How Does Increment/Decrement Operators Work...??

[ajay2nokia]

Can any1 plz tell me how increment & and decrement operators work in C as well as in Linux if known...!
As far as I knwo there are post & pre increment/decrement operators :
I m getting diff results when calculated manually & different when programmed in C ....!!
Suppose for ex :

a = 2b + (- - b) + 3(b++) +b

if b = 3
what is ans of b
manually ans is = 6 + 2 + (3*2) + 3 = 17
but when programmed ans is 19 yyyyyyy..........???
nt only in this case in many cases it happens i want to know the priorities which comes 1st and all ThX in adv....!!

[mdnayeem]

First of all, its a very bad idea to write a single experssion in which one operand depends on the value of another operand.
In general, the expressions are evaluated according to the precedence of the operators. In your case, the b++ operand should be evaluated first then --b followed by * and + respectively. Then one has to worry about associativity. But, there is usually no guarantee in which order the operands are evaluated.
As far as I can tell what is happening in your example is that first the b++ term is evaluated then 2*b follwed by --b and then +b. So, this is what you would get
2*3 + 2 + 3*3 + 2 = 19 (I assume this is not the answer you were looking for but you were looking for 17)
That is why you will have to be very careful when writing a single experssion where one operand depends on the value of another operand. In fact, I must again say that one should avoid using such experssions.

[wje_lf]

What mdnayeem said.

As an illustration, I tried running this program, which evaluates the terms of your expression in all 24 possible orders:

#include <stdio.h>

int main(void)
{
  int a;
  int b;

  b=3;

  a  = 0; a += 2*b;     a += (--b);   a += 3*(b++); a += b;       printf("%d ",a);
  a  = 0; a += 2*b;     a += (--b);   a += b;       a += 3*(b++); printf("%d ",a);
  a  = 0; a += 2*b;     a += 3*(b++); a += (--b);   a += b;       printf("%d ",a);
  a  = 0; a += 2*b;     a += b;       a += (--b);   a += 3*(b++); printf("%d ",a);
  a  = 0; a += 2*b;     a += 3*(b++); a += b;       a += (--b);   printf("%d ",a);
  a  = 0; a += 2*b;     a += b;       a += 3*(b++); a += (--b);   printf("%d ",a);
  a  = 0; a += (--b);   a += 2*b;     a += 3*(b++); a += b;       printf("%d ",a);
  a  = 0; a += (--b);   a += 2*b;     a += b;       a += 3*(b++); printf("%d ",a);
  a  = 0; a += 3*(b++); a += 2*b;     a += (--b);   a += b;       printf("%d ",a);
  a  = 0; a += b;       a += 2*b;     a += (--b);   a += 3*(b++); printf("%d ",a);
  a  = 0; a += 3*(b++); a += 2*b;     a += b;       a += (--b);   printf("%d ",a);
  a  = 0; a += b;       a += 2*b;     a += 3*(b++); a += (--b);   printf("%d ",a);
  a  = 0; a += (--b);   a += 3*(b++); a += 2*b;     a += b;       printf("%d ",a);
  a  = 0; a += (--b);   a += b;       a += 2*b;     a += 3*(b++); printf("%d ",a);
  a  = 0; a += 3*(b++); a += (--b);   a += 2*b;     a += b;       printf("%d ",a);
  a  = 0; a += b;       a += (--b);   a += 2*b;     a += 3*(b++); printf("%d ",a);
  a  = 0; a += 3*(b++); a += b;       a += 2*b;     a += (--b);   printf("%d ",a);
  a  = 0; a += b;       a += 3*(b++); a += 2*b;     a += (--b);   printf("%d ",a);
  a  = 0; a += (--b);   a += 3*(b++); a += b;       a += 2*b;     printf("%d ",a);
  a  = 0; a += (--b);   a += b;       a += 3*(b++); a += 2*b;     printf("%d ",a);
  a  = 0; a += 3*(b++); a += (--b);   a += b;       a += 2*b;     printf("%d ",a);
  a  = 0; a += b;       a += (--b);   a += 3*(b++); a += 2*b;     printf("%d ",a);
  a  = 0; a += 3*(b++); a += b;       a += (--b);   a += 2*b;     printf("%d ",a);
  a  = 0; a += b;       a += 3*(b++); a += (--b);   a += 2*b;     printf("%d ",a);

  printf("\n");

  return 0;

} /* main() */

The output:

17 16 21 17 22 21 15 14 23 17 24 21 17 14 21 15 24 23 17 16 21 17 22 21