在C中合并两个文件

Question

I want to merge two files in C, and stop in the end.

I have a program but it keeps spelling out words even after my file is ended. For example:

File 1:

a
b
c

File 2:

1
2
3
4
5

And the result I get is:

a
1
b
2
c
3
c
4
c
5

And I want:

a
1
b
2
c
3
4
5

I think it's my if statement in my while loop that is my issue. I don't know how to define that.

My code is:

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


int main(void) {

    char d[200];                        //char för da.txt
    char h[200];                        //char för hej.txt
    FILE *hptr, *dptr;

    hptr = fopen("hej.txt", "r");       //Öppna hej.txt => hptr
    dptr= fopen("da.txt", "r");         //Öppna da.txt => dptr

    /*
    if(((hptr) == NULL && (dptr) == NULL)) //Fall 1 
    {
        printf("Error, båda filerna är tom");
        exit(1);
    }


    if ((hptr) == NULL)                 //Fall 2 när hej.txt är tom
    {
        fscanf(dptr,"%[^/]", d);        //Hämtar från da.txt
        printf("\nFil 2:\n%s", d);      //Skriver ut från da.txt
        exit(1);
    }

    if ((dptr) == NULL)                 //Fall 3 när da.txt är tom
    {
        fscanf(hptr,"%[^/]", h);        //Hämtar från hej.txt
        printf("Fil 1:\n%s", h);        //Skriver ut från hej.txt
        exit (1);
    } */

    if(hptr != NULL || dptr != NULL)        //Fall 4 när ingen fil är tom
    {

        while (!feof (hptr) && !feof (dptr))
        {
            if (hptr ***I guess this is the problem*** == feof)
            {
            fgets(d, 200, dptr);
            printf("%s", d);
            }

            if (hptr == feof)  
            {
            fgets(h, 200, hptr);
            printf("%s", h);
            }

        }

        fclose (hptr);
        fclose (dptr);
    }
    //getch();
    return EXIT_SUCCESS;
}

Answer 1

feof only updates when you try to do some IO. When you call fgets, and it reaches EOF and fails, you just print the string anyway, even though it wasn't updated. This is why you get the repeated "c"

Try this instead:

 while (!feof (hptr) || !feof (dptr))
 {
    if (fgets(d, 200, dptr))
    {
        printf("%s", d);
    }

    if (fgets(h, 200, hptr))  
    {
        printf("%s", h);
    }
}

Answer 2

Try to use

!feof (hptr) 

instead of

hptr == feof

Explanation: feof is a function -- you are comparing a file pointer hptr against a function pointer feof which will never work -- but since you can always compare pointers in C the compiler does not give you any errors.

You also appears to have a cut-and-paste error as you are using hptr in both if statements -- Im sure one of them should have been dptr

Answer 3

Actually you need this:

  if (hptr != NULL && dptr != NULL)        //Fall 4 när ingen fil är tom
  {               //^ you need && here, not ||

    do
    {
        if (fgets(d, 200, dptr) != NULL)
          printf("%s", d);

        if (fgets(h, 200, hptr) != NULL)
          printf("%s", h);
    } while (!(feof(hptr) && feof(dptr)));

    fclose(hptr);
    fclose(dptr);
  }

Please see this SO question: Why is “while ( !feof (file) )” always wrong?

Answer 4

You don't need feof() (it is terrible! ) , just use fgets()s return value, alternating between the two input files.

#include <stdio.h>

int main(void)
{
char buff[100];
int state;

FILE * one, *two;
one = fopen("one", "r" );
two = fopen("two", "r" );

if (!one || !two) return 1;

for (state=0; ; state ^=1 ) {
        if (!fgets( buff, sizeof buff, state ? two: one)) break;
        printf("%s", buff);
        }

while (fgets( buff, sizeof buff, state? one : two)) {
        printf("%s", buff);
        }
fclose(one);
fclose(two);

return 0;
}