Easy way to reverse a string

This is a discussion on Easy way to reverse a string within the C Programming forums, part of the General Programming Boards category; What is the easiest way to reverse a sting? I thought there was a function, but I cannot remember it. ...

  1. #1
    Slime Dragoon_42's Avatar
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    Easy way to reverse a string

    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?

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    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/

  3. #3
    Code Goddess Prelude's Avatar
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    >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.
    My best code is written with the delete key.

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

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    C++ Witch laserlight's Avatar
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    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.
    C + C++ Compiler: MinGW port of GCC
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    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Code Goddess Prelude's Avatar
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    >Here is an implementation of strrev
    A good example of the programmer trying to be clever...
    My best code is written with the delete key.

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    huh Prelude ????

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    Code Goddess Prelude's Avatar
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    >huh Prelude ????
    I was just commenting on this:
    Code:
    *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.
    My best code is written with the delete key.

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    aha

  10. #10
    Slime Dragoon_42's Avatar
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    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:
    Code:
    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?
    Last edited by Dragoon_42; 04-16-2004 at 10:09 AM. Reason: Forgot a question

  11. #11
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    hi dragon, i am following this topic. i got segmentation fault while calling your reverse_str function.

    my main code:
    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 ?
    blue_gene

  12. #12
    C Programmer Stack Overflow's Avatar
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    Post

    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:

    Code:
    #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
    Last edited by Stack Overflow; 04-16-2004 at 12:14 PM.
    Segmentation Fault: I am an error in which a running program attempts to access memory not allocated to it and core dumps with a segmentation violation error. This is often caused by improper usage of pointers, attempts to access a non-existent or read-only physical memory address, re-use of memory if freed within the same scope, de-referencing a null pointer, or (in C) inadvertently using a non-pointer variable as a pointer.

  13. #13
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    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

  14. #14
    & the hat of GPL slaying Thantos's Avatar
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    Exclusive OR (XOR) returns true only when the two values are different.
    Truth table for XOR:
    Code:
       0 1
    0| 0 1
    1| 1 0
    ^ is a bitwise XOR so if you have:
    1010 and 1101 you would get: 0111

  15. #15
    & the hat of GPL slaying Thantos's Avatar
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    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:
    Code:
    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:
    Code:
    #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.

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