CODEWITHSUNDEEP
c
Luca Lotifi
Luca Lotifi

Reputation: 89

Cannot access to memory in C

I have to create a function that initializes k cluster choosing the starting center based on the distance between the point. This is the code i wrote:

int *inizialize_centroids(int *p_dataset, int d, int k, int **p_discard_set) {


int* centroids = (int*) malloc(k*((2*d)+1)*sizeof(int)), *point_to_compare = (int*) malloc(d*sizeof(int)),
*cluster_point = (int*) malloc(d*sizeof(int));
float* distance = (float*) malloc(2*k*sizeof(float));
if(centroids == NULL || point_to_compare == NULL || cluster_point == NULL || distance == NULL){
    printf("Something went wrong in inizialize_centroids(), memory allocation failed! (row 94/95)");
    exit(1);
}


centroids[0]=1;
for(int i = 1; i < d; i++){
    centroids[i] = p_dataset[i];
    centroids[i+d] = pow(p_dataset[i],2);
}

*p_discard_set[0] = 1;
*p_discard_set[1] = p_dataset[0];
memcpy(cluster_point, &p_dataset[0], (d + 1)*sizeof(int));


int j;
int i = 1;
int t;
while (i < k){
    
    j = 0;
    while(j < 2*CHUNK ){
        memcpy(point_to_compare, &p_dataset[j/2 * (d + 1)], (d + 1) * sizeof(int));
        
        distance[j] = (float) point_to_compare[0];
        j++;
        distance[j] = compare(cluster_point, point_to_compare, i, d);
        j++;
    }

   
    int id = (int) distance[0];
    float max = distance[1];

    j = 0;
    while (j < 2* CHUNK){
        if(distance[j+1] > max){
            max = distance[j+1];
            id = distance[j];
        }
        j+=2;
    }

    i++;

    *p_discard_set[0] = i;
    *p_discard_set[i] = id; //HERE OCCURE THE PROBLEM


    [....]

}

return centroids;

The problem is that, i can't assign

*p_discard_set[i] = id;

and i don't understand why it gives me "interrupted by signal 11: SIGSEGV"

This is how i use it in main:

int *discard_set = (int*) malloc((k + 1) * sizeof(int)), *compressed_set = (int*) malloc(sizeof(int)), *retained_set = (int*) malloc(sizeof(int));
// discard_set è così fatto: [n, id_1, ... , id_n]

if(discard_set == NULL || compressed_set == NULL || retained_set == NULL){
    printf("Something went wrong in main(), memory allocation failed! (row 39)");
    exit(1);
}

int* centroids = inizialize_centroids(dataset, d, k, &discard_set);

The stange thing is that i can do

*p_discard_set[0] = i;

and k > 2, so it isn't out of bound, I think.

Upvotes: 0

Views: 49

Answers (1)

Chris Dodd
Chris Dodd

Reputation: 126546

The precedence of postfix operators in C is higher than prefix, so when you say

*p_discard_set[i] =  ...

you actually get

*(p_discard_set[i]) = ...

and what you actually want is

(*p_discard_set)[i] = ...

so you need the explicit parentheses to make it work the way you are expecting.

Upvotes: 2

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