perm: %14.12g\n", det(x, 5, 1));
+
+	return 0;
+}

Task/Matrix-arithmetic/C/matrix-arithmetic-2.c

@@ -0,0 +1,76 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <tgmath.h>
+
+void showmat(const char *s, double **m, int n)
+{
+	printf("%s:\n", s);
+	for (int i = 0; i < n; i++) {
+		for (int j = 0; j < n; j++)
+			printf("%12.4f", m[i][j]);
+			putchar('\n');
+	}
+}
+
+int trianglize(double **m, int n)
+{
+	int sign = 1;
+	for (int i = 0; i < n; i++) {
+		int max = 0;
+
+		for (int row = i; row < n; row++)
+			if (fabs(m[row][i]) > fabs(m[max][i]))
+				max = row;
+
+		if (max) {
+			sign = -sign;
+			double *tmp = m[i];
+			m[i] = m[max], m[max] = tmp;
+		}
+
+		if (!m[i][i]) return 0;
+
+		for (int row = i + 1; row < n; row++) {
+			double r = m[row][i] / m[i][i];
+			if (!r)	continue;
+
+			for (int col = i; col < n; col ++)
+				m[row][col] -= m[i][col] * r;
+		}
+	}
+	return sign;
+}
+
+double det(double *in, int n)
+{
+	double *m[n];
+	m[0] = in;
+
+	for (int i = 1; i < n; i++)
+		m[i] = m[i - 1] + n;
+
+	showmat("Matrix", m, n);
+
+	int sign = trianglize(m, n);
+	if (!sign)
+		return 0;
+
+	showmat("Upper triangle", m, n);
+
+	double p = 1;
+	for (int i = 0; i < n; i++)
+		p *= m[i][i];
+	return p * sign;
+}
+
+#define N 18
+int main(void)
+{
+	double x[N * N];
+	srand(0);
+	for (int i = 0; i < N * N; i++)
+		x[i] = rand() % N;
+
+	printf("det: %19f\n", det(x, N));
+	return 0;
+}