Question

i have a question for you!

I have a C program in which i have a dynamic 2d array in my main function:

int **neighbours;

which i initialize based on args from command line.
The problem is that i want to clean up the code a bit. To do that i want to create a function in a header file that takes the array uninitialized as a reference argument and initialize it there.

This is how i call the function:

init(&nodeNum, &neighbours, argv[1], &nodes);

this is how i declare the function in the header file:

int init(int *nodeNum, int ***neighbours, char *arg, struct node **nodes)

and this is how try to allocate the memory:

neighbours=malloc(*nodeNum*sizeof(int *));
        for(i=0;i<*nodeNum;i++){
            neighbours[i] = malloc(*nodeNum*sizeof(int *));
        }

well obviously something doesn’t go well and it crashes! any hints on what i may be doing wrong?

thanks to the help of some nice people the malloc now works, but filling the nodes failes!

here is the whole code of the function:

int init(int *nodeNum, int ***neighbours, char *arg, struct node **nodes){
FILE *file;
char buffer[4];
int i,y,distance;

if(strcmp(arg,"-l")==0){
        file = fopen("C:\\Users\\trendkiller\\thesis\\matrix.txt","r");
        printf("parsing adjacency matrix from file\n\n");
        //file = fopen ("C:\\Users\\trendkiller\\thesis\\matrix.txt", "r" ) ;
        *nodeNum = atoi(fgets(buffer,4,file));
        *neighbours=malloc(*nodeNum*sizeof(int *));
        for(i=0;i<*nodeNum;i++){
            (*neighbours)[i] = malloc(*nodeNum*sizeof(int *));
        }
        printf("this prints\n");
        printf("number of nodes: %s", buffer);
        for(i=0;i<*nodeNum;i++){
            for(y=0;y<*nodeNum;y++){
                fgets(buffer,4,file);
                (*neighbours)[i][y]=atoi(buffer);
                            }
        }


    }
    else{
        *nodeNum = atoi(arg);

        *neighbours=malloc(*nodeNum*sizeof(int *));
        printf("this prints\n");
        for(i=0;i<*nodeNum;i++){
            (*neighbours)[i] = malloc(*nodeNum*sizeof(int *));
        }

        *nodes=malloc(*nodeNum*sizeof(struct node));

        for(i=0;i<30;i++){
            (*nodes)[i].x=rand()%100;
            (*nodes)[i].y=rand()%100;
        }
        printf("this prints\n");

        printf("creating new adjacency matrix\n\n");
        for(i=0; i<*nodeNum; i++){
            for(y=0; y<*nodeNum; y++){
                distance=sqrt((((*nodes)[y].x-(*nodes)[i].x)*((*nodes)[y].x-(*nodes)[i].x))+(((*nodes)[y].y-(*nodes)[i].y)*((*nodes)[y].y-(*nodes)[i].y)));
                if(i==y){
                    (*neighbours)[i][y]=-1;
                }
                else if(distance<=20){
                    (*neighbours)[i][y]=1;
                }
                else {
                    (*neighbours)[i][y]=0;
                }
            }

        }

        file = fopen("C:\\Users\\trendkiller\\thesis\\matrix.txt","a+");
        fprintf(file,"%d\n",*nodeNum);
        for(i=0;i<*nodeNum;i++){
            for(y=0;y<*nodeNum;y++){
                fprintf(file,"%d\n", (*neighbours)[i][y]);
            }
        }
    }

return 0;

}

Thanks in advance for your help!

Answer 1

The problem is that neighbours is passed by value, so modifications to it inside the function have no effect. You need to pass it by pointer, and modify it indirectly:

int init(int *nodeNum, int ***neighbours_ptr, char *arg, struct node *nodes) {
    *neighbours_ptr = malloc(*nodeNum*sizeof(int *));
    for(i=0;i<*nodeNum;i++){
        (*neighbours_ptr)[i] = malloc(*nodeNum*sizeof(int *));
    }
}

You also need to pass &neighbours instead of neighbours as the second argument of init.