Pre and post increment

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Hello,

Here is a program with output in gcc (4.3.2) on pre and post increments:

//--------------------code begin--------------------------------------------
#include <stdio.h>

main () {
    int a;
    a=1; printf ("1. %d %d\n", ++a, a);                     // 1. 2 2
    a=1; printf ("2. %d %d\n", a, a++);                     // 2. 2 1
    a=1; printf ("3. %d %d\n", a++, a);                     // 3. 1 2
    a=1; printf ("4. %d %d\n", a++, ++a);                   // 4. 2 3
    a=1; printf ("5. %d %d\n", ++a, a++);                   // 5. 3 1
    a=1; printf ("6. %d %d %d\n", ++a, a, a++);             // 6. 3 3 1
    a=1; printf ("7. %d %d %d\n", a++, a, ++a);             // 7. 2 3 3
    a=1; printf ("8. %d %d %d %d\n", a, a++, ++a, a);       // 8. 3 2 3 3
    a=1; printf ("9. %d %d %d %d\n", a, ++a, a++, a);       // 9. 3 3 1 3
    a=1; printf ("10. %d %d %d %d %d\n", a, a++, a, ++a, a);// 10. 3 2 3 3 3 
    a=1; printf ("11. %d %d %d %d %d\n", a, ++a, a, a++, a);// 11. 3 3 3 1 3 
}
//--------------------code end--------------------------------------------

The output from the program is listed next to it in comments. I thought
I knew something about pre and post increments, but this program busted
my understanding of the pre and post increments. I would appreciate
if someone could explain me the output.

Thanks