NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/12519

Internet Message Format | 1996-08-05 | 3.4 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: Help - sorting strings
Date: Mon, 01 Apr 96 13:13:27 GMT
Organization: none
Message-ID: <[e-mail]>
References: <4j11ne$kj9$[e-mail]> <4j67ki$[e-mail]>
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In article <4j67ki$[e-mail]>
           [e-mail] "Walt Dixon" writes:

>#include <stdlib.h>
>
>static int alphabetize( const void* arg1, const void* arg2 )
>{
>    return strcmp(*(char**)arg1, *(char**)arg2));
>}

I suggest you write that as:

     return strcmp(*(char*const*)arg1, *(char*const*)arg2));

While you can cast const away don't do it unless you have to. If nothing else
in this case some compilers can warn about asting qualifiers away so the
version with const has more chance of compiling cleanly.

>
>you pass a pointer to your sorting function to qsort.  qsort will then
>envoke your function as required to sort the array of strings.
>Here is how you would envoke the routine.
>
>qsort(words,num_elements,sizeof(char*),alphabetize);
>
>words is declared to be a char**, num_elements is the number of strings,
>sizeof(char*) is the size of each element in the char* array, and alphabetize
>is the pointer to the comparison funciton.
>
>To reverse sort all you would have to do is to reverse the two arguments
>to the strcmp function.
>
>I have attached a routine that reads in a bunch of strings.  One string
>per line, sorts then and prints them out.  I did not clean up
>the memory that was allocated.  It is only an example.
>
>walt
>
>---------------------------------240801314710921
>Content-Transfer-Encoding: 7bit
>Content-Type: text/plain
>
>#include <stdlib.h>
>#include <stdio.h>
>#include <string.h>
>
>int compare(const void* a, const void* b)
>{
>    return(strcmp(*(char**)b,*(char**)a));
>}

I suggest you write that as:

     return(strcmp(*(char*const*)b,*(char*const*)a));

While you can cast const away don't do it unless you have to. If nothing else
in this case some compilers can warn about asting qualifiers away so the
version with const has more chance of compiling cleanly.

>void main(int argc, char** argv)
  ^
 int

>{
>    FILE* fh;
>    char** words;
>    char buffer[132];
>    int i,ii;
>
>    if(argc != 2)
>    {
>        printf("No file specified.\n");
>        exit(1);

1 as an exit status has no defined meaning - use EXIT_FAILURE to indicate
a failure.

>    }
>    fh = fopen(argv[1],"r");
>    if(fh)
>    {
>        words = (char**) malloc(500 * sizeof(char*));
>        i = 0;
>        while(fscanf(fh,"%s \n",buffer)==1)
>        {
>            words[i++] = strdup(buffer);

strdup() isn't a standard library function - this simply won't compile/link
on many systems.

>        }
>        printf("read %d words\n",i);
>        qsort(words,i,sizeof(char*),compare);
>        for(ii=0; ii < i; ii++)
>        {
>            printf("%s\n",words[ii]);
>        }
>        fclose(fh);
>    }
>    else
>    {
>        printf("Unable to open %s\n",argv[1]);
>    }

     return 0; /* Or return EXIT_SUCCESS */
>}

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