[Hash table] simple

#include<stdio.h>
#include<stdlib.h>
#define MAX 1000
#define in "in.txt"
#define out "out.txt"
void insert(unsigned long H[], unsigned long x)
{
    int k;
    k = x % 1000;
    H[k] = x;
}
int search(unsigned long H[], unsigned long x)
{
    return x%1000;
}
void hdelete(unsigned long H[], unsigned long x)
{
    H[x%1000] = NULL;
}
void hwritefile(unsigned long a[MAX], int n)
{
        FILE *f;
        f = fopen(out,"wt");
        if(f==NULL)
        {
                printf("Khong tao duoc file\n");
                exit(0);
        }
        for (int i = 0; i < n; ++i)
        {
                fprintf(f, "%lu\n", a[i]);
        }
        fprintf(f, "\n" );
        fclose(f);
}
void hreadfile(unsigned long H[], int n)
{
        FILE *f;
        unsigned long x;
        int k;
        f = fopen(in,"rt");
        if(f==NULL)
        {
                printf("Khong doc duoc file\n");
                exit(0);
        }
        for (int i = 0; i < n; ++i)
        {
            fscanf(f, "%lu", &x);
            if (x == NULL)
                x = 0;
            insert(H,x);
        }
        fclose(f);
}
void writenumof10digit(int n)
{
        FILE *f;
        f = fopen(in,"wt");
        if(f==NULL)
        {
                printf("Khong tao duoc file\n");
                //getch();
                exit(0);
        }
        for (int i = 0; i < n; ++i)
        {
                fprintf(f, "%lu\n", rand()%1000000000);
        }
        fprintf(f, "\n" );
        fclose(f);
}
int main(int argc, char const *argv[])
{
        unsigned long a[MAX];
        writenumof10digit(1000);
        hreadfile(a,1000);
        hwritefile(a,1000);
        //for(int i=0; i<1000; i++)
        //    printf("%lu\n",a[i]);
        printf("%d\n",search(a,30212));
        hdelete(a,30212);
        printf("%d\n",a[search(a,30212)]);
        system("pause");
        return 0;
}