A-M baby

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;
+}