Thread: gauss elimination method not working

the gauss elimination function seems to be not working, I cannot find the mistake.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
void inverse(int r, int c, int* matrix[]);
void vectorb(int r);
void gauss(int r, int c, int *A[]);
void gauss(int r, int c, int *a[]);


int main()
{
	srand(time(NULL));
	int r;
	int c;
	printf("define the size of the matrix\n number of rows\n");
	scanf_s("%d", &r);
	printf("number of columns\n");
	scanf_s("%d", &c);
	int **matrix = (int**)malloc(r * sizeof(*matrix));
	for (int i = 0; i < r; i++)
	{
		matrix[i] = (int*)malloc(c * sizeof(*matrix[r]));
	}


	for (int i = 0; i < r; ++i)
	{
		for (int j = 0; j < c; ++j)
		{
			matrix[i][j] = rand() % 21 - 10;
			printf("%d     ", matrix[i][j]);




		}


		printf("\n");


	}
	inverse(r, c, matrix);
	printf("\nvector is equal to \n");
	vectorb(r);
	gauss(r, c, matrix);
	
}




void inverse(int r, int c, int* matrix[])
{




	double determinant = 0;
	for (int i = 0; i<r; i++)
		determinant = determinant + (matrix[0][i] * (matrix[1][(i + 1) % r] * matrix[2][(i + 2) % r] - matrix[1][(i + 2) % r] * matrix[2][(i + 1) % r]));


	printf("\nInverse of matrix is: \n\n");
	for (int j = 0; j<r; j++){
		for (int i = 0; i<r; i++)


			printf("%.2f\t", (((matrix[(i + 1) % r][(j + 1) % r] * matrix[(i + 2) % r][(j + 2) % r]) - (matrix[(i + 1) % r][(j + 2) % r] * matrix[(i + 2) % r][(j + 1) % r])) / determinant));
		printf("\n");


	}


}
void vectorb(int r)
{
	int *vector = (int*)malloc(r * sizeof(*vector));
	for (int i = 0; i < r; i++)
	{
		vector[i] = (int)malloc(r * sizeof(*vector));


	}


	for (int i = 0; i < r; ++i)
	{


		vector[i] = rand() % 21 - 10;
		printf("%d     ", vector[i]);
		printf("\n");
	}


}
void gauss(int r, int c, int *a[])
{
	for (int k = 0; k<r - 1; k++)
	{
		for (int i = k + 1; i<r; i++)
		{
[I]			double p = a[k] / a[k][k];
			for (int j = k; j<r + 1; j++)
[I]				a[i][j] = a[j] - p*a[k][j];
		}
	}
	double *x = (double*)malloc(r * sizeof(*x));
	for (int i = 0; i < r; i++)
	{
[I]		x = (int)malloc(r * sizeof(*x));

	}

	x[r - 1] = a[r - 1][r] / a[r - 1][r - 1];
	for (int i = r - 2; i >= 0; i--)
	{
		int s = 0;
		for (int j = i + 1; j<r; j++)
		{
[I]			s += (a[j] * x[j]);
[I]			x[i] = (a[i][r] - s) / a[i];
		}
	}
	printf("\nThe result is:\n");
	for (int i = 0; i<r; i++)
[I]		printf("\nx%d=%lf", i + 1, x);
}

you are right, I think instead of using the matrix variable in the gauss function I should create another function that creates the augmented matrix (adding the vector b to the matrix) and then use it in the gauss function. Any idea how can I create this augmented matrix?

I made this modification in my code, I added the new function and I now I just need to use the function matrix_augmented into the gauss function, is that right?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
void inverse(int r, int c, int* matrix[]);
void matrix_augmented(int *a[], int *v, int r, int c);
void gauss(int r, int c, int *a[]);


int main()
{
	srand(time(NULL));
	int r;
	int c;
	printf("define the size of the matrix\n number of rows\n");
	scanf_s("%d", &r);
	printf("number of columns\n");
	scanf_s("%d", &c);
	int **matrix = (int**)malloc(r * sizeof(*matrix));
	int* vector = (int*)malloc(r*sizeof(*vector));
	for (int i = 0; i < r; i++)
	{
		matrix[i] = (int*)malloc(c * sizeof(*matrix[r]));


	}


	matrix_augmented(matrix, vector, r, c);
	
	inverse(r, c, matrix);
	gauss(r, c, matrix);
	
}




void inverse(int r, int c, int* matrix[])
{




	double determinant = 0;
	for (int i = 0; i<r; i++)
		determinant = determinant + (matrix[0][i] * (matrix[1][(i + 1) % r] * matrix[2][(i + 2) % r] - matrix[1][(i + 2) % r] * matrix[2][(i + 1) % r]));


	printf("\nInverse of matrix is: \n\n");
	for (int j = 0; j<r; j++){
		for (int i = 0; i<r; i++)


			printf("%.2f\t", (((matrix[(i + 1) % r][(j + 1) % r] * matrix[(i + 2) % r][(j + 2) % r]) - (matrix[(i + 1) % r][(j + 2) % r] * matrix[(i + 2) % r][(j + 1) % r])) / determinant));
		printf("\n");


	}


}


void matrix_augmented(int *a[],int *v,int r,int c)
{
for (int i = 0; i < r; ++i)
{
for (int j = 0; j < c; ++j)
{
a[i][j] = rand() % 21 - 10;
printf("%d     ", a[i][j]);
v[i] = rand() % 21 - 10;




}
printf("%d", v[i]);
printf("\n");


}
}
void gauss(int r, int c, int *a[])
{
	for (int k = 0; k<r - 1; k++)
	{
		for (int i = k + 1; i<r; i++)
		{
			double p = a[i][k] / a[k][k];
			for (int j = k; j<r + 1; j++)
				a[i][j] = a[i][j] - p*a[k][j];
		}
	}
	double *x = (double*)malloc(r * sizeof(*x));
	
	x[r - 1] = a[r - 1][r] / a[r - 1][r - 1];
	for (int i = r - 2; i >= 0; i--)
	{
		int s = 0;
		for (int j = i + 1; j<r; j++)
		{
			s += (a[i][j] * x[j]);
			x[i] = (a[i][r] - s) / a[i][i];
		}
	}
	printf("\nThe result is:\n");
	for (int i = 0; i<r; i++)
		printf("\nx%d=%lf", i + 1, x[i]);
}

I think I did not understand what I am supposed to do with the +/-1.

My advice on coding numerics is to first do it in an interactive language like MATLAB, then port to C.

Originally Posted by Xavi Camps

I think I did not understand what I am supposed to do with the +/-1.

What Salem said is that you need to check all the places where you add or subtract one from the variable that represents cells in the array especially around the boundaries of the array (ee 0 and max) to make sure that they are not creating an index outside the memory of the array. For example x[r - 1] is the result of the r-1 between 0 and max cell of X?