How do I add a newline character after every 3 characters in C?

Question

I have a text file "123.txt" with this content:

123456789

I want the output to be:

123
456
789

This means, a newline character must be inserted after every 3 characters.

void convert1 (){
    FILE *fp, *fq;
    int i,c = 0;
    fp = fopen("~/123.txt","r");
    fq = fopen("~/file2.txt","w");
    if(fp == NULL)
        printf("Error in opening 123.txt");
    if(fq == NULL)
        printf("Error in opening file2.txt");
    while (!feof(fp)){
        for (i=0; i<3; i++){
            c = fgetc(fp);
            if(c == 10)
                i=3;
            fprintf(fq, "%c", c);
        }
        if(i==4)
            break;
        fprintf (fq, "\n");
    }
    fclose(fp);
    fclose(fq);
}

My code works fine, but prints a newline character also at the end of file, which is not desired. This means, a newline character is added after 789 in the above example. How can I prevent my program from adding a spurious newline character at the end of the output file?

`while (!feof(fp))` is [always wrong](http://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong). — melpomene, Jul 26 '15 at 14:15
@Sebi: Read **carefully** what [`feof`](http://port70.net/~nsz/c/c11/n1570.html#7.21.10.2) does, then read further and think about the implications. If that does not help, read the q&a linked by melpomene. — too honest for this site, Jul 26 '15 at 14:22

honk · Accepted Answer · 2015-07-26T15:16:33.860

As indicated in the comments, your while loop is not correct. Please try to exchange your while loop with the following code:

i = 0;
while(1)
{
    // Read a character and stop if reading fails.
    c = fgetc(fp);
    if(feof(fp))
        break;

    // When a line ends, then start over counting (similar as you did it).
    if(c == '\n')
        i = -1;

    // Just before a "fourth" character is written, write an additional newline character.
    // This solves your main problem of a newline character at the end of the file.
    if(i == 3)
    {
        fprintf(fq, "\n");
        i = 0;
    }

    // Write the character that was read and count it.
    fprintf(fq, "%c", c);
    i++;
}

Example: A file containing:

12345
123456789

is turned into a file containing:

123
45
123
456
789

score 0 · Answer 2 · answered Jul 26 '15 at 15:15

I think you should do your new line at the beggining of the lopp:

// first read
c = fgetc(fp);
i=0;
// fgetc returns EOF when end of file is read, I usually do like that
while((c = fgetc(fp)) != EOF)
{
   // Basically, that means "if i divided by 3 is not afloating number". So, 
   // it will be true every 3 loops, no need to reset i but the first loop has
   // to be ignored     
   if(i%3 == 0 && i != 0)
   {
     fprintf (fq, "\n");
   }

   // Write the character
   fprintf(fq, "%c", c);

   // and increase i
   i++;
}

I can't test it right now, maybe there is some mistakes but you see what I mean.

How do I add a newline character after every 3 characters in C?

2 Answers2