need help with pointers

[xiansen]

Hi guys, I am trying to program some sample tasks with pointers. Below is the step by step instructions that I have to follow. I am having trouble at step 10 which is to swap the values of 2 variables using pointers. please help, I have tried almost anything out of the book and can't still figure it out.

1. Create to integer variables named x and y
2. Create an int pointer named p1
3. Store the address of x in p1
4. Use p1 to set the value of x to 99
5. Using cout, display the value of x
6. Using cout and the pointer p1, display the value of x
7. Store the address of y into p1
8. Use p1 to set the value of y to -300
9. Create two new variables: an int named temp, and an int pointer named p2
10. Use temp, p1, and p2 to swap the values in x and y
11. Write a function with the following signature: void noNegatives(int *x). The function should accept the address of an int variable. If the value of this int is negative it should set it to zero
12. Invoke the function twice: once with the address of x as the argument, and once with the address of y
13. Use p2 to display the values in x and y
14. Create an int array with two elements. The array should be named a
15. Use p2 to initialize the first element of a with the value of x
16. Use p2 to initialize the second element of a with the value of y
17. Using cout, display the address of the first element in a
18. Using cout display the address of the second element in a
19. Use p1, p2, and temp to swap the values in the two elements of a
20. Display the values of the two elements.

#include "stdafx.h"
#include <iostream>
void noNegatives(int *x);

using namespace std;

int main()
{
//1
    int x, y;
    //2
    int *p1;
    
    //3
    p1 = &x;

    //4
    *p1 = 99;

    //5
    cout << "the value of x is " << x << endl;

    //6
    cout << " the value of x using *p1 is " << *p1 << endl;

    //7
    p1 = &y;

    //8
    *p1 = -300;

    //9
    int temp, *p2;

    //10 I learned how to swap values using 3 variables.but, can't seem to do it 
    // with  pointers. 

     p2 = &temp;
     *p2 = *p1;
     temp = x;
     x= y;
     y= temp;
    

    cout << x << endl;
    cout << y << endl;
    
    //11 this fn is supposed to set pointers to 0 if they have negative numbers.
    
    noNegatives(p1);
    noNegatives(p2);

    // apparently, *p2 comes out the same regardless of the x and y.
    cout << "y value is" << *p2 << endl;
    cout << "x value is" << *p2 << endl;

    //14
    int a[2];

    //15 i think this is wrong since i am not supposed to assigned a new int
    //to the pointer. please help me on this too.
    p2 = &a[0];
    *p2 = 99;

    //16
    p2 = &a[1];
    *p2 = *p1;
	
	
    //17 & 18
    cout << "the first element is " << a[0] << endl;
    cout << "the second element is " << a[1] << endl;

    //19 and 20
    p2 = &temp;
    *p2 = 99;
	
    a[1] = temp;

    p2 = &temp;
    *p2 = *p1;

    a[0] = temp;


     cout << "the first element is " << a[0] << endl;
     cout << "the second element is " << a[1] << endl;



    system("pause");
    return 0;
}

void noNegatives(int *x)
{
    if (*x < 0)
    {
        *x = 0;
        
    }
}

[Narue]

It'll help if you pass the address of the two objects to a function:

void swap(int *a, int *b)
{
  // Swap the objects pointed to by a and b
}

int main()
{
  int x = 10;
  int y = 20;

  swap(&x, &y);
}

This way you won't be confused by the presence of the original variables themselves.

[xiansen]

thanks for the example. but I am to strictly to follow the instructions.otherwise, i would just use three variables way to swap the values.

[Narue]

I get that, but you're clearly confused by the presence of the variables x and y. Using a function, if only as a temporary crutch, helps you understand what's going on and why. Then you can merge the function into main and strictly adhere to the requirements of the assignment.

Use some creativity, man. :icon_rolleyes:

[iw2z]

Well, I would rather not give you the straight answer, but rather the I would think about it:

The task is to swap the values of 2 variables using pointers.

So first, write the code the swaps the values of the variables without the use of pointers (using only variables x, y and temp), and make sure it's working.

i would just use three variables way to swap the values.

I guess that is trivial for you.

Then, replace each variable (except temp, which you're allowed to use) with a pointer that points to it.

I hope I've helped.

[xiansen]

thank you! i have figured it out.