Not understanding the operation pre and post increment

Question

I have following C program and I'm not understanding the output of this program:

#include <stdio.h>

int main()
{
    int a=8;

    printf("%d\n", a++);

    printf("%d\n", ++a);

    printf("%d\n", a*a);

    return 0;
}

The value of first printf is 8 and the value of second printf is 10!! But how??

a is 8; `printf("%d\n", a++);`, a increments to 9 _after_ printing. `printf("%d\n", ++a);` a increment to 10 _before_ printing. — chux - Reinstate Monica, Mar 09 '14 at 03:57
@Scotia It's easier to read the docs, or your textbook, for a question like this. — David Heffernan, Mar 09 '14 at 07:34

Dai · Accepted Answer · 2014-03-09T04:32:52.963

2

a++ is postfix notation, the value is first incremented, but the previous value is returned, which is then passed into prinf.
++a is prefix notation, the value is first incremented before it's passed into printf.

Your code is roughly equivalent to this:

int a = 8;
printf("%d\n", a);
a++;

a++;
printf("%d\n", a);
printf("%d\n", a*a);
return 0;

But it's closer to this (note I'm using the , operator, which evaluates each sub-expression but only returns the final sub-expression):

int a = 8; int aTemp;

// a++
printf("%d\n", (aTemp = a, a = a + 1, aTemp) );

// ++a
printf("%d\n", (a = a + 1) );

edited Mar 09 '14 at 04:32

answered Mar 09 '14 at 03:58

Dai

1

Actually `a` is incremented *before* the call to `printf` is evaluated. `a++` just evaluates to the old value of `a`, not the new value. – sepp2k Mar 09 '14 at 04:07
@sepp2k Yes, I was just rephrasing it to be simpler to understand. – Dai Mar 09 '14 at 04:24
@sepp2k I've reworded my answer and provided a better equivalent expression. – Dai Mar 09 '14 at 04:33
@Scotia it represents the intermediate value of `a++` that would exist in a CPU register or stack-space during the method-call. – Dai Mar 09 '14 at 04:44
How this line `printf("%d\n", (aTemp = a, a = a + 1, aTemp) );` is printing 8??? – opu 웃 Mar 09 '14 at 04:49
1

@Scotia See here: http://en.wikipedia.org/wiki/Comma_operator – Dai Mar 09 '14 at 04:52

score 1 · Answer 2 · answered Mar 09 '14 at 03:58

1

a++ will use a in the current statement (in this case output to screen) and then increment "a" after the statement.

++a will increment "a" before the statement is executed.

answered Mar 09 '14 at 03:58

Russell Jonakin

2 Answers2