Go back when printing a new character

Question

Hi,

in the following piece of code:

.......
 mystring ID;
 char dum[]="_";         
 fprintf(fp_file,"%s%s%d   ",ID,dum,1);
.....

ID is a string like "customers "

so I get the output:

"customers _1"

But my intention is to get

"customers_1"

For technical reasons I can not modify ID in any way, so I wonder how, when the program is printing dum, could it go a character back, overwritting the white space which forms part of ID, so I get my desired result.

Thanks a lot

Answer 1

Why not just loop through your ID string until you find the space, and print every character except the space. Try this:

void print_id(FILE *file, const char *id, char delim, int num)
{
    while(*id && *id != ' ') fputc(*id++, file);
    fprintf(file, "%c%d\n", delim, num);
}

You could use strchr() to find spaces and calculate the length of the ID you want printed with that, because printf()'s %s specifier can be used to print only part of a string, but then you'd be looping over the string twice. This way we only do it once. Usage:

print_id(ID, '_', 1);

Update: If you need all characters but the last one, Nick D's answer is the way to go. It uses one of the more esoteric features of the *printf() family of functions to tell printf() to print only up to the last character. There is a way to do this with a loop and fputc(), but the minor efficiency difference isn't really worth the hassle of working out how to write it and how to document it so that your successor can understand what you're doing. Because you are documenting it, right? :P

Answer 2

Copy ID to a temporary string that you can modify, and strip the space (I'm assuming it is a space) from the end of your new string.

Answer 3

You can bound the string size like this:

fprintf(fp_file,"%.*s%s%d   ",strlen(ID)-1, ID, dum, 1);

Answer 4

Your ID string seems to have a trailing space.

try something like

int i = strlen(ID);
if (ID[i-1] == ' ') ID[i-1] = '\0'

This does edit the string though....

Answer 5

If your ID string always contains trailing space you could add a backspace characters to the fprintf() format string to remove them.

It's a bit hacky, but this small example program Works For Me (TM):

#include <stdio.h>

int main(int argc, char **argv)
{
    FILE * fp;
    const char *id = "customers ";
    const char *dum = "_";
    fp = fopen("foo.txt", "w");
    if (fp) {
            fprintf(fp, "%s\b%s%d\n", id, dum, 1);
            fclose(fp);
    }
    return 0;
}