Subj : Re: Reverse words in a string (Another Interview question)
To   : comp.programming
From : Willem
Date : Tue Oct 04 2005 08:29 pm

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.


SaSW, Willem
-- 
Disclaimer: I am in no way responsible for any of the statements
            made in the above text. For all I know I might be
            drugged or something..
            No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

.