How to change 2 strings using a function, by changing their pointers' value

Question

main()

char *s1="Second";
char *s2="First";
swap(s1,s2);
printf("%s\n",s1);
printf("%s\n",s2);

I have as an exercise, to swap those 2 strings above (so that the one that executes the program will see "First Second" instead of "Second First"), by changing the values of their pointers, using the function swap (that I have to make).

roger_that · Accepted Answer · 2013-05-17 07:45:06Z

3

This would do what you need

void swap(char **s1, char **s2){
  char *temp=*s1;
  *s1=*s2;
 *s2=temp;
}

int main(){
  char *s1="second";
  char *s2="first";
  swap(&s1,&s2);
  printf("%s",s1);
  printf("%s",s2);
 return 0;

}

answered May 17, 2013 at 7:45

roger_that

9,84119 gold badges72 silver badges110 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

xaxxon Over a year ago

This is the best answer that is actually in C. It doesn't use tricks, and it doesn't use c++-only functionality.

Liviu Over a year ago

Wow! This code may be genious for C lovers, but you changed the user's code: swap(&s1,&s2); is not swap(s1,s2);. IMHO, this is more serious than ignoring the tag.

vinod · Accepted Answer · 2013-05-17 08:23:43Z

0

char *a="vinod";
char *b="kumar";

a=(char*)((int)a+(int)b);
b=(char*)((int)a-(int)b);
a=(char*)((int)a-(int)b);
printf("%s%s",a,b);

answered May 17, 2013 at 8:23

vinod

1325 bronze badges

1 Comment

xaxxon Over a year ago

Uhh.. aren't there serious buffer overflow issues here? I think you're really thinking about the XOR trick, but from what I've learned recently, that's actually an undefined behavior for non-integer (including pointer) types. While this may work sometimes, I'd recommend staying away from this solution. It's "tricky", likely not correct, and the compiler will do a good job for you if you do things the common way. Do what Liviu says and use a temporary pointer.

Linus Caldwell · Accepted Answer · 2013-05-17 07:49:48Z

-1

inline void swap(char*& s1, char*& s2)
{
    char* temp = s1;
    s1 = s2;
    s2 = temp;
}

edited May 17, 2013 at 7:49

Linus Caldwell

11.1k12 gold badges48 silver badges61 bronze badges

answered May 17, 2013 at 7:30

Liviu

1,9172 gold badges22 silver badges51 bronze badges

6 Comments

Liviu Over a year ago

Well, I'm kind of new here, but I don't think it's alright to solve your school problems :p

xaxxon Over a year ago

This isn't valid C. This is C++. Either you mis-tagged your question, or this isn't a valid solution.

Liviu Over a year ago

@xaxxon The tag is incorrect, you can't do swap(s1,s2); in C, see the next solution.

xaxxon Over a year ago

i'm guessing the question is tagged correctly but the asker doesn't know he can't do what he wants and needs to change the prototype for swap, not change languages.

Liviu Over a year ago

@xaxxon Or maybe the teacher written the above code (the question) and demanded the implementation. I'm in a middle of a minor polemic right now: it seems it is common these days to learn directly C++ (and skip C entirely)

|

Collectives™ on Stack Overflow

How to change 2 strings using a function, by changing their pointers' value

3 Answers 3

2 Comments

1 Comment

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related