Thread: Writing, passing, and handling matrices???

I cannot do anything w/ matrices in C, and I cant find my mistake. Below is my code. It basically uses two functions, one to dynamically allocate a matrix, and another to initilaze the matrix w/ all zeros! The main program basically just allocates the all zero matrix, and then it prints it. This is a very small and simple program. It compiles w/ no warnings whatsoever. When I run the program it says access violation.

If I do a[0][0], no problems, where a is a float**. If I do a[1][1] then I get access violation. The funny thing is I have seen tons and tons of code that takes a float** and accesses it as if it were a float[][]. So, I must be missing something.

Code:

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

float** newmtrx(int, int); //Allocate space for matrix return pointer
void initmtrx(float**, int, int); //set all entries to zero

void main()
{
    float** a;
    int i=0,j=0;
   
    a = newmtrx(5,5);  //allocate and point to 5x5 matrix
    initmtrx(a,5,5);  //set values in a to zero
   
    for(i=0; i<5; i++)
        for(j=0; j<5; j++)
            printf("%f\t",a[i][j]);
} //end of main

//function definitions
float** newmtrx(int m, int n)
{
  int i;
  float **p;

  p=(float **)malloc((unsigned) (m)*sizeof(float*));
  if (!p) printf("Failure in allocate_real_matrix().");

  for (i = 0; i < m; i++){
    p=(float **)malloc((unsigned) (n)*sizeof(float*));
    if (!p[i]) printf("Failure in allocate_real_matrix().");
  }
  return p;
}

void initmtrx(float** a, int m, int n)
{
    for(;m>0;m--)
        for(;n>0;n--)
            a[m-1][n-1] = 0;
}

Thanks. I made two pretty large errors.

Now, that I have that working, I have another questoin.
All of my functions about 60 of them work w/ pointers. I have matrix inverses, signal processing functions, etc. All of them work w/ pointers since they are the cleanest and most flexible.

If I know the size of my matrix, I declare it this way.

Code:

int A[4][2] = {
{-1,2},
{3,4},
{-5,-5},
{0,8}
};
;

I cant use A for my function calls, because my functions require pointers.

If I used

Code:

int** A ;

I can easily pass A to my functions.

The way i am working around this is I am just dynamically allocating all of my matrices.

My question is, is there a way to allocate space during compile time, and initialize the matrix, while at the same time, associating a pointer w/ it? How do you point a float** to a 2-D array?