Matrix parameter passing error

Question

I am trying to understand the function of pointers for an array. For this, I have elaborated the small program below. I made some mistake, because, in the indicated line, error is occurring (Segmentation fault).

What is the correct way to pass the pointer that points to an array, as I tried in the example?

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

int main()
{
    int i;
    float t1[4][2];
    for (i=0;i<8;i++)
    {
        t1[i%4][i%2]=i+1;
    }
    teste1(t1);
    getchar();
    return 0;
}
void teste1(float ** t1)
{
    int i;
    for (i=0;i<8;i++)
    {
        printf("%f\t",t1[i%4][i%2]);   // Erro nesta linha ("segmentation fault")
    }
}

Answer 1

The problem is in the parameter declaration in the function teste1. If you are passing a two-dimensional * array* (4X2) of the type float, then the function should receive the same type in the parameter.

Actually the code didn’t even compile. In addition to not recognizing the function teste1 there was incompatibility of types in the parameter. You’re probably using a compiler that doesn’t meet the language’s standards, so you were able to compile. Or the actual code you compiled is different from the one posted.

I reversed the two functions so that the teste1 is declared/defined before being used. I could also just declare before and set after but I think the code gets dirtier like this.

I also arranged the calculation of the second index that was wrong as indicated in the comments:

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

void teste1(float t1[4][1]) {
    for (int i = 0; i < 8; i++) printf("%f\t", t1[i % 4 ][i / 4]);
}
int main() {
    float t1[4][2];
    for (int i = 0; i < 8; i++) t1[i % 4][i / 4] = i + 1;
    teste1(t1);
    getchar();
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

In the comment it was said to leave generic. This is possible with arrays if using a compiler that meets the C99 standard (the GCC allows until the dimensions sizes can be passed after the array). The header of the function would have to look like this:

void teste1(int m, int n, float t1[m][n]) {

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

But if you really want to use a pointer as a parameter and want to generalize you have to do this:

In this case the teste1 is receiving as pointer (not pointer to pointer).

The element position calculation is being done manually since it is being treated with pointer and not array.

The call also needs to make a casting to suit the type of array pointer.

And there are also parameters to receive the total rows and columns of the matrix.

I also used #define to avoid getting lost when changes the size of the matrix

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

#define linhas 4
#define colunas 2

void teste1(float * t1, int totalLinhas, int totalColunas) {
    for (int i = 0; i < totalLinhas* totalColunas; i++) printf("%f\t", *((t1 + (i % totalLinhas) * totalColunas) + (i / totalLinhas)));
}
int main() {
    float t1[linhas][colunas];
    for (int i = 0; i < linhas * colunas; i++) t1[i % linhas][i / linhas] = i + 1;
    teste1((float *)t1, linhas, colunas);
    getchar();
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.