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Reputation: 1

Use double pointer to Create a function to do matrix operation

I'm trying to create a library including some functions like create a matrix, do add, sub, transpose and invert matrix, and I need to use double pointer In the beginning, I write this code to assign matrix but it seems does not work and I don't know where is the problem

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

static double P[4][4]={ { 1,   0,   0,   0},
                        { 0,   1,   0,   0},
                        { 0,   0,   1,   0},
                        { 0,   0,   0,   1}                       
                      };
double **P_M;
void show_matrix(int n,int m,double **matrix)
{
    int i,j;
    printf("\n The matrix is:\n");
    for (i=0;i<n;i++)
    {
        for (j=0;j<m;j++);
        printf(" \t",&matrix[i][j]);
        printf("\n");
    }
}

double matrix( int n, int m, double **matrix)
{
    int row;
    /*  allocate N 'rows'. */
    matrix = malloc( sizeof( double* ) * n );
    /*  for each row, allocate M actual doubles. */
    for( row = 0; row < n; row++ )
    matrix[ row ] = malloc( sizeof( double ) * m );

}

void main()
{
    int i, j;
    matrix(4,4,P_M);    
    for(i=1; i<5; i++)
            for(j=1; j<5; j++)
                P_M[i][j] = P[i-1][j-1];    
    //show_matrix(4,4,P_M);

}

Upvotes: 0

Views: 97

Answers (1)

0___________
0___________

Reputation: 67476

Many problems.

  1. Out of bounds - as indexes are from zero.
  2. printf(" \t",&matrix[i][j]); -> printf("%lf \t",matrix[i][j]);
  3. double matrix( int n, int m, double **matrix) -> double **matrix( int n, int m, double ***matrix) and the appropriate changes inside the function + return *martix; at the end if you need it. Otherwise make it void. Call it matrix(4,4,&P_M);

And probably more which I have not noticed. *** pointers are silly and passing the address to the pointer is not necessary. 

double **matrix(int n, int m)
{
    int row;
    double **array;
    /*  allocate N 'rows'. */
    if (!(array = malloc(sizeof(double*) * n)))
    {
        return NULL;
    }
    /*  for each row, allocate M actual doubles. */
    for (row = 0; row < n; row++)
        if (!(array[row] = malloc(sizeof(double) * m)))
        {
            //do something if malloc failed - for example free already allocated space.
            return NULL;
        }
    return array;
}

and in the main P_M = matrix(4,4);

Upvotes: 2

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