#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;
}