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fopen with fscanf in C

Posted on 2007-03-19
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Last Modified: 2012-08-13
I want to read  some data from a file into a data structure, how can I use the scanf function and do error checking?

For example -
FILE *fp;
fp = fopen(filename, "r");
Entry entry = new Entry();
if (fp == NULL)
    {
      fprintf(stderr, "Error: Unable to open file %s\n\n", string);
      return FALSE;
    }
   
 //while( (ch=getc(fptr)) != EOF )
***Normally I would try to get each character and see if I reach the end of file, but if I want to use fscanf, how should I detect the eof?***
***Also, what about if there is error in the input file?  For example, If I am expecting a pattern like john, smith, 1/1/2000, john.smith@email.com
***I would like to know how I can detect fields missing or corrupted error with scanf?

              fscanf(fp, "%s,%s,%s,%s", entry.firstName, entry.lastName,
            entry.DOB, entry.email);

Look forward to hear from you
      
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Question by:4eyesgirl
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4 Comments
 
LVL 24

Expert Comment

by:fridom
ID: 18754312
You can do as in the getch loop

while((fscanf(fp, fmt_string, args...) != EOF)

However you must check the return value, because on read failure (for whatever reason EOF is returned), however it might be a different problem so you have to check errno too if you want to be on the safe side.

Regards
Friedrich
0
 
LVL 53

Expert Comment

by:Infinity08
ID: 18757262
>> ***I would like to know how I can detect fields missing or corrupted error with scanf?

The return value of fscanf() contains the number of successfully read fields. So, if you're expecting 3 fields, and fscanf() returns 2, then you know that there's a problem in the record.

Similarly, when the end of the file is met before fscanf could read any data, EOF is returned.


More information :

http://www.cplusplus.com/reference/clibrary/cstdio/fscanf.html
0
 

Author Comment

by:4eyesgirl
ID: 18787558

char* firstName = (char*) malloc(MAX_LEN);
char* lastName = (char*) malloc(MAX_LEN);
char* DOB = (char*) malloc(MAX_LEN);
char* email= (char*) malloc(MAX_LEN);

when I try to use
while( (fscanf(fp, "%s,%s,%s,%s", firstName, lastName,
            DOB, email) != EOF)) {
//....does not work
// firstName -> read the entire line.  Doesn't looks like fscanf separate with then input format that I provided.
}

however, using space works, why?
while( (fscanf(fp, "%s %s %s %s", firstName, lastName,
            DOB, email) != EOF)) {
//....
}

I don't want to change my file format.  My file format is firstname,lastname,dob,email
Please help!
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LVL 53

Accepted Solution

by:
Infinity08 earned 2000 total points
ID: 18788911
Try something like this :

  char *buff = (char*) malloc(MAX_LEN * 4 + 5);

  /* ... */

  while (fgets(buff, MAX_LEN * 4 + 5, fp)) {
    if (sscanf(buff, "%[^,],%[^,],%[^,],%[^,\n]\n", firstName, lastName, DOB, email) != 4) continue;

    fprintf(stdout, "firstName : %s\n", firstName);
    fprintf(stdout, "lastName : %s\n", lastName);
    fprintf(stdout, "DOB : %s\n", DOB);
    fprintf(stdout, "email : %s\n", email);
    fprintf(stdout, "\n");
  }

The (MAX_LEN * 4 + 5) is the maximum allowed length of a line based on MAX_LEN.

Note that I also filtered out invalid lines by checking the return value of sscanf.
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