Thread: Accessing Data within a struct! Need help please

Code:

#include <stdio.h>
#include <string.h>

#define MAX_NUM 53

//declare struct of type lotto
struct lotto
{
char firstname [19];
char lastname [19];
int ticket_num[5];
int winnings;
};


int main () 
{
   struct lotto *info;
   int winning_nums [5], num_tickets, moneywon;
   int i, q, g, f, r, t, win3, win4, win5, win6;
   int matches=0;
   char filename[20];  

   FILE *ifp;
   
   //Prompt user for filename with lottery ticket info
   printf("Please enter the filename with the ticket information.\n");
   scanf("%s", &filename);
   ifp = fopen(filename, "r");

   //Scan number of lottery tickets on file
   fscanf(ifp, "%d", &num_tickets);
   
   //Allocate dynamic memory
   info=malloc(num_tickets*sizeof(struct lotto));
   
   //Scan first and last name for lottery info file
   for(t=0; t<num_tickets; t++){
      fscanf(ifp, "%s %s", &info[t].lastname, &info[t].firstname);
	  }
	   
    printf("%s\n", info[0].firstname, info[0].lastname);  // HERE IS WHERE I AM HAVING TROUBLE. 
IT SCANS FIRST AND LAST  NAME FROM THE .TXT doc GOOD WHEN I TEST IT WITH 
 "info[0].firstname" but NOT with "info[1].firstname... info[2], etc...

Text Document:

5
Llewellyn Mark
1 15 19 26 33 46
Young Brian
17 19 33 34 46 47
Cazalas Jonathan
1 4 9 16 25 36
Siu Max
17 19 34 46 47 48
Balci Murat
5 10 17 19 34 47


NOTE: I am simply trying to scan all the last names and first names at once and then have access to each last or first name. After attempting to come up with the code, I was trying to test it by printing out different last names that were stored in the array. Please HELP.

You need to tell us what part of your application is not working the way you expect.

I edited my code and added some comments explaining what is not working. Thanks for the reply.

So, basically when I scan the FIRST firstname and lastname on my .txt document and try to print it out, it works!

When I try to scan my second, third, fourth, etc... firstname and lastname AND print them out it does not work.

The way I try to access the first, firstname and lastname is first info[0].firstname and info[0].lastname

therefore, the second firstname and lastname should be info[1].firstname and info[1].lastname But it doesn't work...it does not read the correct data and THATS what I need help with..

Thanks again! (I'm a bit frustrated)

An array of size 5, i.e. declared with [5], has five elements. These elements are 0, 1, 2, 3, and 4. Such an array does not have an element [5] that you can access. Another example: in order to store six numbers you need an array of size 6, which you declare as [6]; to access these elements you start at 0 and keep counting up until you have addressed all six of your elements, which means that the last valid index is at 5.

When you're reading from a file, it's just like reading the user's input. All characters that are present in the file must be processed in the order they are received. "%s" instructs fscanf() to try to interpret the next characters as a string. Basically there's a notion of a current position in the file, and as you read characters the position advances. You can interpret the characters as a string, or a number, or whatever else you like.

Here's an example. Suppose you pass fscanf() a %d, asking it to interpret characters as an integer. If the stream (like a file or user input) contains "123\netc", fscanf() will read the '1' and '2' and '3', do some multiplication and give you the number 123, leaving "\netc" in the input stream. If you then called fscanf() with %d again it would skip the '\n' (scanf() functions skip whitespace when looking for numbers), then hit the 'e' and decide that this character can't be interpreted as part of a number. fscanf() will fail and return an error code to you. If you ignore the error code you'll be using an uninitialized variable in your program.

Notice how I said fscanf() skips over whitespace when it's looking for numbers (and strings, etc.). This means that once it fails to find an item once it's hard to recover from the error and keep going, because you don't really know where on a line you are, whether the line is malformed, etc. For this reason it's often suggested that you read in entire lines at once with fgets(), and then attempt to process the lines individually with sscanf(). If any line fails to be in the correct format, you can keep going and continue to process further lines.

Anyway, here's an example of the kind of code you could use. (I've assumed that the number at the beginning of the file does not exist or has already been processed, because it's not actually necessary.)

Code:

FILE *fp = fopen( /* ... */ );
char line[BUFSIZ];
int count = 0;

while(fgets(line, sizeof line, fp)) {
    count ++;
    if(count % 2 == 1) {  /* odd-numbered line, must be a name */
        char first[BUFSIZ], last[BUFSIZ];
        if(sscanf(line, "%s%s", first, last) == 2) {
            /* since sscanf() returned 2, both items were successfully read */

            /* (names printed in square brackets to make the contents
                of the variables obvious) */
            printf("Hello, [%s] [%s].\n", first, last);
        }
        else {
            printf("Error processing line %d: didn't look like a name\n", count);
        }
    }
}

fgets() returns NULL if it fails to read a line (which normally happens at the end of the file); hence, the while loop executes once per line and allows you to deal with it. On odd-numbered lines (where the first line is line 1), you expect to find a name, so the code uses sscanf() to pull information out of the line and into two strings (first and last). Note that there are two format specifiers given, "%s%s" (and there is no space between them -- as I said before *scanf() skip whitespace when converting input into the expected format). sscanf() returns the number of format specifiers it was able to successfully satisfy; in this case, 2 if both strings were present on the line, 1 if only one of them was, and 0 if neither were.

Anyway, CBoard has logged me out now and I'm tired of typing anyway. If you want to know more, try searching the boards or ask another question.

[edit] One other thing I forgot to mention: you should fclose() a file when you're done with it. [/edit]

Of course there are other ways to solve this problem. I suppose the simplest is to just read all the data into your structure at once, including names and numbers. After you read in the first and last name as you have done, read in the remaining six numbers into ticket_num[5] and winnings respectively of the current structure. That way your fscanf() %s%s will always be at a first/last name pair.