Thread: swapping values with pointers

this program is supposed to take in two values, then use a function [void swap(( double *, double *);] to swap the variables and print them out. I was working on it with a friend of mine and he said something about needing another variable to do the swap and I just didn't understand what he was trying to say. Thanks for the help.

Code:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    void swap (double *, double *);
    
    double *ptrA, *ptrB;
    double a, b;
 
    
    ptrA = &a;
    ptrB = &b;
    
    printf ("Please enter two numbers.");
    scanf ("&#37;d %d", &a, &b);
    
    printf ("\nNumber 1 is: %d",a);
    printf ("\nNumber 2 is: %d",b);
    
    
    swap (&a, &b);
    
    system("PAUSE");
    return 0;
}

    void swap (double *ptrA, double *ptrB) {

         
         *ptrA = *ptrB;
         *ptrB = *ptrA;
         

         
         printf ("\n\nNew number 1 is %d:",*ptrA);
         printf ("\nNew number 2 is %d:\n\n",*ptrB);
}

Seriously?

Suppose I want to exchange the contents of two cups. One has coffee, the other has tea. I begin by pouring the tea into the coffee...

Is it that hard to grasp the need for a third cup?

i would also recommend moving the printf statements that show the swapped values. it wont change anything in the situation now, but it helps to show that a and b in main have actually changed.
for example, if you did a program like this in the future and somehow it changed the variables in your function so it seemed to work, but in fact maybe something is wrong and the variables you meant to change in main have stayed the same.

so what i would do is move the 2 printf statements in swap and put them in the main function after calling swap but of course before the system("pause") command.

just a suggestion

You could also use bitwise XOR:

Code:

*ptrA ^= *ptrB;
*ptrB ^= *ptrA;
*ptrA ^= *ptrB;

If you don't know what it is, look it up on google. If this is an assignment, you should definitely use XOR, it'll give you some extra credit for sure.

i doubt it would give extra credit--it would look like an attempt from someone who was confused. it looks messy and, since ive never used bitwise operators but would consider myself experienced, it looks confusing when what they are trying to do is actually simple. i think it over complicates things.

again, just my thoughts.

Originally Posted by IceDane

If you don't know what it is, look it up on google. If this is an assignment, you should definitely use XOR, it'll give you some extra credit for sure.

XOR'ing two doubles... That's rich.

I got my ass handed to me when I dared to mention the XOR trick on this forum. I mentioned it as a bit of trivia, not an actual SUGGESTION to use it. And here you are actually telling somebody to use it.

You're setting him up for an awful embarrassment when the instructor asks how it works.

Originally Posted by brewbuck

XOR'ing two doubles... That's rich.

I got my ass handed to me when I dared to mention the XOR trick on this forum. I mentioned it as a bit of trivia, not an actual SUGGESTION to use it. And here you are actually telling somebody to use it.

You're setting him up for an awful embarrassment when the instructor asks how it works.

Didn't notice it was doubles we were talking about.

I told him to look it up before he used it, of course. There's nothing wrong with the XOR trick, it's faster and more efficient(Not that it really matters in this case), but it saves you extra x bytes of memory. Also, it is hard for the compiler to detect that you're swapping two variables.

Robatino: Whether or not the variables have the same address is irrelevant - You're swapping the values stored at the addresses.

thank you very much for the help dwks. yes i am about as novice as they come to C, which is why i didn't get the whole third variable needed the way my friend was trying to explain it to me. I'm trying to get my language skills better (to get a job in web design, the only language i know right now is html and very little c++ i did in high school 5 years ago).

brewbuck, is it hard to grasp the concept of not being a dick? dwks and others answered my question with respect which i why i came here for help with coding, thanks again for the help in past/present/future yall.

Originally Posted by mackieinva

brewbuck, is it hard to grasp the concept of not being a dick? dwks and others answered my question with respect which i why i came here for help with coding, thanks again for the help in past/present/future yall.

I guess that was ruder than I was shooting for. But don't just turn your brain off because you're dealing with a computer. Too many people have a habit of doing that. When you want to exchange two items, be it variables or physical objects, it should be apparent that you need some "space" in order to do it.