Swap two variables using macro

Ziaur Rahman's Avatar author of Swap two variables using macro
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This is an article on Swap two variables using macro in C.
The code to swap two variables using macro expansions

Code: C
#include<stdio.h>
#include<conio.h>
#define SWAPE(x,y) int t;t=x;x=y;y=t;
main()
{
    int a,b;
    
    printf("\n Enter two number");
    scanf("%d%d",&a,&b);
    printf("\n Before swaping the Value of a=%d and b=%d",a,b);
    SWAPE(a,b);
    printf("\n After swap value of a=%d and b=%d",a,b);
    return 0;
}
Newbie Member
2Nov2006,13:35   #2
nicenaidu's Avatar
//Code for swapping two numbers without using temp variable
Code: CPP
#include <iostream>
using namespace std;
int main ()
{
  int a = 10;
  int b = 5;

  cout << "before swap: a = " << a << " b = " << b << endl;
  
  //Swapping numbers without using a temp var : method1
  a = a ^ b;
  b = b ^ a;
  a = a ^ b;

  cout << "after swap using method1: a = " << a << " b = " << b << endl;

  //Swapping numbers without using a temp var : method2
  a = a + b;
  b = a - b;
  a = a - b;
  cout << "after swap using method2: a = " << a << " b = " << b << endl;

  return 0;
}
Light Poster
22Dec2006,14:42   #3
aurnk's Avatar
better method is
usage of
#define swap(a,b) a^=b^=a^=b;
Light Poster
22Dec2006,14:43   #4
aurnk's Avatar
better method is
usage of
#define swap(a,b) a^=b^=a^=b;

one line swap without using temporary variable.
Contributor
12Apr2007,11:15   #5
Peter_APIIT's Avatar
Hello nicenaidu, i have compiler the file you post in MS VS 2005 but the IDE complaint that cstiod.h error .

Thanks for your help.


Your help is greatly appreciated by me and others.
Newbie Member
24May2007,15:43   #6
Vikrant Singh's Avatar
Here is a suggestion...

Using ^ operator is not the best way of swapping two values.
Just consider is soemone try to SWAP same variable, like SWAP(A,A) what will happen?
A^A will clear the all of the bits of A, because of which A will lost its original value on calling SWAP macro.

Further swapping value using equation like A+B is not advisable , if A and B are very large values than result of this addition may cross the maximum value which an integer can store. We may end up with a incorrect result.

So we should not try to play smart with fancy instruction as far as swapping is concerned.
Go4Expert Member
25May2007,13:30   #7
oleber's Avatar
Lets speak of C/C++

Why use a Macro and fix a type?

Template functions can do the work cleaner.

Code: CPP
template <class TYPE>
void swamp(TYPE &var1, TYPE &var2) {
    TYPE var_temp = var1;
    var1 = var2;
    var2 = var_temp;
}
Newbie Member
25May2007,18:59   #8
Vikrant Singh's Avatar
This can be done in Macro also…
Try this…
#define SWAP (A, B) struct tempStruct { char C[sizeof(A)];} swap_tmp;\
swap_tmp = *( struct tempStruct*) &A;\
*( struct tempStruct*) &A = *( struct tempStruct*) &B;\
*( struct tempStruct*) &B = swap_tmp;
Go4Expert Member
25May2007,20:04   #9
oleber's Avatar
Correct to

Code:
    #define SWAP(A, B) {struct tempStruct { char C[sizeof(A)];} swap_tmp;\
    swap_tmp = *( struct tempStruct*) &A;\
    *( struct tempStruct*) &A = *( struct tempStruct*) &B;\
    *( struct tempStruct*) &B = swap_tmp;}
Other wise you get problems when using diferent types

Code: CPP
#include <cstdio>
    #include <iostream>
    
    using namespace std;
    
    #define SWAP(A, B) {struct tempStruct { char C[sizeof(A)];} swap_tmp;\
    swap_tmp = *( struct tempStruct*) &A;\
    *( struct tempStruct*) &A = *( struct tempStruct*) &B;\
    *( struct tempStruct*) &B = swap_tmp;}
    
    int main () {
        
        char v1 = 'a';
        char v2 = 'b';    
        SWAP (v1, v2);
        cout << v1 << "   " << v2 <<"\n";
    
        float f1 = 45;
        float f2 = 123;
        SWAP (f1, f2);
        cout << f1 << "   " << f2 <<"\n";
    }
Last edited by oleber; 25May2007 at 20:32..
Newbie Member
4Jul2007,15:39   #10
keith12125's Avatar
>better method is
>usage of
>#define swap(a,b) a^=b^=a^=b;
>
>one line swap without using temporary variable

what exactly does the assignment operator ^= do?
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