How to use EOF to read all data from file

Question

I'm learning about file i/o in C language and I wrote this program that reads a file, and then for every even number found, it has to print * to the screen.

My problem is that my program keeps printing * forever.

I have tried different ways,some from this website, but I can't seem to understand how to read until end of a text file using EOF.

I want to learn how to read a text file until the end of the file please. How do I read until the end of a text file? EOF in C.


int main(void)
 {

     int num;
     FILE *ifp;

     ifp = fopen("numbers.txt", "r" );

     if(ifp == NULL)
     {
         exit(1);
     }

     do
     {
         fscanf(ifp, "%d", &num);       

            if(num%2 == 0)

                {
                printf("*\n");
                }
        } while(num != EOF);


    fclose(ifp);

    return 0;
}

You are printing ```*``` for the even number though. – Shubham Jun 05 '20 at 06:45 — Shubham, Jun 05 '20 at 06:45
@Lucas sorry about that! I have fixed the typo – Kakarotto Jun 05 '20 at 06:48 — Kakarotto, Jun 05 '20 at 06:48

score 2 · Accepted Answer · answered Jun 05 '20 at 06:20

2

you need to check the result of the scanf

     do
     {
         int result;
         result = fscanf(ifp, "%d", &num);       
         if(result == EOF) break;

         if(result != 1) 
         {
             printf("scanf error\n");
             break;
         }
         if(num%2 == 0)
         {
              printf("*\n");
         }
      } while(1);

answered Jun 05 '20 at 06:20

0___________

60,014
4
34
74

hey! thx, I made those changes, and now is not printing `*`. I checked with he debugger and noticed that goes into the `scanf error` in line 25 – Kakarotto Jun 05 '20 at 06:34
2

so you have the problem with your file. It fails to scan. ***Always*** check the result of any I/O operation – 0___________ Jun 05 '20 at 06:35

Joseph Pilliner · Answer 2 · 2020-06-05T07:37:19.167

0

It's doing this because while(num != EOF) is testing whether int num, the number read from the file, is EOF, rather than whether the end of file has been reached.

~~To test whether the EOF flag has been set on FILE *ifp, use while(!feof(ifp)) instead.~~

edited Jun 05 '20 at 07:37

answered Jun 05 '20 at 06:07

Joseph Pilliner

61
7

Hey! thank you for your help. Should I do while loop or add it tho the end of the do-while loop? thx – Kakarotto Jun 05 '20 at 06:16
3

NO NO NO https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong – 0___________ Jun 05 '20 at 06:21
1

Yes, I strongly suggest you to read that post, which is linked by P_J_ – Shubham Jun 05 '20 at 06:37
@P__J__ I wasn't considering concurrency, and I will edit my post to reflect that what I suggested was not best practice. However, the first part of what I said is still correct. – Joseph Pilliner Jun 05 '20 at 07:35

score 0 · Answer 3 · answered Jun 05 '20 at 06:43

Instead, you should try while loop.

#include <stdio.h>
#include <stdlib.h>

int main(void)
 {

     int num;
     FILE *ifp;

     ifp = fopen("numbers.txt", "r" );

     if(ifp == NULL)
     {
         perror("Read");
         exit(1);
     }

     while(fscanf(ifp, "%d ", &num) != EOF)
     {

         if(num % 2 != 0) // For every odd number.

                {
                printf("*\n");
                }
     }
    fclose(ifp);

    return 0;
}

score 0 · Answer 4 · answered Jun 05 '20 at 07:02

0

Have you tried this:

while (!feof(ifp)) {
   if (fscanf(ifp, "%d ", &num) > 0) {
      if(num % 2 != 0) // For every odd number.
                {
                printf("*\n");
                }
   }
}

answered Jun 05 '20 at 07:02

Catch22

391
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5

How to use EOF to read all data from file

4 Answers4