Pass matrix by reference using pointer of pointer

[cadubentzen]

Hello, i really don't know why has a error in my code, that pass a pointer of pointer (name of a matrix with 2 dimensions). Here is the source code of a simple example where appears segmentation fault when execute (but compiles normal):

#include <stdio.h>

#define LINHAS 3
#define COLUNAS 5

float a[LINHAS][COLUNAS];

void zeros(float **p,float m, float n){
int i,j;
for(i=0;i<m;i++)
for(j=0;j<n;j++)
p[i][j]=0;
}

void exibe(float **p,float m, float n){
int i,j;
for(i=0;i<m;i++){
for(j=0;j<n;j++)
printf("%f ",p[i][j]);
printf("\n");
}
}

int main(int argc, char **argv){
float **ptr_a = (float**)a;
zeros(ptr_a,LINHAS,COLUNAS);
exibe(ptr_a,LINHAS,COLUNAS);
return 0;
}

[keskiverto]

float ** ptr_a = ...
Dereferencing a pointer to a pointer will return a pointer.
The type of ptr_a[i] is thus float *.
However, that memory location does not have a pointer, and that
dereference does not return the address &a[i][0].

Second. Float indices (m, n)? No, please.

How about:

void zeros( float p[][COLUNAS], int m, int n);
void exibe( float p[][COLUNAS], int m, int n);

[iQChange]

Então temos um BR/PT no fórum? Achei que estava sozinho.

[cadubentzen]

iQChange, sou Brasileiro sim :D

keskiverto, isn't interesting to me use functions like you suggested, because the number of columns can change, like a function to multiply matrices. However, i use a declaration of matrix in this way: int a[LINHAS*COLUNAS], so i can pass a pointer as parameter and manipulate.

Thank you.

[keskiverto]

the number of columns can change

That was not apparent from your example, which was using statically allocated 'a' (although only the dynamically determined m*n block of it).

You seem to be using C. C supports VLA and pointer to array. See
http://collaboration.cmc.ec.gc.ca/science/rpn/biblio/ddj/Website/articles/CUJ/2002/0201/meyers/meyers.htm
(particularly Listing 3)

[iQChange]

@cadubentzen You have PM deactivated. If you activate it, we can talk about C++ stuff ;)

[frycex]

It works:

#include <stdio.h>
using namespace std;
#define LINHAS 3
#define COLUNAS 5

float a[LINHAS][COLUNAS];

void zeros(float **p, short m, short n) {
short i, j;
for (i = 0; i < m; i++)
for (j = 0; j < n; j++)
p[i][j] = 0;
}

void exibe(float **p, float m, float n) {
int i, j;
for (i = 0; i < m; i++) {
for (j = 0; j < n; j++)
printf("%f ", p[i][j]);
printf("\n");
}
}

int main(int argc, char **argv) {

float **ptr_a = new float* [LINHAS];
for (int i = 0; i < LINHAS; i++) {
ptr_a[i] = new float[COLUNAS];
}

zeros(ptr_a, LINHAS, COLUNAS);
exibe(ptr_a, LINHAS, COLUNAS);

for(int i = 0; i < LINHAS; i++){
delete [] ptr_a[i]; //deleting pointers ptr_a[i]
}
delete [] ptr_a; //deleting pointer ptr_a

return 0;
}