fscanf C编程错误

Question

I am not new to programming, but I encountered this small problem and I can't seem to get it. I want to read a file with dates and put them in another file with another format

Input example : 18.08.2015
Output example : 18-08-2015

Here is the code (dat1 has "r" permission and dat2 "w"):

char d[3];
char m[3];
char g[5];
while(fscanf(dat1,"%s.%s.%s\n",&d,&m,&g)==3)
{
    fprintf(dat2,"%s-%s-%s\n",d,m,g);
}

On the other hand, this works fine if I use [space] instead of a [dot] in the input file. (18 08 2015)

What am I missing? The solution has to be as simple as possible and with using fscanf, not fgetc or fgets, to be explained to students that are just beginning to learn C. Thanks.

Answer 1

The %s pattern matches a sequence of non-white-space characters, so the first %s will gobble up the entire string.

Why use char arrays at all, why not int?

int d;
int m;
int g;
while(fscanf(dat1,"%d.%d.%d\n",&d,&m,&g)==3)
{
    fprintf(dat2,"%d-%d-%d\n",d,m,g);
}

The %d in fprintf will not output leading zeros though. You'll have to teach your students a little bit extra or leave it for extra credit.

Answer 2

Since the scanf format %s reads up to the next whitespace character, it cannot be used for a string ending with a . . Instead use a character class: %2[0-9] or %2[^.] . (Change the 2 to the maximum number of characters you can handle, and don't forget that the [ format code does not skip whitespace, so if you want to do that, put a space before the format code.)

Answer 3

Change

  fscanf(dat1,"%s.%s.%s\n",&d,&m,&g)

to

  fscanf(dat1,"%[^.].%[^.].%[^.]\n",d,m,g);