Page 2 of 2<12 Show 40 post(s) from this thread on one page

Go4Expert (http://www.go4expert.com/)
-   C (http://www.go4expert.com/articles/c-tutorials/)
-   -   Swap two variables using macro (http://www.go4expert.com/articles/swap-variables-using-macro-t1732/)

 Vikrant Singh 4Jul2007 15:43

Re: Swap two variables using macro

a^=b is equivalent to a = a ^ b

 keith12125 5Jul2007 15:51

Re: Swap two variables using macro

^= is a binary XOR assignment;

It seems to run really slow however...
I wrote 2 versions of the same sorting function, 1 using a temp variable and the other using this method. When i ran it on my P4 sort_xor took 24 seconds while sort_temp took 15.

Code:

```#include <stdio.h> #include <stdlib.h> #define SIZE 50000 void sort_xor(int *a, int size) /* Sorts the terms of the given array into numerical order. */ {         bool stop = false;         while (!stop)         {                 stop = true;                 for(int i = 0; i < size - 1; i++)                         if (a[i] > a[i + 1])                         {                                 a[i] ^= a[i + 1] ^= a[i] ^= a[i + 1];                                 stop = false;                         }         } } void sort_temp(int *a, int size) /* Sorts the terms of the given array into numerical order. */ {         bool stop = false;         int temp;         while (!stop)         {                 stop = true;                 for(int i = 0; i < size - 1; i++)                         if (a[i] > a[i + 1])                         {                                 temp = a[i];                                 a[i] = a[i + 1];                                 a[i + 1] = temp;                                 stop = false;                         }         } } int main() {         int a[SIZE];         for (int i = 0; i < SIZE; i++)                 a[i] = SIZE - i;         printf("Sorting with sort_xor...\n");                    /* I start timing here*/         sort_xor(a, SIZE);         printf("Finished Shorting with shor_xor.\n");            /* Stop here */         printf("Press any key to sort with sort_temp\n");         getchar();         for (int i = 0; i < SIZE; i++)                 a[i] = SIZE - i;         printf("Sorting with sort_temp...\n");                  /* I start timing here*/         sort_temp(a, SIZE);         printf("Finished Shorting with shor_temp.\n");          /* Stop here */ }```

 Vikrant Singh 5Jul2007 18:26

Re: Swap two variables using macro

Yes you are correct. and following is the reason...

analyze following assembly code

temp = x
mov eax,dword ptr [x]
mov dword ptr [temp],eax

x = y;
mov eax,dword ptr [y]
mov dword ptr [x],eax

y = temp;
mov eax,dword ptr [temp]
mov dword ptr [y],eax

6 mov instruction....

x ^= y ^= x ^= y;
mov eax,dword ptr [x]
xor eax,dword ptr [y]
mov dword ptr [x],eax
mov ecx,dword ptr [y]
xor ecx,dword ptr [x]
mov dword ptr [y],ecx
mov edx,dword ptr [x]
xor edx,dword ptr [y]
mov dword ptr [x],edx
6 Mov + 3 Xor

 sharma_atul13 16Jul2007 15:34

Re: Swap two variables using macro

the below logic also works as follows for swapping of nos without external variable introduction
Code:

```main() {   int a,b;   printf("enter the nos");   scanf("%d %d",&a,&b);   printf("nos before swapping a=%d ,b=%d", a,b);   a=a*b;   b=a/b;   a=a/b;   printf("nos after swapping  a=%d ,b=%d", a,b); }```

 All times are GMT +5.5. The time now is 14:02. Page 2 of 2<12 Show 40 post(s) from this thread on one page