Adding spaces to existing file

Can anyone help? For some reason this code won't work. The file remains unchanged.

Code:

---

#include <stdio.h>

int main ()
{
        FILE *f1;
        int ch;

        f1 = fopen("file2.txt", "r+");

        if (f1 == NULL)
        {
                printf("File access error.\n");
                return;
        }

        while((ch = fgetc(f1)) != EOF)
        {
                if (ch == '\n')
                fputc('\n', f1);
        }

        printf("File processing complete.\n");

    if (fclose(f1) != 0)
    printf("File close error.\n");

        return 0;
}

---

This kind of question is asked not very long ago.
Next time, make sure that you search the forum before posting.
Question 19.14
http://cboard.cprogramming.com/c-pro...ying-file.html

I'm sorry, but those links don't help. I know the algorithm, but the reason the code won't work is most likely due to careless mistakes.

Code:

---

while((ch = fgetc(f1)) != EOF)
{
    // replace all '\n' with '#'
    if (ch == '\n')
        //fputc('\n', f1);
        unget('#', f1);
}

---

Never mind. That won't work.

Is that C++? I'm using C, so unget will not work.

It's actually ungetc(). I made a typo.

What were you trying to do?

This is a task where you need to open two files... one as your input file which is read sequentially and the other as an output file that is written sequentially. The problem being that you can't insert text into a sequential file without overwriting some part of it's existing content.

I thought that's what r+ was supposed to do? I wanted the program to insert the spaces while going through the file sequentially. Even though it's supposed to modify the file in some way, I fail to see any change at all. Or is there no other choice but to open up another file for writing?

EDIT: I just checked and the program indeed works if I choose to output a second file.

The thing is that when you have a sequential file, there is no way to push characters back to make room for your insertion... you end up overwriting it's contents with your insertions.

Conceptually...

Code:

---

aaaaaaaaaaaaaabzzzbbbbbbbbbbbbbbbbbbbcccccccccccccccddddddddddd
              ^ file pointer here... insert ///

aaaaaaaaaaaaaab///bbbbbbbbbbbbbbbbbbbcccccccccccccccddddddddddd
                  ^ file pointer now here.

---

Where'd your zzz go?

The only way around this is to open a second "output" file....
To clean things up you can delete the original and rename the new one.

No need for a separate output file. I would use fread and fwrite... but anyways, you’d need to do an fseek to backup one byte before overwriting with a substitute character. Although your first post indicates you are substituting with the SAME character.

Quote:

---

Originally Posted by nonoob

No need for a separate output file. I would use fread and fwrite... but anyways, you’d need to do an fseek to backup one byte before overwriting with a substitute character. Although your first post indicates you are substituting with the SAME character.

---

I'd still go with the two file system... that way if the program crashes, the input file isn't trashed.

I did try using fseek, in addition to printing an extra newline, but both did not manage to alter the file at all. Here's the section I changed:

Code:

---

        while((ch = fgetc(f1)) != EOF)
        {
            fputc(ch, f1);
                if (ch == '\n')
                {
                    fseek(f1, -1L, SEEK_CUR);
                    fputc('\n', f1);
                    fputc('\n', f1);
                }
        }

---

Oh wow, man. Reading a char, then immediately writing it... I have no idea if the system keeps separate file positions for read vs. write. I doubt it. So you may be duplicating a character into the NEXT position. Then you're backing up one, writing it again (why?), then writing a subsequent one. I think you need to relax and figure out what you're trying to accomplish.

I'm with CommonTater, two files should be used. here is some code that will read, find and replace, then remove the input file and rename the output file to the input.
Disclaimer: i haven't tested this yet. Correct me if i'm wrong

Code:

---

#include <stdio.h>

int main(int argc, char** argv) /* you don't need these args, but i like putting them in regardless */
{
        char line[255]; /* or however long you want */
        FILE* in = fopen("file2.txt", "r");
        FILE* out = fopen("_file2_.txt", "w");
        if (in == NULL)
        {
                printf("File access error.\n");
                return 0;
        }
        while(fgets(line, sizeof line, INSTREAM) != NULL) /* if you only have 1 line, you can comment this out, but if the file contains multiple lines, leave this in */
        {
                for(i = 0; line[i] != NULL; i++)
                {
                        if (line[i] == "#") /* finds "#" */
                        {
                                fprintf(out, "%c", "*";        /* writes "*" */
                        }
                        else
                        {
                                fprintf(out, "%c", line[i]); /* writes the letter that isn't "#" */
                        }
                }
        }
        fclose(in); fclose(out);
        remove("file2.txt"); /* deletes the input file */
        rename("_file2_.txt", "file2.txt"); /* names the output file, what the input file was */
        return 0;
}

---

Quote:

---

Originally Posted by nonoob

Oh wow, man. Reading a char, then immediately writing it... I have no idea if the system keeps separate file positions for read vs. write. I doubt it. So you may be duplicating a character into the NEXT position. Then you're backing up one, writing it again (why?), then writing a subsequent one. I think you need to relax and figure out what you're trying to accomplish.

---

Reading and writing uses the same position? That still doesn't explain why the file doesn't change. I mean, it still should have changed (new lines, spaces, deletions, etc.) but it doesn't.

Quote:

---

Reading and writing uses the same position? That still doesn't explain why the file doesn't change. I mean, it still should have changed (new lines, spaces, deletions, etc.) but it doesn't.

---

The file is not changed at all?
I don't believe that. I compiled and run your code(post #1 code). The file is *changed*.

My eyes must be deceiving me. Which part changed?

Code:

---

$ cat file2.txt
this is a test file
ust for testing only
bye bye...

$ cc rw.c
$ ./a.out
File processing complete.
$ cat file2.txt
this is a test file

st for testing only

ye bye...

---

rw.c source code is exactly your code. copy and pasted.
It's time to post your program output...

Are you on Linux? Perhaps it's a compiler problem.

Code:

---

#include <stdio.h>

int main ()
{
        FILE *f1;
        int ch;

        f1 = fopen("file2.txt", "r+");

        if (f1 == NULL)
        {
                printf("File access error.\n");
                return;
        }

    while((ch = fgetc(f1)) != EOF)
        {
        printf("%c", ch);
        }

        rewind(f1);

        while((ch = fgetc(f1)) != EOF)
        {
                if (ch == '\n')
                fputc('\n', f1);
        }

        printf("\n\nFile processing complete.\n\n");

        rewind(f1);

        while((ch = fgetc(f1)) != EOF)
        {
        printf("%c", ch);
        }

    if (fclose(f1) != 0)
    printf("File close error.\n");

        return 0;
}

---

The output:

Code:

---

aaaaa
aaaaa
aaaaa

File processing complete.

aaaaa
aaaaa
aaaaa
Process returned 0 (0x0)  execution time : 0.011 s
Press any key to continue.

---

When I switch to the two-file system, the algorithm works correctly, so I suppose I'll do just that, now that I know how to remove and rename files. Thanks, guys.

Quote:

---

Originally Posted by nonoob

Oh wow, man. Reading a char, then immediately writing it... I have no idea if the system keeps separate file positions for read vs. write. I doubt it.

---

It doesn't.