Writing a Binary file's name avoiding sprintf() in C

Question

I have been given a project which monitors 3 apartment's water usages. I need to write these usages into a binary file and the binary filename must be different for each apartment: The names must be:

"compact_usages_%d.bin"

where %d is either apartment 1, 2 or 3 and I am not allowed to use the following code:

sprintf(filename, "compact_usage_%d.bin", apartment);

Is there another way to do this without using sprintf() ?

Answer 1

Since your substitution is always the same size and is a constant offset from the beginning of the string, you can use array arithmetic to edit it directly:

#define BASE     "compact_usage_"
#define END      ".bin"
#define NAME     BASE "0" END
int main (void) {
    static char filename[] = NAME;
    unsigned char aptNo = 1;
    filename[sizeof(BASE)-1] = '0' + aptNo;
    printf("%s\n", filename);
    return 0;
}