Subj : Re: Reverse words in a string (Another Interview question)
To   : comp.programming
From : Roger Willcocks
Date : Wed Oct 05 2005 11:02 am

"Willem" <willem@stack.nl> wrote in message 
news:slrndk5m0d.ege.willem@toad.stack.nl...
> Roger wrote:
> ) That seems like a lot of work... it's hard to beat the canonical 
> solution:
> )
> ) #include <stdio.h>
> )
> ) void revtok(char* str, char sep)
> ) {
> )     char *tmp, *end;
> )     do {
> )         while (*str && *str == sep) str++;
> )         end = str;
> )         while (*end && *end != sep) end++;
> )         tmp = str; str = end;
> )         while (--end > tmp) { char t = *tmp; *tmp++ = *end; *end = t; }
> )     } while (*str);
> ) }
> )
> ) void main(int argc, char* argv[])
> ) {
> )     char str[] = "when in the course of human events";
> )     revtok(str, '\0');
> )     revtok(str, ' ');
> )     printf("%s\n", str);
> ) }
>
> I'll give it a go:
>
>  #include <stdio.h>
>
>  char *reverse(char *str)
>  {
>    char *ret, *src, *tgt;
>    int len, wordlen;
>
>    len = strlen(str);
>    ret = malloc(len);
>
>    src = str; tgt = ret + len;
>    while (*src) {
>      for (wordlen = 0; isspace(src[wordlen]); wordlen++)
> ;
>      tgt -= wordlen + 1;
>      tgt[0] = src[wordlen];
>      memcpy(tgt+1, src, wordlen);
>      src += wordlen + 1;
>    }
>    return ret;
>  }
>
>  int main(int argc, char *argv[])
>  {
>    if (argc < 2)
>      return -1;
>    return printf("%s\n", reverse(argv[1]));
>  }
>
> Which is not the canonical solution, but it seems to me
> that it's a bit easier to understand what it does.
>
> It's probably a bit slower too, but that's because it's not inline.
> If the function is speced to return a copy, then it should be faster.

Okay, how about:

#include <stdio.h>
#include <memory.h>

char* revstr(char* str, char* out)
{
    char* end;
    while (*str == ' ') str++;
    end = str;
    while (*end && *end != ' ') end++;
    if (*end) {
        out = revstr(end, out);
        *out++ = ' ';
    }
    memcpy(out, str, end - str);
    *(out += end - str) = 0;
    return out;
}

void main(int argc, char* argv[])
{
    char out[100];
    revstr("when in the course of human events", out);
    printf("%s\n", out);
}

--
Roger

.