How to make changes in an array through a function - c

Question

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define SIZE 10
// A hashtable is a mixture of a linked list and array
typedef struct node NODE;
struct node{
int value;
NODE* next;
};
int hash(int);
void insert(int,NODE **);
int main(){
NODE* hashtable[SIZE];
insert(12,&hashtable[SIZE]);
printf("%d\n",hashtable[5]->value);
}
int hash(int data){
return data%7;
}
void insert(int value,NODE **table){
int loc = hash(value);
NODE* temp = malloc(sizeof(NODE));
temp->next = NULL;
temp->value = value;
*table[loc] = *temp;
printf("%d\n",table[loc]->value);
}
The above code prints :
12 and
27475674 (A random number probably the location.)
how do I get it to print 12 and 12 i.e. how to make a change in the array. I want to fill array[5] with the location of a node created to store a value.

Answer 1

The expression *table[loc] is equal to *(table[loc]) which might not be what you want, since then you will dereference an uninitialized pointer.
Then the assignment copies the contents of *temp into some seemingly random memory.
You then discard the memory you just allocated leading to a memory leak.
There's also no attempt to make a linked list of the hash-bucket.
Try instead to initially create the hashtable array in the main function with initialization to make all pointers to NULL:
NODE* hashtable[SIZE] = { NULL }; // Will initialize all elements to NULL
Then when inserting the node, actually link it into the bucket-list:
temp->next = table[loc];
table[loc] = temp;

Answer 2

This is just a simple change which I have made to your program which will tell you what you are actually doing wrong.
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define SIZE 10
// A hashtable is a mixture of a linked list and array
typedef struct node NODE;
struct node {
int value;
NODE* next;
};
NODE *hashtable[SIZE] = { NULL };
int hash(int);
int insert(int); //, NODE **);
int main(void)
{
int loc = insert(12); //, &hashtable[SIZE]);
if (loc < SIZE) {
if (hashtable[loc]) {
printf("%d\n", hashtable[loc]->value);
} else {
printf("err: invalid pointer received\n");
}
}
return 0;
}
int hash(int data)
{
return data%7;
}
int insert(int value) //, NODE *table[])
{
int loc = hash(value);
printf("loc = %d\n", loc);
if (loc < SIZE) {
NODE *temp = (NODE *) malloc(sizeof(NODE));
temp->value = value;
temp->next = NULL;
hashtable[loc] = temp;
printf("%d\n", hashtable[loc]->value);
}
return loc;
}
Here I have declared the hashtable globally just to make sure that, the value which you are trying to update is visible to both the functions. And that's the problem in your code. Whatever new address you are allocating for temp is having address 'x', however you are trying to access invalid address from your main function. I just wanted to give you hint. Hope this helps you. Enjoy!