Reverse String without strrev function

Question

How can I reverse a string in C by just giving a char* and the length as input parameters and recieving the reversed string as output? Also, without using the strrev function. Just by usign loops?

EDIT: Okay thats how I did it now:

char*  __stdcall Reverse(char *str, int length, char* *out)
{

 char *s = str;

    char *end = s + length - 1;
    for (; s < end; ++s, --end) {
        char ch = *end;
        *end = *s;
        *s = ch;
    }
      *out = str;
}

If this is for a class, you should come up with more than that on your own (or at least try searching first - [reverse a string in c](https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=reverse%20a%20string%20in%20c)) — crashmstr, Mar 26 '15 at 15:18

score 2 · Accepted Answer · answered Mar 26 '15 at 15:19

2

#include <string.h>

char *strrev(char *str)
{
  char *p1, *p2;

  if (! str || ! *str)
        return str;
  for (p1 = str, p2 = str + strlen(str) - 1; p2 > p1; ++p1, --p2)
  {
        *p1 ^= *p2;
        *p2 ^= *p1;
        *p1 ^= *p2;
  }
  return str;
}

#include <stdio.h>

int main(int argc, char *argv[])
{
  while (--argc)
  {
        printf("\"%s\" backwards is ", *++argv);
        printf("\"%s\"\n", strrev(*argv));
  }
}

You can implement your own strrev (I took it from here). Try c strrev source as a Google query.

answered Mar 26 '15 at 15:19

ForceBru

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Why downvote? What's the problem here? – ForceBru Mar 26 '15 at 15:21
See, i got confused because your code is hard to read, it's ok! Could it be `p1 = str; p2 = str + strlen(str) while (p2 > p1)`? wouldn't it be a lot more readable? – Iharob Al Asimi Mar 26 '15 at 15:22
5

You might want to check [this answer](http://stackoverflow.com/a/249469/91757) for why using the XOR swapping isn't actually a good idea these days. – Frerich Raabe Mar 26 '15 at 15:23
@FrerichRaabe right, and it doesn't hurt to use an extra `char` for this. – Iharob Al Asimi Mar 26 '15 at 15:24
Oh also, why `*++argv`? just `argv[i]` and a for loop? `for (i = 1 ; i < argc ; ++i)`? – Iharob Al Asimi Mar 26 '15 at 15:24
@iharob, there's no difference as compilers convert `smh[1]` to `*(smth+1)` – ForceBru Mar 26 '15 at 15:26
Exactly so why would you go through the trouble of reading `*(smth + 1)`? Instead of the much clearer `smth[1]`? Don't get me wrong, sometimes pointer arithmetic is the correct choice to express something in an algortithm, but like 90% of the time it's better to use subscript notation. – Iharob Al Asimi Mar 26 '15 at 15:28
@iharob, you see, it's not my code, actually. I've just removed some unnecessary `#ifdef`s that's all. – ForceBru Mar 26 '15 at 15:31
For avoiding UB with `strrev("")` --> +1 – chux - Reinstate Monica Mar 26 '15 at 15:39

Reverse String without strrev function

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