Matrix in dynamic allocation with execution failure

Question

Hello!

The following code compiled, but shows error in execution:

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

typedef struct 
{
    char *nome;
    int valor;
    int peso;
} objeto;

objeto obj[] = {
    {"map",      10,   2},
    {"compass",   7,   1},
    {"water",    25,   6},
    {"tin",      24,   5},
};

 // max{ f(Xn-1, W-Pn) + Vn , f(Xn-1, W) }    

int solver(objeto *obj, int n, int capac )
{
    int i, j, taDentro, taFora;
    int **matriz = (int **) calloc(n+1, sizeof(int *));

    matriz[0] = (int *) calloc(capac+1, sizeof(int)); 
    for (i=1; i<=n; i++) 
    {
        matriz[i] = (int *) calloc(capac+1, sizeof(int));

        for (j=1; j<=capac; j++)
        {
            taFora = matriz[i-1][j]; //coloca o item anterior?
            if (obj[i-1].peso > matriz[i][j]) matriz[i][j] = taFora;
            // -1 pq os itens em 'matriz' comecam em 1 
            else
            {
              taDentro = matriz[i-1][ j-obj[i].peso ] + obj[i].valor;
              if (taDentro > taFora) matriz[i][j] = taDentro;
              else                   matriz[i][j] = taFora;
            }
        }
    }
    int resultado;
    resultado = matriz[i][j];

    for (i = 0; i < n; i++) free(matriz[i]);
    free(matriz);
    return resultado;
}

int main()
{
    int n = 3;
    int capac = 7;
    int resultado;
    resultado = solver(obj, n, capac);
    printf("%d\n", resultado);
}

The windows window shows this information:

Problem Event Name: APPCRASH

I don’t know if I’m trying to record or read information in wrong space, or if I’m using the pointers wrong. Or both.

Gentlemen, could you help me?

Answer 1

I recommend that you use GDB to debug your program if you cannot solve the problem.

According to the debugger, segmentation failure occurs on the following line:

resultado = matriz[i][j];

Values of variables after segmentation failure:

i = 4

j = 8

Capac = 7

n = 3

What we can see is that:

The following tie:

for (i=1; i<=n; i++) 

The variable i starts at 1, the loop rotates while i is less than or equal to 3 (value of n), so far so good, but note the condition i<=n, this condition will only be false when i is greater than n, that is, when it has the value 4.

In the following line: int **matriz = (int **) calloc(n+1, sizeof(int *)); you reserve n+1 spaces for your array, in case 3+1 = 4, as vector addressing starts at 0, legal positions for your vector would be between 0 and 3 (4 positions), to correct this failure, just decrement the value of i because your vector does not have line 4.

Note that the variable j also suffers from this same problem.

You have two options:

int resultado;
resultado = matriz[--i][--j]; // --i é diferente de i--

Or

i--;
j--;
int resultado;
resultado = matriz[i][j];

I recommend you read:

GDB Tutorial (gnu Debugger)

What is segmentation failure

Answer 2

I believe the problem lies on this line:

resultado = matriz[i][j];

For right now the i will be n+1 and the j will be capac+1, which are unallocated positions.