creating an input function

How do I create an input function that opens the input file and reads in the data from the file? And does that file have to be stored in a certain location? Can I do this just saving it on my desktop as a text file? This is what I have so far

#include <stdio.h>  /* NULL is defined here */
#include <stdlib.h> /* for malloc() */
#include <string.h> /* for string related functions */

#define  NAME_LEN  10

struct data {
   char  name[NAME_LEN];
   int   age;
   int   weight;
};

typedef  struct data  DATA;

struct linked_list {
   DATA                d;
   struct linked_list * next;
};

typedef  struct linked_list  ELEMENT;
typedef  ELEMENT *           LINK;

/* function prototypes */
LINK create_list_from_file(const char *);
void print_list_to_file(LINK, const char *, const char *);
int count(LINK, const int, const int);
LINK lookup(const DATA, LINK);
void insert(LINK, LINK, LINK);

nt main(int argc, char *argv[])
{
   LINK head, found, to_add;
   char *in_file_name = "infile";   /* default input file name */
   char *out_file_name = "outfile"; /* default output file name */
   int a = 20, w = 150;  /* will be used as the age limit and weight limit */


   /* The input and output files can be given as command line arguments */ 
   switch (argc) {
   case 1:
      printf("The default input and output files are %s and %s.\n",
              in_file_name, out_file_name);
      break;
   case 2: 
      printf("The input file is %s and the default output file is %s.\n",
              argv[1], out_file_name);
      in_file_name = argv[1]; 
      break;
   case 3:
      printf("The input file is %s and the output file is %s.\n",
              argv[1], argv[2]);
      in_file_name = argv[1]; 
      out_file_name = argv[2];
      break;
   default:
      printf("The input file is %s and the output file is %s.\n",
              argv[1], argv[2]);
      in_file_name = argv[1]; 
      out_file_name = argv[2];
      printf("The remaining arguments are not used.\n");
   }

/*
1. invoke create_list_from_file() function to create a linear linked list 
   from the data in the input file,
*/
   head = create_list_from_file(in_file_name);
/*
2. invoke print_list_to_file() function with the writing mode ("w"),
*/
   print_list_to_file(head, out_file_name, "w");
/*
3. invoke the count() function,
4. output the counted result to the screen,
*/
   printf("The number of people with age over %d and weight over %d is %d.\n", 
           a, w, count(head, a, w));
/*
5. invoke the lookup() function and insert() function, and
*/
   /* prepare an element to be looked-up and added */
   to_add = malloc(sizeof(ELEMENT));
   strcpy(to_add -> d.name, "Janet");
   to_add -> d.age = 21;
   to_add -> d.weight = 150;
   to_add -> next = NULL;

   found = lookup(to_add -> d, head);
   insert(head, found, to_add);
/* 
6. invoke print_list_to_file() function with the append mode ("a").
*/
   print_list_to_file(head, out_file_name, "a");

/* 
repeat step 5 with an element that does not exist in the current list 
*/
   /* prepare an element to be looked-up and added */
   to开发者_如何学Python_add = malloc(sizeof(ELEMENT));
   strcpy(to_add -> d.name, "Jerry");
   to_add -> d.age = 24;
   to_add -> d.weight = 220;
   to_add -> next = NULL;

   found = lookup(to_add -> d, head);
   insert(head, found, to_add);
/* 
repeat step 6: invoke print_list_to_file() function with the append mode ("a").
*/
   print_list_to_file(head, out_file_name, "a");

   return 0;
}

You can open with standard library almost any file on your computer, including accessible network storage. Either text or binary files could be read, including simple ".txt" files. For example you can read one integer from file :

#include <stdio.h>
...
int i;
FILE *f;
f= fopen("D:\Temp\SomeFile.txt", "r");
fscanf(f, "%d", &i);
fclose(f);

You can use line-oriented semantic if you want (as in the example above) - look C library reference for fopen etc. There should be some example which may be useful also.