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c - Using fscanf() using feof()

问题描述:

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.

网友答案:

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.

网友答案:

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.

网友答案:

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)

网友答案:

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);
    }
}
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