Thread: C program - inverse of a matrix

hi, this is my first time on this board, so i apologise in advanced if i broke any rules or whatever...
so basically, im supposed to use C to find theinverse of a matrix of whose size and values are entered by the user. I am supposed to do so using the following steps:


Step 1: Set up an n × n matrix B and initialize it to the n × n identity matrix.

Step 2: In the matrix A, interchange the top row with the nearest row below it, if necessary,
to bring a nonzero entry to the row 1, column 1 position. (This is a Row Interchange
elementary row operation.) Apply the same Row Interchange elementary
row operation to the matrix B.

Step 3: In the matrix A, if the entry in the row 1, column 1 position is a, multiply the first
row by 1/a in order to make the entry in the row 1, column 1 position have the value
1. (This is a Row Scaling elementary row operation.) Apply the same Row Scaling
elementary row operation to the matrix B.

Step 4: In the matrix A, add suitable multiples of the top row to the rows below so that
all entries below the 1 in the row 1, column 1 position become zero. (These are Row
Addition elementary row operations.) Apply the same Row Addition elementary
row operations to the matrix B.

Step 5: Repeat Steps 2 through 4 to make each diagonal entry Ai,i have the value 1, for
i = 2, 3, 4, . . . , n, and each off-diagonal entryAi,j have the value 0, i, j = 2, 3, 4, . . . , n,
i 6= j. In Step 2, if it is necessary to interchange rows to make element Ai,i nonzero,
interchanging row i with the nearest row below it having Aj,i nonzero. In Step 4,
when zeroing elements in column i, add suitable multiples of row Ri to rows Rj , for
j = 1, . . . , i − 1, i + 1, . . . , n (that is, both above and below the 1 in element Ai,i).

and here is what i have done so far ( sorry i have to post the entire code)

Code:

#include <stdio.h>
#include <stdlib.h>

int main ()
{	
	void printMatrix (char name, double **matrix, int n);
	
	int input, entries, i, j;
	double **A, **B, temp;
	printf("Enter dimension of square matrix:");
	scanf ("%d", &input);
	entries = input * input;
	printf ("Enter the %d entries of the matrix:", entries);
	
	A = (double **) malloc (input* sizeof (double *));
	for (i=0; i<input; i++)
	{
		A[i] = (double *) malloc (input* sizeof (double));
	}
	
	B = (double **)malloc (input* sizeof (double *));
	for (j=0; i<input; i++)
	{
		B[i] = (double *) malloc (input* sizeof (double));
	}

	for (i=0; i<input; i++)
	{
		for (j=0; j<input; j++)
		{
			scanf("%lf", &A[i][j]);
		}
	}
	
	for (i=0; i<input; i++)
	{
		for (j=0; j<input; j++)
		{
			if ( i == j)
				B[i][j] = 1;
			else
				B[i][j] = 0;
		}
	}
	
	if (A[0][0]==0)
		{
		for(i=0;i<input;i++)
			{
				temp=A[0][i];
				A[0][i]=A[1][i];
				A[1][i]=temp;
		
				temp=B[0][i];
				B[0][i]=B[1][i];
				B[1][i]=temp;
			}
		}

	temp = A[0][0];
	
	for(i=0;i<input;i++)
	{
		for(j=0;j<input;j++)
		{
			A[i][j]/=temp;
			B[i][j]/=temp;
		}
	}

	for (i=1;i<input;i++)
	{
		temp = A[i][0];
		for(j=0;j<input;j++)
		{
			A[i][j]-=A[0][j]*temp;
			B[i][j]-=B[0][j]*temp;
		}
	}

			
		
	for (i=0; i<input; i++)
	{
		printf ("\n");
		for (j=0; j<input; j++)
		{
			printf("%f ", A[i][j]);
		}
	}
		
	for (i=0; i<input; i++)
	{
		printf ("\n");
		for (j=0; j<input; j++)
		{
			printf("%f ", B[i][j]);
		}
	}


return 0;	
}

As you can see, i am up to step 5. I have 2 problems - first, for some really odd reason it only works with matrix of size less then 2x2. anything larger it will just ask for input then the program will end. i suspect there is something wrong with the nested for loops of inputting 0s and 1s into B to make it the identity. however, havin said that i cant find anything wrong with anything.
secondly, i am unsure how to proceed with step 5, any help is appreciated.
i am half asleep when im typing this, so ignore and any grammar or spelling or if i havnt explained myself properly.

This may not be the only problem, but look closely at this part:

Code:

    for (j=0; i<input; i++)
    {
        B[i] = (double *) malloc (input* sizeof (double));
    }

wow thx!! that solved the problem straight away!

how did it solve the problem?. it still wouldnt work for higher matrices.

I am also looking for this code. After lot of searches I have found the answer in below website.

C program inverse of a Matrix of any size (NXN)