Reading and Writing to a file in C - c

Question

I am slowly learning C, but not very well. I have been reading over the countless topics and questions on reading and writing, but I have yet to be able to find anything that makes this all click for me.
I was given the following code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 100
struct YouTubeVideo {
char video_name[1024]; // YouTube video name
int ranking; // Number of viewer hits
char url[1024]; // YouTube URL
};
struct YouTubeVideo Collection[MAX];
int tail = 0;
//-- Forward Declaration --//
void printall();
void insertion();
void branching(char option);
void menu();
int main()
{
char ch;
// TODO: Add code to load save data from file
printf("\n\nWelcome to CSE240: YouTube Classic Hits\n");
do {
menu();
fflush(stdin); // Flush the standard input buffer
ch = tolower(getchar()); // read a char, convert to lower case
branching(ch);
} while (ch != 'q');
return 0;
}
void menu()
{
printf("\nMenu Options\n");
printf("------------------------------------------------------\n");
printf("i: Insert a new favorite\n");
printf("p: Review your list\n");
printf("q: Save and quit\n");
printf("\n\nPlease enter a choice (i, p, or q) ---> ");
}
void branching(char option)
{
switch(option)
{
case 'i':
insertion();
break;
case 'p':
printall();
break;
case 'q':
// TODO: Add code to save data into a file
break;
default:
printf("\nError: Invalid Input. Please try again...");
break;
}
}
void insertion()
{
if(tail < MAX)
{
printf("\nWhat is the name of the video? (No spaces characters allowed)\n");
scanf("%s", Collection[tail].video_name);
printf("\nHow many viewer hits does this video have?\n");
scanf("%d", &Collection[tail].ranking);
printf("\nPlease enter the URL: ");
scanf("%s", &Collection[tail].url);
tail++;
}
else
{
printf("\nERROR: Your collection is full. Cannot add new entries.\n");
}
}
void printall()
{
int i;
printf("\nCollections: \n");
for(i = 0; i < tail; i++)
{
printf("\nVideo Name: %s", Collection[i].video_name);
printf("\nRanking (Hits): %d", Collection[i].ranking);
printf("\nURL: %s", Collection[i].url);
printf("\n");
}
}
I am suppose to write the code that will store the collection into a file and the likewise right the code that will load the file and read from it.
Thanks to a fairly helpful TA I was able to formulate the following code for each
void store()
{
FILE * fileName;
fileName = fopen ( "Ranking.dbm" , "wb" );
fwrite ( Collection, sizeof(struct YouTubeVideo), MAX, fileName);
fclose (fileName);
}
and
void read()
{
FILE *fileName;
fileName = fopen("ranking.dbm", "rb");
if (fileName != NULL){
fread ( Collection, sizeof(struct YouTubeVideo), MAX, fileName);
}
else {
printf("ERROR");
}
}
I believe these each to function but the real problem is I don't think I quite understand how and I believe that since I dont even know how they function, I dont know how to use them in the code.
I added both methods to the given code and came up with this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define MAX 100
struct YouTubeVideo {
char video_name[1024]; // YouTube video name
int ranking; // Number of viewer hits
char url[1024]; // YouTube URL
};
struct YouTubeVideo Collection[MAX];
int tail = 0;
//-- Forward Declaration --//
void printall();
void insertion();
void branching(char option);
void menu();
void store();
void read();
int main()
{
char ch;
read();
printf("\n\nWelcome to CSE240: YouTube Classic Hits\n");
do {
menu();
fpurge(stdin); // Flush the standard input buffer
ch = tolower(getchar()); // read a char, convert to lower case
branching(ch);
} while (ch != 'q');
return 0;
}
void menu()
{
printf("\nMenu Options\n");
printf("------------------------------------------------------\n");
printf("i: Insert a new favorite\n");
printf("p: Review your list\n");
printf("q: Save and quit\n");
printf("\n\nPlease enter a choice (i, p, or q) ---> ");
}
void branching(char option)
{
switch(option)
{
case 'i':
insertion();
break;
case 'p':
printall();
break;
case 'q':
store();
break;
default:
printf("\nError: Invalid Input. Please try again...");
break;
}
}
void insertion()
{
if(tail < MAX)
{
printf("\nWhat is the name of the video? (No spaces characters allowed)\n");
scanf("%s", Collection[tail].video_name);
printf("\nHow many viewer hits does this video have?\n");
scanf("%d", &Collection[tail].ranking);
printf("\nPlease enter the URL: ");
scanf("%s", &Collection[tail].url);
tail++;
}
else
{
printf("\nERROR: Your collection is full. Cannot add new entries.\n");
}
}
void printall()
{
int i;
printf("\nCollections: \n");
for(i = 0; i < tail; i++)
{
printf("\nVideo Name: %s", Collection[i].video_name);
printf("\nRanking (Hits): %d", Collection[i].ranking);
printf("\nURL: %s", Collection[i].url);
printf("\n");
}
}
void store()
{
FILE * fileName;
fileName = fopen ( "Ranking.dbm" , "wb" );
if (fileName != NULL)
{
fwrite ( Collection, sizeof(struct YouTubeVideo), MAX, fileName);
fclose (fileName);
}
else {
perror("Following error occurred(): ");
}
}
void read()
{
FILE *fileName;
fileName = fopen("Ranking.dbm", "rb");
if (fileName != NULL)
{
fread ( Collection, sizeof(struct YouTubeVideo), MAX, fileName);
fclose(fileName);
}
else {
perror("Following error occurred with fopen(): ");
}
}
Now I am sure anyone who has read this has probably already face palmed themselves cause they see the problem, but I do not. The code does not create the file to write to and likewise it has nothing to read from so I cant even begin to see what is wrong with that.
Now I am not looking for a given answer, but I would really like to know what it is I am doing incorrectly, what concepts I appear to not understand, and how I can go about fixing these. I have done a few hours of research on this already and I realize it is elementary, but I really would like a hand in learning. Its frustrating spending hours on a topic that the professor said should only take a couple of hours to complete at most.

Answer 1

You really should check the return value of fopen() against NULL - if there is a problem opening the file, it will return NULL and set errno. This is probably a permissions mistake, and by checking the return value and printing the error if one is set, you'll get more information on what went wrong.

Answer 2

Minor: Make sure to check the return values for things like fopen, fread, fwrite etc. more often.
Mild: You've got a potential typo in the filename (some operating systems have case-sensitive file names)
Severe: read() doesn't set up a value for tail... :)

Answer 3

I believe you may have a problem with the read function, which does not call fclose on the file handle (by the way, calling it fileName is a little misleading).
Because you leave the file open, it's entirely likely that you won't be able to overwrite it when you eventually call store. You haven't output any error messages if the file cannot be opened in that function, so it's quite easy to slip under the radar... At least until you wonder why your file date doesn't change.
Otherwise the code looks okay. All it does is dump the entire contents of your array out of memory and read it back in again. What you'll probably want to do is also write out the value of tail, since it is keeping track of how many elements you are keeping. So with minimal code changes, do this:
fwrite ( &tail, sizeof(int), 1, fileName);
fwrite ( Collection, sizeof(struct YouTubeVideo), MAX, fileName);
And of course the corresponding calls to fread in your read method.
fread ( &tail, sizeof(int), 1, fileName);
fread ( Collection, sizeof(struct YouTubeVideo), MAX, fileName);
To reiterate: don't forget to close your file!!!!
fclose(fileName);