NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/3357

Text File | 1996-08-05 | 2.3 KB | 97 lines

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: SORTing problem c++
Date: Sun, 28 Jan 96 02:09:32 GMT
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In article <[e-mail]> [e-mail] "heggie" writes:

>I'm used to normal c
>just recent i switched to c++, now i am stucked
>with a program that did work in c but doesn't
>in c++.
>it is a program that sorts the 'argv' array,
>what can I DO???
>
>
>
>#include <stdio.h>
>#include <stdlib.h>
>#include <string.h>
>
>
>int sort_function(char **a, char **b)
>{
>   return( strcmp(*a, *b));
>}
>
>
>int main(int argc, char **argv)
>{
>   int
>                x;
>
>   qsort(argv, argc, sizeof(char*), sort_function);
>
>   for (x = 0; x < argc; x++)
>         printf("%s\n", argv[x]);
>   return 0;
>}

Your program isn't valid C, let alone C++ (the fact that it happened
to work on your particular system is not relevant to this). The most
obvious problem is that qsort() requires a comparison function of the form

int sort_function(const void *, const void *);

which yours clearly isn't. An ANSI C compiler should have complained about
this, as will a C++ compiler. Instead use something like:

static int sort_function(const void *a, const void *b)
{
    return strcmp(*(const char *const *)a, *(const char *const *)b);
}

Your other problem is that it is not legal in C to modify the argv array
of pointers, so you need to make a copy of it and sort that.


int main(int argc, char **argv)
{
   int    x;

   if (argc > 0) {
      char **const argp = malloc(argc * sizeof(*argp));

      if (argp == NULL)
          exit(EXIT_FAILURE);

      memcpy(argp, argv, argc * sizeof(*argp));

      qsort(argp, argc, sizeof *argp, sort_function);

      for (x = 0; x < argc; x++)
         printf("%s\n", argp[x]);

      free(argp);
   }

   return 0;
}

Note that, like your code, this includes argv[0] in the sort which is the
program name.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
-----------------------------------------