String Swap Not Working

[rcbhat]

Hi, this is the code that I have to swap two strings-

void swap(char *,char *);

main()
{
  char *x="new";
  char *y="word";
  char *t;

  swap(x,y);
  printf("(%s,%s)",x,y);
 
}


void swap(char *x,char *y)
{
    char *t;

    t=x;
    x=y;
    y=x;



}

output- (new,word)

Why is the swap function not swapping the strings once the control comes out of swap? I am passing it using pointers (by reference).

[wildgoose]

Because you're only swapping within the scope of the function. Instead you need to pass the address of the pointers, not the memory referenced by those pointers!

Despite that, line 21 was wrong!

void swap(char *,char *);

main()
{  
   char *x="new";
   char *y="word";

   swap( &x, &y );

    printf("(%s,%s)",x,y);
}

void swap( char **x,  char **y)
{
    char *t;

     t = *x;
    *x = *y;
    *y = t;
}

[mvmalderen]

And don't forget to change line 1 of your code to: void swap(char **,char **); :)

[wildgoose]

oops, good catch tux4life.

I personally hate prototypes unless they're in header files. I intentionally order my functions in dependence order thus main is at the bottom of the file. But didn't do it in this case!

[Dave Sinkula]

Hi, this is the code that I have to swap two strings

Note also that you are not swapping strings; you are swapping pointers.

[rcbhat]

Thanks for all your help, I realized that you cannot actually 'pass by reference' in C, and the pointer manipulation is a cool way to accomplish the effect. That is what I was missing.

[kvprajapati]

rcbhat, Array of chars and pointer to chars is not same thing.
Declaration of your code is pointer to chars.

char *x="new";
  char *y="world";
  ...

'x' points to the first char of "new" which is an
array of constant chars in C++. A special rule in C++ allows the
assignment of the address of a string literal (array of const char) to
a pointer to modifiable char(s). Attempting to modify any const
object in C++, including string literals, produces undefined behavior.

If you are talking about C, not C++, the type of the string literal is
array of chars and NOT array of constant chars. Attempting to modify
a string literal in C produces undefined behavior not because they are const qualified chars, but because the C standard specifically states that it is undefined behavior.

http://c-faq.com/charstring/index.html
http://www.parashift.com/c++-faq-lite/
http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html

OP has another thread with different question for pointer to characters. - http://www.daniweb.com/forums/thread203375.html

[rcbhat]

Attempting to modify a string literal in C produces undefined behavior not because they are const qualified chars, but because the C standard specifically states that it is undefined behavior.

In the swap function that I wrote, I did not try to modify the string literal, I just wanted to interchange the pointers to the array of chars.

I realized that this cannot be accomplished using pass by values unless I am able to work with the actual pointers to the pointers to the chars (i.e. the pointers to the first elements of the string literals)

So, correct me if you think I may be wrong, but your reply is out of context for the question.

[jephthah]

So, correct me if you think I may be wrong, but your reply is out of context for the question.

it's correct, and totally within context.

because your original question and code made it appear that you were operating on a misconception about string literals. even if you did not actually have this misconception, it's a common problem and an unclear question could easily confuse others.

since these threads here will be available for a long time, his clarification is as much for future reference and is entirely appropriate.

[Kurt Kubing]

I think there's only one reason why your code will never work as you expected: you declare read-only strings. Although you didn't declare it with the const keyword, declaring string that way makes your string variable untouchable.

Here are my suggestions:

You either write your code this way:

 1.... char szStr1[] = "Hello";
 2.... char szStr2[] = "About";

 Take note that your strings have the same length. Although it should not be necessary, you have to make sure your string arrays' length can accomodate the length of the longer one.

 3.... void strSwap(char* str1, char* str2)
        {
              char szTemp[1024];
              strcpy(szTemp, str1);
              strcpy(str1, str2);
              strcpy(str2, szTemp);
        }

...or write your function this way:

 1.... void strSwap(char** ppStr1, char** ppStr2)
        {
              char * szTemp;
              int cch1, cch2;
              cch1 = strlen(*ppStr1);
              cch2 = strlen(*ppStr2);
              szTemp = (char*)malloc(cch1+1);
              strcpy(szTemp, *ppStr1);
              *ppStr1 = (char*)malloc(cch2+1);
              strcpy(*ppStr1, *ppStr2);
              *ppStr2 = szTemp;
        }

In the method, you have to declare you strings like this:

    char * szStr1 = "Help";
    char * szStr2 = "Anonymous";
    strtSwap(&szStr1, &szStr2);
    ...
    // use your szStr1 and szStr2
    ...
    free(szStr1);
    free(szStr2);

Well, the second method I think is costly. I just hope I can help you solve, or at least give a hint in solving your problem.

[Dream2code]

just like to add a point here,When we use
strcpy() function it actuaylly copies the 2nd string to the 1st one.

but when we assign
*p=x;
x=*q
*q=*p
like this here we only interchange the address variable.

these two things gives the same output but internal operation is different.

[tuse]

I think there's only one reason why your code will never work as you expected: you declare read-only strings. Although you didn't declare it with the const keyword, declaring string that way makes your string variable untouchable.

Its possible that what the original thread starter meant by 'swap' was to actually interchange the pointers to the read-only strings as Dave and Dream2Code have suggested.