Easy way to reverse a string

[Dragoon_42]

What is the easiest way to reverse a sting? I thought there was a function, but I cannot remember it. Is there a page that lists all functions of all classes I could look at?

[noob2c]

easy way would be to copy is to use a loop and some functions from the string header file

http://www.cplusplus.com/ref/cstring/

[Prelude]

>What is the easiest way to reverse a sting?
If your compiler supports strrev or anything similar then that's the easiest way. Otherwise, a loop that walks from either end of the string toward the middle and swaps until the two indices are the same would come a close second.

[noob2c]

Here is an implementation of strrev
http://64.233.167.104/search?q=cache...hl=en&ie=UTF-8

[laserlight]

You could easily implement one yourself, e.g.
1. Iterate from the last character to the first, placing them into a new string.
2. Swap the nth character with the (length - n)th character.

[Prelude]

>Here is an implementation of strrev
A good example of the programmer trying to be clever...

[noob2c]

huh Prelude ????

[Prelude]

>huh Prelude ????
I was just commenting on this:

*p1 ^= *p2;
*p2 ^= *p1;
*p1 ^= *p2;

It's cute, but shouldn't be used in any real code without an exceptionally good reason. It's hard to use correctly, and very few people understand the nuances.

[noob2c]

aha

[Dragoon_42]

Wow. Thanks for all the help! I thought there was a built in function, but aparrently my GCC compiler does not support it. So I just wrote my own:

char* reverse_str(char *m)/*reflects rotor*/
{
	char rev[26];
	strcpy(rev,m);/*stores rotor for reversal*/
	unsigned length=strlen(m);
	unsigned count;/*counter for string reversal*/
	for(count=0;count<length;count++)
	{
		m[count]=rev[length-count];
	}
	return m;
}

BTW, what does the ^= operator do?

[blue_gene]

hi dragon, i am following this topic. i got segmentation fault while calling your reverse_str function.

my main code:

int main()
{
char*s ;
s = reverse_str("mango");
printf("%s",s);

}

output > segmentation fault

why ??


another question, what is unsigned ?? it should be unsigned int. r u sure ur code is ok ?

N.B : is it not necesarry to null terminate inside ?

[Stack Overflow]

Hi all,

Just for my opinion, and the nice implentation, I'd definetly go for the strrev() code. It's really nice in my opinion, and it worked perfectly first time I compiled it:

#include <stdlib.h> // for printf

char* my_strrev(char *);

char* my_strrev(char *string1) {
	char *p1, *p2;

	if (!string1 || !*string1)
		return string1;

	for (p1 = string1, p2 = string1 + strlen(string1) - 1; p2 > p1; ++p1, --p2) {
		*p1 ^= *p2;
		*p2 ^= *p1;
		*p1 ^= *p2;
	}

	return string1;
}

int main () {
	char str[256] = {"!looc si sihT"};

	printf("%s\n", my_strrev(str) ); // call on function in here for ex.

	return 0;
}

Code 1.1: Using my_strrev()

Now I only called the function my_strrev() because any previous header file or DLL linkage might try figthing over which function to use and etc...

I take my hat off to noob2c for finding this function I'm making a replica of the string library, so many thanks, I'll have to add this implementation to my project.

Edit (after noob2c post): Ah, ok. Much thanks to you too Prelude


Hope this helps,
- Stack Overflow

[noob2c]

no problem..... it was prelude who knew the name of the function .... i did a search to find it. As for ^= that means exclusive or or XOR

[Thantos]

Exclusive OR (XOR) returns true only when the two values are different.
Truth table for XOR:

   0 1
0| 0 1
1| 1 0

^ is a bitwise XOR so if you have:
1010 and 1101 you would get: 0111

[Thantos]

I always prefer to take the easist route until I'm sure of what is going on. As such I would recommend what Prelude suggest and avoid the codes that use ^= until you are 125% sure you know what it is doing.

Below is a quicky I threw together that uses 0 tricks and should be easy enough to walk through. Hope it is of help:

char *strrev(char *str)
{
  unsigned len = strlen(str) - 1; /* Minus one so we don't move the null character */
  int count;
  char ch;
  for (count=0; count < len; count++, len --)
  {
    ch = str[count];
    str[count] = str[len];
    str[len] = ch;
  }

  return str;
}

A test program could look like this:

#include <stdio.h>
int main (void)
{
  char msg1[]="Hello my baby hello my darling hello my ragtime gal!";
  char msg2[]="Save me a kiss by wire, baby my heart is on fire!";

  puts("Before");
  puts(msg1);
  puts(msg2);
  puts("After");
  puts(strrev(msg1));
  puts(strrev(msg2));

  return 0;
}

Note: Changing the declaration of msg1 and msg2 from char [] to char * will result in a segmentation fault since that area of memory will be marked as read only.