Using fscanf() using feof()

Question

Here's my code.

#include<stdio.h>
void main(){
    FILE *fp;
    int a,b;
    fp=fopen("hello.txt","r");
    while(!feof(fp)){
      fscanf(fp,"%d %d",&a,&b);
      printf("%d %d\n",a,b);
    }
}

My hello.txt is

1   2
3   4

My Output is

1   2
3   4
4   4

Why is my last line being printed twice. Has not fp reached EOF?

Also,the tag in stackoverflow says Usually, when it is used, the code using it is wrong. What does it mean?

Thanks.

`main` should return `int` in C and C++ (your compiler may permit it, but don't do it.) — Jacob Parker, Mar 30 '13 at 14:06
That's an awesome tag description. `eof` is indeed almost never what you want. — Kerrek SB, Mar 30 '13 at 14:07
You are abusing `feof`: http://stackoverflow.com/questions/5431941/while-feof-file-is-always-wrong — William Pursell, May 15 '13 at 14:20

score 6 · Accepted Answer · answered Mar 30 '13 at 14:07

You must never perform an input operation without immediately checking its result!

The following should work:

while (fscanf(fp,"%d %d",&a,&b) == 2)
{
    printf("%d %d\n",a,b);
}

This will stop at the first conversion failure or end of the file. Alternatively, you could distinguish between conversion failures (to skip the erroneous line) and end-of-file; see the documentation of fscanf.

score 3 · Answer 2 · answered Mar 30 '13 at 14:10

Also,the tag in stackoverflow says Usually, when it is used, the code using it is wrong. What does it mean?

It means that the way the feof() function (and other functionality with regards to EOF in general) is used is often misunderstood and wrong. So is your code.

First, fscanf() doesn't always do what you think it does, and getting lines from a file is better performed using fgets(). However, if you're really inclined to use fscanf(), then check if it could read someting at all, else when it couldn't, you will print the variables one time more than needed. So what you should do is:

fp = fopen("hello.txt", "r");

while(fscanf(fp, "%d %d", &a, &b) == 2) {
    printf("%d %d\n", a, b);
}

fclose(fp);

Also, please do use whitespaces, your code is very hard to read.

teppic · Answer 3 · 2013-03-30T16:27:34.547

2

The reason you're getting an extra line is that EOF isn't set until after fscanf tries to read a third time, so it fails, and you print the results anyway. This would do the sort of thing you've intended:

while(1){
  fscanf(fp,"%d %d",&a,&b);
  if (feof(fp))
     break;
  printf("%d %d\n",a,b);
}

(Note that this example does not check for errors, only for EOF)

edited Mar 30 '13 at 16:27

answered Mar 30 '13 at 14:24

teppic

8,039
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This will break on ill-formed input [such as this one](http://pastebin.com/n1bViVDT). – Kerrek SB Mar 30 '13 at 16:12
@KerrekSB This is simply an example of how `feof` works, using the original code - which answers the question. – teppic Mar 30 '13 at 16:16

Johnny Mnemonic · Answer 4 · 2013-03-30T16:12:21.837

0

You can do the following:

#include <stdio.h>

void main(){
    FILE *fp;
    int a,b;
    fp=fopen("file.txt","r");
    while(fscanf(fp,"%d %d",&a,&b)==2)
    {
      printf("%d %d\n",a,b);
    }
}

edited Mar 30 '13 at 16:12

answered Mar 30 '13 at 14:09

Johnny Mnemonic

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5
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Using fscanf() using feof()

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