c - grouping strings in a struct

Question

I have a bunch of strings that look like:

'Hello1-FOO', 'Aello2-FOO', 'Bye1-BAR', 'Bye3-BAR', 'Hello22-FOO', 'Bye4-BAR', 'Welcome-BAR' ...

All of them are stored on a struct.

struct str {
    char *strings;
}
...
struct str **t_str;
size_t j;
t_str = malloc(sizeof *t_str * 20);
for (j = 0; j < 20; j++)
    t_str[j] = malloc(sizeof *t_str[j]);
...
t_str[0]->strings = "Hello1-FOO";
t_str[1]->strings = "Aello2-FOO";
....

What I would like to do is to display (sort) them by category, so they look similar to this:

FOO:
 Hello1-FOO
 Aello2-FOO
 Hello22-FOO

BAR:
 Bye4-BAR
 Welcome-BAR
 Bye1-BAR
 Bye3-BAR

Basically group them by the token after the '-'

What would be a good way of doing this? Should I store them on a second struct after processing the string? Any idea will be appreciated. Thanks

The first thing to fix is that your struct allows for only one string. If that's the real code, you haven't gotten far enough to have this problem yet. — David Thornley
– David Thornley, Commented Aug 4, 2010 at 15:08
@David Thornley: I have already allocated memory for all the strings mentioned above. I can display them (ungrouped) without problem. I just wanted to comment where they being held. — Frank
– Frank, Commented Aug 4, 2010 at 15:14
So do you have one str instance for each string? That's what your code looks like, but the description made it sound like you wanted to store all the strings in a single str instance — Michael Mrozek
– Michael Mrozek, Commented Aug 4, 2010 at 15:23
@Michael Mrozek: Sorry for the wrong description. Yes I have one str instance for each string. — Frank
– Frank, Commented Aug 4, 2010 at 15:27

torak · Accepted Answer · 2010-08-04 15:33:20Z

1

Just use qsort. The following code makes some assumptions but you should be able to change it to suit your needs.

int categoryComparitor(const void * a, const void * b)
{
  char *string1 = (char *)a;
  char *string2 = (char *)b;

  string1 = strrchr(string1, '-') + 1;
  string2 = strrchr(string2, '-') + 1;

  return strcmp(string1, string2);
}


{
  ...
  char *strings[];  // Array of string pointers
  int stringCount;  // Holds current number of valid elements in strings.
  ...
  qsort(strings, stringCount, sizeof(char *), categoryComparitor);
}

edited Aug 4, 2010 at 15:33

answered Aug 4, 2010 at 15:27

torak

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1 Comment

schot Over a year ago

+1 for using standard libary functions instead of reinventing.

Jerry Coffin · Accepted Answer · 2010-08-04 15:28:24Z

As David Thornley already pointed out, your struct isn't really defined well to handle this situation (well at all). Since your input is two separate logical pieces, you really want to define the struct accordingly -- containing two separate strings, one for each part of the input.

struct record { 
    char *category;
    char *string;
};

Then you want to read each piece into one of those two strings:

record read_line(FILE *infile) { 
    char buffer1[128], buffer2[128];
    fscanf(infile, "%[^-]-%s", buffer1, buffer2);
    record ret;
    ret.string = dupe_string(buffer1);
    ret.category = dupe_string(buffer2);
    return ret;
}

Then, to sort those records, you'll want to define a comparison function with the signature expected by qsort, that does a comparison on the category member:

int cmp(void *a, void *b) { 
    return strcmp(((record *)a)->category, ((record *)b)->category);
}

Then you'll sort your array of records using that comparison function.

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